81724134 planets by Elen Fro

A GOLDEN GUIDE速

Complete your collection of Golden Guides and Golden Field Guides! GOLDEN GUIDES BIRD LIFE

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PLANETS

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FISHES

GEOLOGY

MAMMALS

POND LIFE

REPTILES AND AMPHIBIANS ROCKS AND MINERALS SEASHELLS OF THE WORLD SEASHORES

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SKY OBSERVER'S GUIDE

SPIDERS AND THEIR KIN TROPICAL FISH

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STARS

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TREES

VENOMOUS ANIMALS

WEATHER

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WEEDS

WHALES AND OTHER MARINE MAMMALS

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FOREWORD Welcome to the solar system! Over the past three decades of the space age, we have learned more about our celestial neighbors than in any other period in history. We have discovered that each planet and satellite is a world in its own right and that so me of the minor me mbers of the solar syste m preserve information about the formation and evo lution of the Sun, the planets, and even life itself. This book is for everyone who wants to explore these new worlds. It provides information about the m and tells you where and when they can be seen in the sky. This book, a co mpanion to the Golden Guide Stars, is both an ar m chair guide and a field guide you can take with you wher ever you go. The author would like to express his thanks to many people, but particularly to the following: Caroline Green berg and Re mo Cosentino at Golden Press, who initiated the book and tirelessly gave their expertise, encourage ment, and friendship; Bonny lee Michaelson for help and encouragement; colleagues at the National Space Society, especially Leonard David; and Ron Miller for his fine illus trations. A special thanks is also owed to the scientists, managers, and associated investigators of the National Aeronautics and Space Ad ministration for achieving the spectacularly successful planetary exploration missions that made possible many of the photographs in this book. And acknowledgment is due to the A merican public for making these explorations possible through their support. M. R. C � 1990 Western Publishing Company, Inc. Illustrations <C> 1990 Ron Miller. All rights reserved, including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical

device, printed or written or oral, or recording for sound or visual reproduction or for

use in any knowledge retrieval system or device, unless permission in writing is obtained from the copyright proprietor. Produced in the U.S.A. by Western Publishing Company, Inc. Published by Golden Press, New York, N.Y. Library of Congress Catalog Card Numbe" 89-051364.1SBN 0-307-24077-0

CONTENTS IN TRODUCTION

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12 14

The Copernican Revolution

The Solar System in the Universe OBSERVING THE SKY The Zodiac

Early Theories of the Planets

Finding Things in the Sky

Planetary Phenomena

Effects of the Atmosphere The Moon Illusion

SOLAR SYS TEM EXPLORATION

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FORMATION OF THE SUN AND PLANETS

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INTRODUCTION TO THE PLANETS Planetary Orbits Planetary Sizes

Planetary Motion Kepler's Laws Planet Types THE SUN

MERCURY

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20 25

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VENUS

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EARTH

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THE MOON

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MARS

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AS TEROIDS: THE MINOR PLANETS

COMETS

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114

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132

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140

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146

150

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PLANETS BEYOND PLUTO? BIBLIOGRAPHY .

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INDEX

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NEPTUNE

PLUTO

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SATURN URANUS

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JUPITER

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156

157

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158

Ancient s kywatchers

INTRODUCTION The planets and their satellites, asteroids (the minor planets), and the Sun make up the solar system, our cosmic neighborhood. If we could travel at the speed of light, 186,000 miles a second, it would take only eight minutes to reach the Sun, and only half a day to cross the solar syste m fro m one side of Pluto's orbit to the other. The nearest stars, by co mparison, are several light-years away. MYTHOLOGY is the way early peoples tried to explain what they saw in the sky. They could easily see the Sun and Moon, five bright planets, occasional co mets, and frequent meteors, but they did not know what they were. To them, everything in the sky was magical, some sort of god or demon. Astrology, the false clai m that events in the heav足 ens control our lives and predict the future, grew out of these myths. 4

The ancients tried to find patterns in the sky and the movement of the objects in it. A mong the fixed stars they sow "pictures"-the constellations. They noticed that the Sun, Moon, and planets wondered a mong the stars. Many ancient peoples built monumental structures-Stonehenge is an exa mple-to observe the sky. ASTR O N OMY is the science of observing the universe, using logical procedures and continual testing of theories. As we refine our knowledge, so me of the facts and nu mbers in this book will hove to be revised. Astrono my is on international endeavor. You see the so me sky the ancients sow, but you hove the benefit of much more knowledge to explain what you observe. Modern astrono my is only a few centuries old. SK YWATCH I N G is on enjoyable pastime. Field guides, such as Stars and Skyguide (see the bibliography, p. 157), con help you identify the stars and constellations, while astrono my magazines keep you in touch both with events in the sky and with recent discoveries of astrono my and the space program. Binoculars, preferably 7 X 50 size, ore very useful for sca nning the sky. They are also comparatively i nexpensive. later you may wish to step up to a telescope. Before you buy one, consult a good guide for advice and avoid poorly mode depart ment-store telescopes. PLAN ETA R I UMS ore great places to beco me familiar with the sky. Often planetariu ms also offer public observing through telescopes, a nd courses in astronomy. A mateur astro nomical societies have freq ue nt meeti ngs a nd "star parties" where you con look at the planets through s mall telescopes. Professional observatories ore usually too busy to allow the public to use the instruments at night. 5

The Ptolemaic system

EARLY THEORIES OF THE PLANETS

The earliest skywatchers who kept records were the Su mer足 ians, Babylonians, and Egyptians. They were followed by the Greeks, who continued to try to find order in the sky and its motions. Everything in the sky was thought to be some sort of star : fixed stars that didn't change and didn't move relative to one another, shooting stars ( meteors) that flashed across the sky, hairy stars (comets) that appeared occasionally. They gave the na me planetes, meaning "wanderers," to the Sun, Moon, and planets that move against the background of fixed stars. TO TH E G R E EK S, the Earth was the center of the universe, and everything moved around it. In their philosophy, everything above the Earth was celestial and perfect. The perfect shape was the sphere, so they thought that the planets (including Sun and Moon) were attached to huge turning transparent crystalline spheres centered on the Earth, causing planetary motions. 6

THE PTOL EMAIC SYSTEM was set forth by Claudius Ptol e my, a Greek astrono mer working in Alexandria, Egypt, in the 2nd century A. D. He proposed that planets move on little circles, called epicycles, while the center of each epicycle moves in a circular path, a "cycle" or "deferent," around the Earth. By choosing the proper sizes and speeds for the cycles and epicycles, this theory could account for irregularities in the observed motions of the planets. Ptol emy's scheme was thought to be true for almost 1 ,400 years. TH E TYCH O N I C SYSTEM was proposed by Tycho Brahe, a Danish astrono mer, in the late 16th century. He thought the Moon and Sun revolved around the Earth, but that all the planets revolved around the Sun. Tycho was not right, but his careful naked-eye observations led Johannes Kepler to figure out just how they do move (see p. 3 1). P R E-TEL E S C O P I C I N ST R U M E NTS consisted of mechani cal devices for measuring angles, and hence positions, of things in the sky. Sundials and hourglasses measured the passage of ti me, and accurate clocks did not co me along until the 17th and 18th centuries. The most co m mon astro nomical observi ng ins tr u ments were astrolabes, which were often beautiful works of crafts manship as well as practical tools. Tycho and others also used large quadrants, al most the size of a roo m, to measure a n g l e s in t h e s k y v e r y accurately. The Tychonic system

Copernicus' Sun-centered system

THE COPERNICAN REVOLUTION

The Polish-Germa n astro nomer Nicola us Copernicus, who died i n 1543, revolutionized our ideas abo ut the u niverse when he proposed, correctly, that the S u n is the ce nter of the solar system, and the Earth is j ust another planet. His controversial ideas eve ntually prevailed. Several decades later, Tycho disproved two of the key beliefs the a ncie nts had about the sky, and so helped prove Copernicus right. One a ncie nt belief was that the fixed stars were u nchanging, but in 1572 Tycho discovered a new star, called a nova. (We know now that novas are actually older stars blowing up and get ting temporarily brighter.) Tycho also disproved a belief that comets were really hot gasses high in Earth's atmosphere. Careful mea足 surements of a comet showed it was beyond the Moo n. O ne 8

of Tycho's assistants, Johannes Kepler, used Tycho's obser足 vations to deduce the laws by which planets move around the Sun (see p. 3 1 ). THE T EL E S C O P E was adapted for astronomical use by Galileo Galilei, in Florence, about 16 10. With it, distant objects were brought nearer and clearer, fainter objects became visible, and measurements were more precise. Among Galileo's first discoveries were four satellites revolving about Jupiter. This discovery also helped dis足 prove the older ideas. For these contributions, which upset many conservative beliefs, Galileo was arrested and imprisoned by the Catholic Church until the end of his life, and his books were long banned. Over the next few decades, Galileo and others, such as Cassini, Roemer, and Huygens, discovered many new things: the rings of Saturn, more satellites, sunspots, and many of the phenomena we know today. With them began the modern age of astronomy. Galilee Galilei and his new telescope

THE SOLAR SYSTEM IN THE UNIVERSE

The solar system is only a very, very small part of the universe. We do not know how big the universe is, but astronomers can see several billions of light-years away with their largest radio and optical telescopes. (A light year, the distance light can travel in a year, is approxi mately 5.8 trillion miles.) The universe is between 10 and 20 billion years old. Our solar system and Sun are only about 4.5 billion years old. TH E LARGEST STR U C T U R E S we know of are clusters of galaxies, islands of stars, tens or hundreds of millions of light-years across, containing from a few dozen to thou sands of galaxies. GALAX I E S are of different shapes and sizes. Some are flat disks called spiral galaxies (our own Milky Way galaxy is a spiral). Others are globular or ellipsoidal, while still others are irregular. Galaxies contain from a few billion to a trillion stars and range in size from a few tens of thou sands of light-years across to several hundred thousand light-years in diameter. Galaxies are typically spaced a few million light-years apart. STARS, together with clouds of dust and gas (called nebu las), typically are spaced a few light-years apart. Stars are huge gaseous balls, generating energy through nuclear fusion in their cores. The Sun is an average-sized star. Other ordinary stars range in size from about a tenth the size of the Sun to perhaps ten times its size. Extraordinarily small stars, such as neutron stars, may be only a few miles across, the size of a large city, while the largest stars are billions of miles in diameter, the size of the orbit of Jupiter or Saturn in our solar system. 10

The solar system in the galaxy

PLAN ETS are cold, relatively s mall bodies (compared to most stars) circling the Sun (and perhaps other stars). The diameter of the solar syste m, measured across the orbit of the outermost planet, Pluto, is only about 1 1 light-hours. SATELLITES, loosely called " moons," are s maller bodies orbiting the planets. Some of the m may be as large as the smaller planets. Asteroids ( minor planets) are s mall bodies orbiting the Sun. NAM E S of celestial bodies such as newly found asteroids and satellites, and names for such features on planets and satellites as mountains and craters, are proposed by their discoverers and must be approved by the International Astronomical Union (IA U). Comets are na med for their discoverer(s). S maller bodies may not have names, only catalog nu mbers. 11

The Sun, Moon, and planets look as though they move against the background of the constellations of the zodiac.

OBSERVING THE SKY THE ZODIAC The zodiac is the region of the sky through which the Sun, Moon, and planets pass, as seen from Earth. It is a band about 30 degrees north and south of the celestial equator, the projection of the Earth's equator into the sky. For simplicity, we pretend that all the stars are attached to a great "celestial sphere" some large, indefinite distance away. These stars, and the patterns of the stars called constellations, are the background against which the planets move. Following the practice of the ancients, the band of the zodiac is divided into 1 2 constellations, most of which were thought to represent living creatures. "Zodiac" means "cir足 cle of the animals" in Greek. The 12 constellations of the 12

zodiac are: Aries, Taurus, Gemini, Ca ncer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, a nd Pisces. A conve nient way of locati ng the planets is to give the name of the co nstellatio n of the zodiac they are "i n," which really mea ns "i n front of." Because of cha nges in the sky over the past 2,000 years, which altered the orientatio n of the co nstellatio ns, today the pla nets actually travel through more tha n the 12 classical constellations. As see n from the Earth, the Su n seems to travel duri ng a year around the zodiac, always alo ng a line or path in the sky called the ecliptic, the pla ne of the Earth's orbit around the Su n projected out o nto the celestial s phere. Because the Earth is tilted over 23 degrees, the ecliptic a nd the celestial equator are i nclined by the same amount. The orbits of the Moo n a nd planets are tilted slightly to the ecliptic, so their paths in the sky do not lie exactly alo ng the ecliptic, but they are always close-exce pt for Pluto. 13

PLAN ETARY PH E NOMENA refers to the appearance in the sky and the relative positions of planets with respect to the Sun or each other. The angle between the Sun and a planet, as we see them from Earth, is a planet's elongation. " EV E N I NG STAR" is a term given to any planet that remains in the sky after the Sun has set. Such a planet will be east of the Sun-that is, it will have an easterly elongation. "MO R N I NG STAR" refers to a planet that is in the sky at dawn. It will be west of the Sun, and so have a westerly elongation. All the planets are at times morning stars and at other times evening stars. I N F E R I O R PLAN ETS-Mercury and Venus-are those with orbits closer to the Sun than Earth's orbit. They are never seen at a great angle from the Sun. When an inferior planet is aligned with the Sun, it is said to be in conjunction. Inferior conjunction ("I" on diagram below) occurs when the planet is on the near side of the Sun, superior conjunc足 tion ("S") when it is on the far side of the Sun. The greatest angular distance between an inferior planet and the Sun occurs when our line of sight to the planet is tan足 gent to the planet's orbit, called g reatest eastern elongation (" E") when the planet is east of the Sun and greatest western elongation ("W") when it is west of the Sun. Inferior planet phenomena

Inferior planets show phases, like the Moon, related to where they are in their orbits. They are at full phase at the time of su perior conjunction, halfphase at times of greatest elongation, and new phase (hence invisible) at the time of inferior con junction. S U P E R I O R PLAN ETS -Mars, Ju piter, Saturn, Uranus, Nep tune, and Pluto-are those with orbits larger than Earth's, and they may have any elongation. When on the far side of the Sun, as seen from Earth, they are said to be in conjunction ("C" on diagram below). When o p posite the Sun, they are in opposition ("0"). When they are at right angles to the Sun, they are said to be in quadrature ( Q ) Such planets have only a narrow range of phases, being almost fully illuminated as seen from Earth at all times. "

TH E SYNOD I C P E R IOD of an object is the length of time it takes for the object to return to a previous relative position, such as op position or conjunction. V I EW I N G TH E PLAN ETS For the inferior planets, times of greatest elongation are best for viewing. The su perior planets are brightest and visible longest at night around the time of opposi Earth-� tion-when the Earth is directly between the planet and the Sun. Planets are not visible ', close to the times of con Sun junctions, for then they are too close to the Sun in the sky.

0\ ( /. \c ,:f-1

Superior planet phenomena

Q��

An observer an d the horizon

FINDING THINGS IN THE SKY

What you see depends upon when you look. Sometimes the things you want to see are up in the daytime. The Moon is bright enough to be seen in the daytime, but the stars and planets are overpowered by the brightness of the daytime sky. As the Earth turns, the sky seems to revolve about us, moving west 15 degrees every hour. To locate things in the sky, you must first know which direction is north. You can use a magnetic compass, or, at night, look for the North Star. For those in the northern 16

he misphere, the celestial equator will start at the horizon in the east, rise upward and to the right toward the south, reaching its highest point due south, and then decline downward to the right, intersecting the horizon due west, opposite east. All planetary objects will appear within 30 degrees on either side of this invisible celestial path. AZ IMUTH is a measure of direction around the horizon starting at 0 degrees due north, moving clockwise through 90 degrees at east, 180 degrees at south, 270 degrees at west, and 360, or 0, degrees again at north. ELEVAT I O N of an object is its angle above the horizon, fro m 0 degrees at the horizon to 90 degrees directly overhead (a point called the zenith). A useful rule of thumb is that your fist, held out at arm's length, is about 10 degrees across. F I ND I N G CHART S Each planet in this book has a "finding chart" that shows the planet's path through the zodiac during upco ming years or in the morning and evening skies. (The chart for each planet can be found in the section on "Observing.") Tables give the best times for viewing each planet. For Mercury, this would be the week or so around the dates of great足 est elongations; for Venus, several weeks around the greatest elon gations; for the other planets, the sev足 eral months around the times of opposition. Sky motion a round Earth

The atmosphere distorts light rays

EFFECTS OF THE ATMOSPHERE on what we see in the sky are due to our looking through miles of turbulent air. When we look at objects near the horizon the effects are greater, because we are then looking through more air. TWINKLING, or scintillation, is caused by air layers slightly bending the light as it passes toward us. Stars, which are so far away they appear as mere points of light, twinkle more than planets do. Planets, which are closer to us, appear as small disks of light; they usually twinkle less. For objects close to the horizon, the air may act like a prism, causing a star or a planet to display flashing colors. REDDENING occurs because the atmosphere is not equally transparent to all colors of light. As a beam of light travels, the air removes some of its blue light, scattering it in all directions and causing the sky to appear blue. The remain足 ing light reaching your eye looks redder. The more air a light beam travels through, the greater the reddening, which is why the Sun and Moon appear reddish when they are low in the sky. Air pollution may accentuate this effect. REFRACTION is a bending of the light from an object in the sky, making the object appear slightly higher in the sky than it is. The effect is greatest closer to the horizon, where it seems to flatten the rising or setting Sun and Moon.

THE MOON ILLUSION

The moon illusion is the na me given to the optical illusion that the Moon (or Sun) appears larger when it is near the horizon than when it is high in the sky. This seems to be caused by the fact that, when near the horizon, the Moon is seen near fa miliar foreground objects, such as trees or buildings or the horizon itself. Because the brain cannot determine how far away the Moon is, it incorrectly interprets what we see. The effect seems most noticeable for the Moon when it is near full phase. One way to prove that the Moon does not change size is to photograph it rising, and then again, with the sa me camera and lens, when it is high in the sky. The i mage of the Moon on the slide or negative will measure the sa me linear size. Full moon rising

CARN EGI E

Apollo 11 astronauts on the Moon (1969)

NASA

SOLAR S YS TEM EXPLORATION Until the space age, we could only study the universe from Earth with instruments that had to look upward through miles of turbulent, filtering, polluted atmosphere. Now we send our robot observers throughout the solar system. TH E S U N has been examined by several special space observatories near Earth, including the Orbiting Solar Observatory and the Solar Maximum Mission. The distant effects of the solar wind have also been studied by probes called I M P, for Interplanetary Monitoring Platform, and by sensors on space probes heading to other planets. One of the I M P craft was Voyager passing Saturn (1977) later used to study a comet. NASA M E R C U R Y has been explored by only one space probe as of 1989, Mariner 10, in 1973 and 1974. Its flight allowed it to pass this innermost planet three times, mapping the surface.

V E N U S has been extensively investigated. The U. S. probe Mariner 2 flew by in 1962, revolutionizing our knowledge of this cloud-wrapped world. Since then, other American visits have been made by Mariner 5 in 1967, Mariner 10 in 1974, and Pioneer Venus, which in 1978 orbited the planet and sent a probe into its atmosphere. Most recently the Magellan spacecraft made detailed maps of the surface. By 1989, the Soviet Union had sent 13 spacecraft, all named Venera, to Venus. Some have even landed on the surface and taken a few photographs before the hostile atmosphere of that planet destroyed the probes. These are the only surface photographs we have of any planetary body besides the Moon and Mars. EARTH, too, has been explored with spacecraft. Meteo足 rological satellites monitor our atmosphere. Closer-orbit足 ing landsat earth resources satellites have also greatly improved our knowledge of our planet and its biological, geological, and hydrological systems. Hubble Space Telescope

NASA

THE MOON, the only plan足 etary body besides Earth explored by humans in per足 son, was visited during the American Apollo program from 1969-72. More than 800 pounds of moon rocks were brou ght back for study. Before that, several U. S. and Soviet probes orbited or landed on the Moon to pave the way. The U. S. S. R. has sent robot landers to gather and bring From Viking Lander 2, showing bock rock samples from the frost on Mars ( 1977) NASA surface. MARS was first photographed close up by the American Mariner 4 in 1965. Three other Mariner croft and several Soviet spacecraft have visited the Red Planet, although the Iotter were not successful. The climax of Martian investi足 gation came in 1976 when two U. S. Viking spacecraft orbited the planet and sent landing croft down to the surface. These landers operated for years, sending bock photographs, soil analyses, and meteorological data. JUPITER sow the flybys of Pioneer 10 in 1973, Pioneer 1 1 in 1974, and Voyagers 1 and 2 in 1979. They discovered new satellites and a ring, and photographed and measured details in Jupiter's atmosphere invisible from Earth. The computer-enhanced photographs of Jupiter ore some of the most beautiful ever token. SATURN was visited by Pioneer 1 1 in 1979, by Voyager 1 in 1980, and then by Voyager 2 in 198 1, after their swings by Jupiter, whose gravity helped get them out to the Ringed Planet. They discovered new moons and myriad rings.

URANUS was visited by Voyager 2 in 1986, after a gravity assist from Saturn. It discovered many new sat ellites, new rings, and a puzzling tilted magnetic field that has yet to be fully explained. NEPTUNE was visited by Voyager 2 in August, 1989, the last Voyager encounter before leaving the solar system. TWO COMETS have been p r o b e d . T h e fi r s t w a s Comet Giacobini-Zinner in 1985, reusing one of the IM P spacecraft, renamed I C E ( International Come tary Explorer). Data from ICE helped space scientists from Europe, Ja pan, and the S o v i e t Un i o n send probes to Comet Halley. STARS,

NEBULAS,

AND

as well as planets have been observed by the Orbiting Astronomi cal Observatory from near Earth orbit, and by other specialized instruments that can receive kinds of light GA L AXI E S

Landsat 4 photograph of Earth (1982)

NASA

that never reach the surface of Earth. The latest is the Hubble Space Telescope, which promises to revolu tionize our knowledge of the cosmos. Giotto, the European comet mission (1986)

ESA

FORMATION OF THE SUN AND PLANETS Our universe began about 15 billion years ago with a tremendous explosion astronomers call the " Big Bang." The lighter elements, mostly hydrogen and helium, were formed in this explosion and spread throughout the uni足 verse. Later, most of these gases collected together into large bodies we call the galaxies. Our Milky Way galaxy is one of these, a giant pinwheel of stars, gas, and dust over 100,000 light-years across. Our Sun is one of several hundred billion stars in the Milky Way. The first stars to form in our Milky Way galaxy, perhaps 10 billion years ago, were almost pure hydrogen and helium, and could not have formed hard, rocky planets because there were no heavier elements. Some of the more massive original stars lived out their lives and exploded as supernovas. In the extreme heat of these explosions, heav足 ier elements such as carbon, oxygen, silicon, and iron were formed and spewed out to enrich the remaining gas. ABOUT 4.5 BILLION YEARS AGO one such supernova caused a cloud of interstellar gas and dust to condense, and the cloud continued to collapse under the force of its own gravity. As it shrank, it grew hotter and began to spin faster. The inner portion shrank faster, and got even hotter, spinning off and leaving behind smaller orbiting clouds of gas. These smaller clouds condensed to form large lumps of gas with some heavy material at their centers. The central blob of gas collapsed still more, until it was hot enough to cause nuclear fusion, the combining of atoms of hydrogen into helium, which releases energy.

Formation of the solar system

THE BIRTH OF THE SUN, when fusion started, created a blast of radiation that blew the rest of the surrounding gas away from the center. Smaller bodies near the Sun were stripped of their gases, while the more distant ones remained massive enough to keep theirs. Thus the inner planets are s mall, hard, and have little or no at mosphere; the outer planets are large and mostly pri mordial gas. The gravitational pulls of these large proto-planets attracted more material, and moreover they were hotter. Thus each of the large outer planets formed extensive satellite sys足 tems-with inner satellites that are rocky and dense, and outer moons that are icy and less dense. In the region just beyond proto-Mars, the gravity of proto-Jupiter kept any large bodies fro m forming, leaving behind the asteroid belt. Comets are probably the re mains of material that didn't get caught by the proto-planets. THE PLANETS and many of their satellites later became hot enough so that they partially or totally melted, letting denser ele ments sink to form a core and lighter ele ments float up to form a crust. This heat ca me fro m several sources. One source was the heat created by their contrac足 tion fro m a giant sphere of gas to a s maller object. Another was heat from radioactive decay of elements such as ura足 niu m and thorium. And a third source was the energy fro m impacts of s maller bodies falling onto the m, converting their energy of motion into heat. Over eons, so me satellites have escaped their planets, others have collided (some re-formed afterwards), and so me minor objects揃 have been captured by the major planets to become moons. Thus the solar syste m was born and evolved to what we see now.

Formation of the planets

PLANETARY ORBITS

Astronomers use t he astro足 n omi cal unit, a . u . , the average distance of approxi mately 93 million miles between t he Earth and t he Sun, as a unit of mea足 sure. It is almost i mpossible to show bot h t he sizes of the planets and t heir orbits to the sa me scale. If the Sun really were t he size s hown in the illustration on the fac足 ing page, the Earth to the s a m e scale would be 4 inches away, and Pluto would be 14 feet away. Most planet orbits have an inclination within a few degrees of Eart h's orbit, called the ecliptic. All the planets orbit in the sa me direction: Counterclock足 wise as seen from far above the North Pole of Earth. Planetary orbits are almost circles, except for Pluto. A n u mber d e s c r i b i n g t h e a mount an o r b i t differs fro m a perfect circle is called eccentricity. The planets, with the Planets compared i n size

exception of Venus and Uranus, all spin on their axes in the same counter足 clockwise direction. Most of the satellites of the planets revolve in the sa me way.

PLANETARY SIZES

The planets range in size from tiny Pluto, only 1,429 miles in diameter and with a mass 0.003 ti mes that of Earth's, to giant Jupiter, 88,732 miles across and with a mass 3 1 8 ti mes Earth's. The Sun, with a mass of 2 billion billion bil足 lion metric tons, contains more than 99.99 percent of all the material in the solar syste m, and is 864,949 miles in dia meter. Minor bodies of the solar syste m range fro m the larg足 est satellite, Jupiter's Gan足 y m e d e , w h i c h is a b o u t 3,268 miles across, through the largest asteroid, Ceres, so me 600 miles in size, down to microscopic dust particles. Planet orbits compared

Gravity keeps planets in orbit

PLANETARY MOTION

All objects in the solar system are in motion. The planets revolve around the Sun, and the satellites revolve around the planets. At the same time, the entire solar system is moving t hroug h space, ap proximately toward t he stars of the constellation lyra, at a speed of 44,000 miles per hour. (But since those stars are also moving, we will never "get there.") A PLANET'S YEAR, or period of revolution (also called orbital period), is the time it takes to orbit the Sun. The inner planets move faster, the outer ones slower. This is caused by a balancing of the force of gravity between the Sun and each planet and the planet's speed, which would tend to carry the planet away in a straight line. A PLANET'S DAY, or rotation period, is the time it takes to rotate once on its own axis. While it was possible to describe the motion of planets accurately (see next page) as early as 16 10, it took until the end of that century for

Sir Isaac Newton to explain why they move as they do. To accomplish that he had to discover the law of Gravity, the laws of Motion, and the mathematical technique called calculus. THE LAW OF GRAVITY states that the force pulling two bodies together is proportional to the product of their masses and inversely proportional to the square of their distance apart. Mathematically, this is expressed by F G (M1 X M2}/d2â&#x20AC;˘ =

KEPLER'S LAWS

In the early 1600s, Johannes Kepler used Tycho's observaÂ tions to derive the rules by which planets (and all other bodies) move. These are summarized as Kepler's Laws: I. The orbit of each planet is an ellipse and lies in a plane, with the Sun at the focus of the ellipse. (A circle is a special case of an ellipse with no eccentricity.)

II. A planet moves faster when closer to the Sun, and slower when farther away.

Ill. There is a fixed mathematical relationship between the size of the planet's orbit, A, expressed in a.u.'s, and the period of time it takes to go around the Sun once, Kepler's Laws

T, measured in years. The relationship is T' =A'.

PLANET TYPES

Astronomers often classify the planets (except Pluto, which is an anomaly) into two categories, depending on their size, structure, and composition. TERRESTRIAL- TY PE PLANETS are those somewhat like Earth. They are Earth itself, Mercury, Venus, and Mars. Such planets are basically hard, small, rocky worlds with little or no atmosphere (even the thick atmosphere of Venus, see p. 52, is thin in comparison to the atmosphere of Jupiter). Mars' density is 3.9; the others are about 5.5. Earth is the largest of these planets, and has the only nitrogen-oxygen atmosphere of any planet. Mercury has very little atmosphere, while the atmospheric pressure on Mars is only a few percent that of Earth's. Before the Sun began to shine, today's terrestrial-type planets had rocky cores surrounded by hydrogen-helium atmospheres. They are all close to the Sun, so eruptions of the new-formed Sun blew away these gasses, leaving them bare rocky worlds. Over the eons, Venus, Earth, and Mars acquired their present atmospheres from gasses emitted by their rocks and from volcanic emissions. Mercury is too close to the Sun, and hence Terrestrial planet cross section too hot, to retain much of an atmosphere. Terrestrial-type planets also rotate comparatively slowly on their axes. Earth and Mars take about 24 hours to rotate once, Mer足 cury takes two months, and Venus takes eight Earth months for one of its "days."

GAS-GIANT PLANETS, also called Jovian planets (after Jupiter), are large, low-density (between 0.6 and 1.6), mostly gaseous bodies. They are Jupiter, Saturn, Uranus, and Neptune. While these planets may have rocky cores (even larger than some terrestrial planets), the bulk of their material is gaseous, largely hydrogen and helium with smaller amounts of methane and ammonia. Their atmo足 spheres, the cloud tops of which are the only thing we can observe of them, are thousands of miles thick. At the top they are very cold, hundreds of degrees below zero Fahrenheit. Gas-giants all rotate on their axes more quickly than terrestrial planets. Jupiter and Saturn take only about ten hours for one day. Sometimes Uranus and Neptune are put into a class of their own, intermediate between terrestrial and gas-giant types, but closer to the latter. PLUTO does not fit either class. It is small, probably rocky and icy, has little atmosphere, and spins slowly. It is more like one of the satellites, and indeed may be an escaped satellite of Neptune. SATELLITES of

the planets, which are loosely called "moons," may be classified as either rocky or as icy, with some of them being both. A few of the satellites, most notably Titan (pp. 128- 129), which is one of Saturn's 18 satellites, have atmospheres.

Jovian planet cross section

THE SUN The Sun is a star and the dominant body in the solar system. It is the body that gives the solar system its name (from Sol). It is the major source of energy for all the planets, and it is by its reflected light that we are able to see them. The ancients thought the Sun a god, and gave it various names: Apollo, Ra, Sol, Helios, Shamash, Savitar. The Sun was the most important thing in the sky for the ancients, who quickly and correctly connected it with the weather, crops, and indeed their very existence. It was their time足 keeper and season marker. The fundamental direction for our ancestors was the east, where the Sun comes up. Our language today recalls that when we say we "orient" ourselves. Even as our knowledge grew, it was difficult to determine just what the Sun was, how far away it was, its size, and the origin of the Sun's energy. Not until the early 20th century were atomic fusion processes discovered that could explain the energy of the Sun and other stars. The Sun is about 4.5 billion years old, midway through its life. In a few billion more years it will expand almost to the size of Venus' orbit and cool, becoming what astrono足 mers call a red giant star. In the process it will end all life White-light image of the Sun NASA on Earth. After more hundreds of millions of years, the Sun will shrink, eventually, to a small, extremely dense star the size of Earth, a white dwarf star. Then it will slowly cool over billions of years, fading to invisibility. 34

Prominences seen in X-roy light

NASA

The Sun undergoes very slight di m ming and brightening. Even a 0. 1 percent change has a noticeable effect on Earth's climate. A few percent change could have serious consequences for Earth. Fortunately, the Sun seems to be fairly stable. When we observe the Sun in different colors of light (so me of which we can't see with our eyes, like ultraviolet and X rays), we are looking at different outer layers of the Sun. THE SUN is a ball of gas 864,000 miles in dia meter. It is about 78 percent hydroTheSun in redlight NASA gen, 20 percent heliu m, and 2 percent heavier ele足 ments, the most abundant of which are oxygen, car足 bon, nitrogen, and neon. The overall density of the Sun is only 1.4 ti mes that of water. The Sun rotates on its axis about once a month. Its strong magnetic and electrical fields control the appearances of many of the features we observe.

Structure of the Sun

THE PHOTOSPHERE, the "surface" we see, is a layer about 300 miles thick, with a pressure only about one足 tenth that of Earth's atmosphere at the surface and a density less than a millionth of our atmosphere. The tem足 perature of the gases in the photosphere is about 10,000 degrees Fahrenheit, which is why the Sun appears yellow. Cooler areas, which appear dark, are called sunspots. INSIDE the Sun the density, pressure, and temperature increase. The outer 15 percent of the Sun is in a state of turbulent motion, like boiling water, carrying the internal energy up to the surface. THE CORE of the Sun, only a few percent of the volume of the Sun. has temperatures of 30,000,000 degrees. The

core is over eight times denser than gold, but the material is still gaseous because of the very high temperature. Here, hydrogen atoms (" H" in diagram) are combined by nuclear fusion to make helium atoms (" He") plus energy. To pro足 duce the energy we see as sunlight, more than 1 1 billion pounds of hydrogen turn into energy each second. THE CHROMOSPHERE is the layer lying over the photo足 sphere. It is about 1,000 miles thick. As we go upward, the density falls rapidly, but the temperature increases. Small jets of gas a few thousand miles high, called spicules, stick above the chromosphere. THE CORONA, the Sun's hot, thin outer atmosphere, reaches at least a hundred million miles out into the solar system. PROMINENCES are huge clouds of glowing gas above the photosphere. Some form like clouds and fall down to the surface. Others spring up from near the surface and may travel millions of miles into space. THE SOLAR WIND is a high-speed flow of atomic particles out of the Sun. As they pass the Earth they are moving about a million miles per hour. Some may hit the Earth's atmosphere and cause the auroras. The solar wind can be detected far out in the solar system.

Nuclear fusion reaction

How to project theSun safely

TO VIEW THE SUN SAFELY, project an image of it with a pinhole, a long focal length lens, or a telescope, onto a piece of white paper. You may use an eyepiece at the end of the telescope to make a bigger image, but do not use an eyepiece with lenses that are cemented together; to do so can ruin the eyepiece. Because all binoculars have cemented eyepieces, do nc;>t use them either. Remember: Do not look through the lens, telescope, or binoculars. To do so can result in permanent eye damage, even blindness. look only at the projected image. A SOLAR FILTER can be made by sandwiching two or more layers of fully exposed and developed black-and-white film negative (no t color film) Making a sun filter between two pieces of glass, and taping the edges together. Never stare at the Sun for long even through such a filter. Photographic filters are not safe for this. S U N S POTS , t h e m a i n observable features on the

1800

1825

1850

1875

1900

1925

1950

1975

The sunspot cycle

Sun, are cooler areas in the photosphere. The darker, inner part of a sunspot is called the umbra; the outer, lighter part the penumbra. The number of sunspots varies, reaching a maximum on average every 1 1 years. This is called the sunspot cycle. The next predicted sunspot maximum is in the early 1990s, and the next minimum is expected near the turn of the century. Each day, as the Sun rotates on its axis, you may observe new sunspots on the western limb (edge) of the Sun. Day by day they are carried off eastward, to disappear off the eastern limb. Sunspots may last a few days or even weeks if they are large. Sunspots show solar rotation

Day1

Day5

Day 10

Day 15

••••

TotoI eclipse of the Sun

NASA

SOLAR ECLIPSES A total eclipse of the sun is among the most dramatic and bea utif ul sights in nature. It occurs when the Moon moves directly in front of the Sun, hiding the light from the Sun's s urface (the photosphere) and allowing us to view the faint, ghostly corona, or outer atmosphere, of the Sun. If a total solar eclipse occurs anywhere near yo u, try to go and see it. It will be one of the most awesome experiences of yo ur life. The Sun is 400 times larger than the Moon, b ut it is also abo ut 400 times farther away from Earth. Thus these two objects appear to occupy the same angle in the sky, called angular size. The Earth is the only planet in the solar system whose satellite is exactly the same angular size as the S un as seen from the planet's surface. There are at least two solar eclipses each year, and some years as many as five. A minimum of zero and a maximum of three are total eclipses. A total eclipse can be seen only from inside a narrow band along the Earth, 160 miles wide at most, which is the track of the Moon's inner shadow, the 40

umbra. It may be thousands of miles long. If you were to stay in one location, you would see, on average, a total eclipse about once every 350 years. Most peo足 ple have to travel to see a total eclipse. Outside the path of total足 ity, observers see o partial eclipse, in which the Sun is not completely covered. Even a 99 percent partial eclipse is not anywhere near as spectacular as a total one. Sometimes the umbra does not to uch the Earth. Then there is only a partial e c l i p s e . If t h e M o o n ' s shadow is too short to reach the Earth, the Moon may appear too small to cover the Sun completely, and at mid-eclipse there will be a ring of light, or annulus, around the Moon. This is called an annular eclipse. Solar eclipses occur only at the times of new moon, when the Moon is between the Sun and Earth. An eclipse does not occur each So lar ec lipse geometry

TOTAL SO LA R ECLIPSES D u ration of Tota l ity

Date

Reg ion of Visibi l i ty

1990

Ju122

2m33â&#x20AC;˘

Finland, Siberia, North Pacific

1991

Ju111

654

Pacific, Hawaii, Mexico, Central and

1992

Jun 30

5 20

Atlantic Ocean

South America

1994

Nov3

424

South America, South Atlantic

1995

Oct24

2 10

Asia, Borneo, Pacific

1997

Mar9

2 50

Siberia

1998

Feb26

408

Pacific, northern S. America, Atlantic

1999

Aug11

2 23

Atlantic, Europe, SE and southern Asia

2001

Jun21

456

Atlantic, southern Africa, Madagascar

month because the orbit of the Moon is tilted compared to the path the Sun seems to take around the Earth. Eclipses thus occur in "eclipse seasons" that are about 51/2 months apart. A solar eclipse begins when the Moon just touches the Sun, called first contact. For about an hour of partial eclipse the Moon moves to cover more and more of the Sun. You can observe it with a filter or by projection (see p. 38). The beginning of totality, called second contact, is the instant the last remaining sunlight may stream through a valley on the Moon's edge, producing the beautiful "dia足 mond ring" effect. During totality all of the Sun is covered, and it is safe to look at the total eclipse with your unaided eye. You will see the filmy corona surrounding the dark Moon, and " Bailey's Beads" of prominences around the rim. The length of totality can be from just a few seconds to slightly over 7 minutes. When totality is over, at third contact, there is another diamond ring, the Sun appears again, and you should again use a filter for protection. For another hour of partial eclipse the Moon slowly uncovers the Sun, and as it leaves the Sun, at fourth contact, the eclipse is over. Eclipses are not only awesome, but useful as well. We have records of solar eclipses going back more than 2,500 years. They are so spectacular that many civilizations kept accurate records of them. Since eclipses can be accurately predicted as to time and areas of visibility, they provide reliable markers of time throughout history. Thus they are of use to historians in dating other events, such as the reigns of kings and dates of battles. Astronomers use them to study details of the motion of the Moon and Earth.

Paths of upcoming total solar eclipses

Surface of Mercury

MERCURY Known to the Greeks as Hermes, Mercury of Roman myth 足 ology was the messenger of the gods and the god of co m merce and thievery. It was said that his father was the god Jupiter and his mother Maia, daughter of the Titan Atlas. The Teutonic version of this god, Woden, gives his na me to Wednesday. The sy mbol for the planet is a stylized caduceus, the staff entwined with serpents carried by Mer足 cury. This planet has been known since prehistoric times, at least since the third millenium B. C. The planet was given this na me because it moves the swiftest of all. Mercury moves in an orbit with an average distance from the Sun of 36 million miles, or 0. 39 a. u. Its orbit is very eccentric, exceeded only by that of Pluto, and it is inclined 7 degrees to Earth's orbit. It takes only 88 Earth days to revolve around the Sun once-the length of Mercury's "year." It never gets closer than 50 million miles fro m 44

Earth. Beca use it is always within 27 degrees of the S un, it is hard to see. Mercury rotates on its axis very slowly, taking almost 59 Earth days to spin once-the length of Mercury's "day. " Mercury does not keep one face always toward the S un, as was once thought. The strong gravity of the Sun has caused Mercury's rotation and revolution to be locked together in a fixed ratio : The planet rotates three times on its axis for every two revolutions about the Sun. Because of this, and the irregular motion caused by its eccentric orbit, days and nights on Mercury must be very strange. As seen from some locations on Mercury, the Sun would rise, stop, set where it rose, then rise again and continue across the sky. Then it would set, rise, and set again. Several times d uring the past few centuries, astronomers claimed to have observed a planet closer to the Sun than Mercury. One astronomer named it V ulcan, and it was thought to explain why Mercury's motion is not exactly in accord with Newton's and Kepler's laws. The existence of such a planet has never been confirmed, and later astro足 physicists showed that the Theory of Relativity explains the small discrepancies in Mercury's orbit. Mercury in orbit

M E RC U RY FACTS D i stance to S u n 35 .98 million miles or 0 .387 a . u . Le ngt h o f year 8 7 .9 7 Earth days O r b i t ecc e n t r i c i t y 0 .2 06 O r b i t i n c l i n a t i o n 7 .0 degrees D i a meter 3 ,031 miles or 0 .382 X Earth's Mass 0 .055 X Earth's D en s i t y 5 .43 Gravity 0 .38 X Earth's Le n gt h of day 58 .646 Earth days T i l t of a x i s 0 .0 degrees

Mercury as seen by Mariner 10

NASA

THE SURFACE OF MERCURY is much like that of Earth's Moon. There are three major kinds of terrain: smooth plains, cratered plains between large craters, and heavily cratered land. These features are not evenly distributed over Mercury's surface. Several very large features domi足 nate the planet. Most conspicious is the Caloris Basin, which appears to be the remains of a huge impact. Concen 足 tric rings seem to be frozen ripples in the surface. Other smaller features include cracks and cliffs that are the result of the planet cooling, shrinking, and cracking. Because the planet rotates slowly, and there is no atmo 足 sphere to carry heat around the planet, the Sun-facing side is very hot, reaching several hundred degrees Fahr足 enheit. T he coldest measured temperature, on the side facing away from the Sun, is about 300 degrees below zero. Mercury-is only a little over 3,000 miles in diameter, about 38 percent the diameter of Earth. Mercury's density

is h igh, about 5.4 t imes that of water, about equal to Earth's. Th is implies that Mercury has a large core of iron, wh ich may occupy the inner three-quarters of the planet. Because of its mass, Mercury has a surface grav ity such that a 100-pound person on Earth would we igh 38 pounds on Mercury. It has no known satellites. Astronomers believe that there were five stages in the evolution of Mercury. 1) The planet's materials came together from the solar nebula, and part ial melting allowed the heavier elements, notably iron, to s ink to the center. 2) The planet was bombarded w ith meteroids, wh ich eroded some of the early features. 3) The largest feature on the planet, the Caloris Basin, was made by a huge meteroid impact. 4) lava flooded the Caloris Basin and other areas, smooth ing the surface. 5) Occasional meteoro ids h i t the smooth pla ins, produc ing l ight cratering. Mercury has a slight magnetic field, b ut it is not known if th is is the remains of earlier magnetism, or if it is caused by dynamolike motions in the molten core. The magnet ic field is not strong enough to repel the solar w ind during solar storms, so the surface is constantly bombarded by atomic part icles. No Mercurian sur face Close-up of Mercury NASA features are v is ible from Earth. Our only close-up photographs of Mercury's surface came from the Mar足 iner 10 spacecraft wh ich flew by the planet three times in 1974-75. Tak ing more than 10,000 photo足 graphs, Mar iner 10 mapped about 57 percent of Mercury's surface. 47

Finding chart for Mercury in the morning sky (above) and in the evening sky (opposite page)

O B S E R V I N G M E R C U RY is difficult because it is never more than 27 degrees from the Sun, and thus always seen within about an hour of sunrise or sunset, while the sky is still light. The best times to see Mercury are the few days just before and after greatest elongations, denoted in the table by " E" for greatest eastern elongation (Mercury as an evening "star"), and by "W" for greatest western elongation (Mercury as a morning " star"). The illustrations show ty pical positions of Mercury sev足 eral days before (minus) and after ( plus) greatest elonga足 tions, relative to the point along the horizon where the Su n rises and sets. TRANS ITS O F M E R C U RY-when the planet is seen to cross the face of the Sun-occur a bout 13 times a century, when Mercury is at inferior con junction near the time its orbit crosses t he-ecliptic. Transits in this decade occur on Novem 足 ber 6, 1993, and November 15, 1999. 48

BEST TIMES F O R VIEWING MERC U R Y

1 990 Feb Apr

May 3 1 Aug

1 1

Jun

17 E

Oct

Aug

4 W

Dec

Oct

14 E

Nav 22 W 1 994 Feb

Sep 24

Dec

Mar

1 99 1 Jan

Mar 27 May 12 Jul Sep

25 7

4 E 19

Sep

May 30 Jul

Sep 26 E

Nav 1 995 Jan

6 W 19 E

Nav 19

Mar

Dec 27

May 12 E

J un

Sep

Oct 20

1 992 Mar

Apr 23 Jul

Apr

Feb

Apr 23

Jun

Aug 2 1 E

Oct 3 1

1 993 Feb 2 1

1 996 Jan

Aug 2 1 Dec

1 W

6 E

May 22 Aug

Apr

1 997 Jan 24 w w

4 E

16 w Nav 28 E

1 998 Jan

6 W

Mar 20

May

4 W

Jul

17 E

w Nov 1 1 E Dec 20 w

Aug 3 1

1 999 Mar

Apr

16 w

Jun

Aug

Oct 24 E Dec

28 E 14 3

w W

S u rface of Venus

VENUS The goddess of love and beauty gave her latin name to this the brightest of the planets. To the Greeks this was Aphro足 dite, and before them it was lshtar to the Babylonians. For many years, Venus was thought to be two planets: Eospho足 rus, the morning star, and Hesperus, the evening star. Venus, which becomes closer and brighter than any major celestial body other than the Sun and Moon, has been known since prehistoric times. The symbol for Venus is supposed to be a hand mirror. The orbit of Venus is almost circular, 67 million miles from the Sun, or 0. 72 a.u., bringing Venus within 26 million miles of Earth at times of inferior conjunction. It takes 225 Earth days to revolve around the Sun once (Venus' "year") at a speed in orbit of 22 miles per second. Venus' orbit is inclined about 3 degrees to the ecliptic. It is by far the 50

slowest-rotating planet, taking 243 Earth days to turn once (Venus' "day"). Venus comes closer to Earth than any other planet, about 25 million miles. It takes 584 days to co mplete a synodic period, during which all its phases, seen fro m Earth, are repeated. It has no natural satellites. Venus has been explored from Earth by telescopes, which see only the tops of the clouds, and by radar, which can penetrate its thick clouds. U. S. and Soviet spacecraft have investi足 gated Venus, and a few have landed on its surface. Others still orbit the planet, mapping its surface with radar. In the past, science fiction writers have i magined its surface to be everything from a primeval swa mp where dinosaurs roa m, to totally water-covered land, to a desert. The last is closest to the truth. Because the planet is just slightly s maller than Earth, it was once called our sister planet. We now know that size is about the only similarity it has to Earth.

Ven u s in orbit

VENUS FACTS Distance to S u n 67.23 million miles or 0 .723 a . u . Le n gt h of y e a r 224 7 . 0 Earth days Orbit ecce n t r i c i t y 0 .007 O r b i t i n c l i n a t i o n 3 .4 degrees D i a m eter 7,521 miles or0 .949 X Earth's Mass 0 .815 X Earth's Density 5 .24 Gravity 0 .91 X Earth's Le n gt h of day 243 .017 Earth days T i l t of a x i s 177 .3 degrees

Venus seen by spacecraft in u l traviolet light

NASA

VENUS is much like the ancients imagined hell to be. On the rocky surface the temperature is 900 degrees Fahren足 heit. There is little variation across the planet or from season to season. At that temperature there can be no liquid water.

is the cause of the high temperature. It is 96 percent carbon dioxide, about 3 percent nitrogen, and 1 percent gases such as argon, water vapor, sulfur dioxide, and carbon monoxide. At the surface the atmo足 sphere has 90 times the pressure of Earth's atmosphere. This dense layer of gases holds in the heat Venus receives from the Sun. The thick clouds are made mostly of hydro足 chloric and sulfuric acid droplets. Wind speeds at the surface are only a few miles per hour, but at high altitudes reach several hundred miles per THE ATMOSPHERE

hour, blowing from east to west . Despite the fact that Venus has no magnetic fie l d , it seems to have a u roras high i n its atmosphere; their origin is not understood . is tilted so that its north pole i s oriented a l most the way the south poles o f most o f the other pla nets are. This mea ns that the Sun rises i n the west and sets i n the east-once every 243 days . H owever, the Sun wou l d be d istorted and hard to see from the surface of Venus because of the thick atmosphere . Venus is a bout 7 , 500 m i les across, and has a density s i m i l a r to the other terrestrial planets, 5. 2 times that of water. It is 8 1 percent as massive as Earth . A 1 00-pound person on E a rth wou ld wei g h 91 pounds on Ven u s . The geology of Venus is u n l i ke Earth's . There a re craters, some from the i m pact of meteoroids and some from volca足 noes, some of which may sti l l be active . Both i m pacts and vu lcanism have a l tered Venus' surface during the last 4 . 5 billion years . T H E AXI S O F V E N U S

are named after famous rea l or fictional women . About a fifth of the surface is low plains, a bout 70 percent ro l l i ng uplands, and about 1 0 percent h i g h l a nd s . One of the la rgest features , l shtar Terra , is l a rger than the U nited States and more than two mi les high . The largest highland, ca l led Aphrodite Terra , i s h a l f t h e s i z e of Africa . T h e hig hest mounta i n , Maxwe l l Montes, is seven m i les h i g h , higher than Mt . Everest on Earth . Many long and wide canyons that a re m i les deep sca r the surface . Most of Venus has been mapped by radar. The very few photographs of the Venusian surface, taken by Soviet spacecraft, show plains of rocks stretching i n a l l d i rections. Given the surface temperatu re and pressure on Venus, there can be no liquid water and thus no living orga nisms . MAJ O R S U R FAC E FEATU R E S

F i n d i n g c h a rt for Venus i n the morn i ng s k y 4/1 to 12/7, 1993

O B S E RV I N G V E N U S is easy because it is so bright . It gets close enough to us so that through good binocu l a rs or a telescope we can see it as a sma l l disk, and watch it change phase . The best ti mes to observe Venus a re the couple of months before and after greatest elongati ons . When Venus is brightest, a bout a month after greatest eastern elonga足 tion, and a month before greatest western elongation, it is a s l i m crescent . Venus never gets more than 47 degrees from the S u n , b u t that a l l ows it to rema i n i n t h e evening s k y wel l after sunset and to rise wel l before sunrise . It is often the first or last celestia l object seen during the night. Venus rema ins a morning or evening "star" for seven months . I l lustrations above show the changing position of Venus i n the morning and evening sky relative to the point a l ong the horizon where the Sun rises or sets. ( Remem ber: this point moves a long the horizon as seasons change . ) The numbers refer to Venus' position days before ( m inus) or after (plus) the g reatest elongations. These a re plotted for specific dates this decade, but other elongations w i l l be simi lar. 54

Greatest eastern elongation• +10 .

-10• -20·

+20 •+30 Brightest

•+40

-30• •+50 -40• -50• -60• -70· •+60 -80· -9o• -100 ••-120 -110 • -150 -13.9140 •• -.170 1 - 6� 180 itSuriset point •

F i n d i n g chart for Venus in the eve n i n g sky 7/2311992 to 3/3011993

VENU5-when the planet is seen from parts of the Earth to pass across the disk of the Sun-occur very rarely, when Venus is at i nferior conjunctio n . We a re for tunate that transits will occur early i n the 2 1 st century, on J une 8, 2004 , visible i n E u rope, and o n June 6, 2 0 1 2 , visible i n Asi a . The last pa i r o f transits were i n 1 87 4 and 1 882 . The next w i l l occur i n 2 1 1 7 and 2 1 25 . T R A N S ITS O F

Inferior Conjunction

PHENOMENA OF VENU S Greatest Western Superior Elongation Conjunction

Greatest Eastern Elongation

1 990

Jan 1 8

1 990

Mar 30

1 990

Nov 1

1 99 1

Aug 22

1 99 1

Nov2

1 992

Jun 1 3

1 993

Jan 19

1 993

Apr 1

1 993

Jun 1 0

1 994

Jan 1 7

1 994

Aug 24 Apr 1

Jun 1 3

1 994

Nav2

1 995

Jan 1 3

1 995

Aug 20

1 996

Jun 1 0

1 996

Aug 20

1 997

Apr 2

1 997

Nov6

1 998

Jan 1 6

1 998

Mar 27

1 998

Oct 30

1 999

Jun 1 1

1 999

Aug 20

1 999

Oct 30

2000

Jun 1 1

200 1

Jan 1 7

Pla net Earth as photographed by Apo l l o 16 astron auts

NASA

EARTH Our planet is the only one to have oceans and a breathable atmosphere . And it is the only planet not na med after a Greek or Roman god . The name comes from O l d E n g l ish and its predecessor Germa nic l a nguages . Earth is a lso just the right distance from the Sun so that its average temperature is between the freezi ng point and the boi ling point of water. liquid water is a major condition that makes "l ife possible. life on Earth began around 3 b i l l ion years ago, when primitive chemica l s combi ned i n such a way that they could take energy from their environment and reproduce them足 selves . Over the eons organisms g rew more complex. Humans a rose a few m i l lion years ago. give us our major u n its of time. The the Sun is the year (also cal led The time it takes to spin on our axis, or period of rotation, is the day. EARTH'S MOT I O N S

period of revolution a round the planet's orbital period).

Some of the things we have d iscovered from our c lose study of Ea rth help us understand our solar system neigh足 bors . It is very usefu l to study the Earth from space, for in that way we can see things on a large sca le that we recogn i ze only with d ifficulty from the ground . Earth generates a strong magnetic field that extends out i nto space where it i nteracts with the solar wind . Atomic particles travel down this field to strike the atmosphere and cause aurora s . Others a re trapped i n donut-shaped reg ions a round the E a rth cal led the Van Allen belts . EARTH FACTS Dista nce to Sun 92,9 1 7,93 1 . 7 Diameter 7,926 m iles m iles or 1 .000 a.u.

Length of year 365.2422 days Orbit ecce ntricity 0.0 1 7 Orbit i nc l i nation 0.00 degrees Mass 1 .000 X Earth 's

Earth a n d its magnetic fie l d

(th r ou g h the e quat or )

Density 5 .52 Gravity 1 .000 X E a rth's Length of day 23' 56m 04' Tilt of axis 2 3 .44 1 5 deg rees

wou l d b e th e m o s t c on s p i c u o u s feature o f the planet t o a v i s i t o r f r o m s p a c e . They cover about 70 percent of the a rea of the gl obe, and if a l l the water were spread o u t , i t s a v e r a g e d e p th wou l d be pi most 21/2 m i l es . O f the water o n Earth , most is seawater. If 1 2 g a l 足 l on s r e p r e s en t e d a l l th e water on Earth , only 5 cups of t h a t w o u l d be f r e sh water, and 31/2 cups of that wou l d be frozen i n g laciers . Of the remai nder, not quite l1/2 cups is either too deep, too pol luted , or exists as water vapor in the atmo足 sphere, leaving just 6 d rops of useable fresh water. E A R TH'S O C E A N S

is about 3.5 percent sa lt, mostly sod i u m ch loride, l i ke our table sa l t . Sma l l amounts o f elements such as ca l c i u m , magne足 s i u m , a n d m u ch s m a l l e r amounts o f other elements a lso a re in sol ution . More tha n 80 percent of a l l photosynthesis, the turn足 ing of sunl ight and nutrients i n t o p l an t m a t te r , t a k e s S EAVVAT E R

place in the oceans, making them the principal a bode of l ife on Earth . OCE A N CUR R E N TS, such as the Gulf Strea m , carry huge amounts of the Sun's energy, which fa l l s mostly on equato足 rial reg ions, i nto the temperate zones of the planet. F ive major c i rc u l ation patterns and 34 named major ocean currents affect c l i mates a l l over the globe. There is a complex i nteraction with the atmosphere that produces the c l i mate we experience on our planet .

The Ea rth's ocea n s with water removed

is a thi n l ayer of gases shield ing the surface from space . It is composed of 78 percent nitrogen , 20 percent oxy gen, and sma l l but signifi c a n t a m o u n t s of c a r b o n d i oxide, a rgon, and water vapor, plus traces of other gases . A column of air 1 inch squa re reach i ng from the surface to space wou ld weigh 1 5 pounds, so atmo s ph e r i c p r e s s u r e i s 1 5 p o u n d s p e r s q u a re i n ch . P ressure decreases as you go higher. The atmosphere can be d ivided i nto layers by tem perature and chemical com position . The lowest layer is the troposphere, extend ing up a bout 8 miles . This is where all weather occurs . N ext is the stratosphere, e x t e n d i n g up a b o u t 3 0 m i l e s . A b o ve th a t , th e mesosphere rises to about 50 m i les, and above that is the thermosphere . The out ermost layers of the atmo sphere s l owly th i n out to bec o m e space . A bove about 6 0 m il es the a i r is so EARTH'S ATMO S P H E R E

Structure of Ea rth's atmosphere

thin that a sate l l ite can make at least one orbit without being s lowed down by air drag . Many consider this the boundary between atmosphere and space. Between 8 and 30 m i les up the oxygen is i n the form of ozone, havi ng three oxygen atoms per molecu l e . Ozone absorbs ha rmfu l u ltravio let l ight and prevents it reaching Earth's surface. Some chem ica l s d rift up from the surface and red uce the amount of ozone, a l l owing more dangerous u ltravio let l ight to reach E a rth . Above 40 m i les, sunl ight hits atoms a nd knocks electrons from them , ionizing them , and creating the ionosphere. Rad io waves reflected from this layer travel a round the world . There is no "top" of the atmosphere . The density of materia l just gets lower as you go higher. Although there is no lega l defi nition of where the atmosphere ends and "space" beg ins, it is usua l l y taken to be a bout 60 m i les up. i n the northern hemi sphere and aurora australis i n the southern hemi sphere a re produced at the top of the atmosphere when atomic parti- Atmospheric ci rculation patterns cles from the Sun strike mol足 ecu les of the a i r and cause them to g l ow. These a re best seen from high lati足 tudes, but occasiona lly a re seen c l oser to the equator. In the lower layers, cur足 rents of a i r corry heat a ro u n d the p l a net . The main heat carrier is water vapor. Differences in tem足 perature i n d i fferent parts of the g lobe cause most of the weather we experience. A U RORA B O R E A L I S

Ea rth's i ntern a l structure

EARTH'S I N TE R I O R is a place we can only explore remotely. Even our deepest mi nes and d r i l l s only penetrate a few m i l es i nward toward the core, which is a lmost 4, 000 m i les below. As we go deeper, temperatures i ncrease. The heat inside the Earth is due to the rad ioactive decay of such e lements as uranium and thori u m , plus heat left over from Earth's formatio n . TH E C RUST of t h e E a r t h i s as thin as t h e skin o f a n apple i n compariso n . Its main ingred ients are s i l icon and oxygen , which make up many materi a l s . Continents a nd other "plates" of this l i g hter material float on the mantle, some足 what l i ke icebergs on water. They slowly move, sometimes colliding with one a nother, a process ca l l ed plate tectonics. 62

T H E MAN T L E l ies below the crust. It is denser rock that is somewhat pl a stic, kept from melting by the high pressure from overlying rocks . When this rock finds a weak place in the crust, it may l i q u ify, becoming magma . Magma that reaches the s urface through volcanic activity is cal led l ava . Rising magma a long the ridges at the centers of the oceans produces more crust and pushes the p lates apart. Along ocea n edges, the plates collide and s l i p under one a nother, melting back i nto the mantle. Such reg ions produce earth quakes, vol ca noes, and mountains.

is liquid i n its outer reg ions, com posed mostly of iron and nicke l , l i ke some meteoroids . Motions i n the i ron core act l i ke a dynamo to produce Earth's mag netic field . At the very center o f the E arth i s the i n ner core, proba bly solid nicke l - i r o n . The temperature at the core is a round 1 0 , 000 degrees Fa hrenheit. T H E EARTWS C O R E

prod uced by e a r t h q u a kes, trave l d i ffe r e n t l y t h r o u g h different k i n d s o f rock . By ana lyzing the waves, geo l ogists c a n deduce t h e i nter nal structure of the planet . Ea rthquakes a re caused by breakage and s l i ppage of rocks inside the Earth a long weak reg ions ca l led fau lts. Hundreds of m icroquakes occur dai ly. large earth quakes a re rare, but can be very destructive .

S E I SM I C WAV E S,

Vol c a n i c activity

USGS

Ea rth orbiting the S u n

THE SEASONS Beca use the Earth is tilted on its axis by 2 31/2 degrees, different parts of the planet get different a mounts of heat from the Sun . When the northern hemi sphere is tilted towa rd the S u n , t h e days there a re longer and t h e Sun is high i n t h e sky. Then that hemisphere gains more heat in the day than it rad i ates away i nto space at night. Hence the weather grows war m , and we have summer. Six months later, the northern hemisphere is tilted away from the S u n , days a re short, and the Sun is low in the sky. More heat is lost at night than is gained , and tem peratures fa l l. It is winter. It is this tilted axis, not the s l ight changing d istance between Earth and Sun, that makes the seasons. Ea rth is closest to the Sun about J a nuary 3 , and fa rthest from the Sun, about 3 percent fa rther away, about J u ly 4 each yea r. 64

S u n 's motion across the sky

Seasons i n the southern hemisphere are the opposite of those in the northern hemisphere . are the ti mes when the Sun is fa rthest north or south . The summer solstice (for the northern hem足 isphere) , about J une 2 1 , is the beg i n n i ng of summer. The winter solstice, a bout December 22, is the sta rt of that seaso n . T H E SOLST I C ES

T H E EQU I N OXES a r e the ti mes when the Sun crosses the equator. The northwa rd crossing, the vernal equinox, is about March 2 1 , the beg i n ning of spring . The autumnal equinox is a bout September 2 3 . T h e hottest a n d coldest months do n o t correspond t o the Sun's northern or southern extremes because it takes time to heat up and to coo l off the land, the atmosphere, and the oceans.

Mete or oid s wa r m cr oss ing Earth 's orb it

METE ORS a re streaks of lig ht seen in the night sky; they are caused by sma l l particles from space whizzing i nto the upper atmosphere at speeds of 25 mi les per second. The friction of the i r motion heats the air around them to i ncan足 descence . On a typica l dark night, about five to ten mete足 ors per hour can be see n . About ten tons of such particles hit the Earth each day. Most are only the size of a grain of sand . Even bright meteors are caused by particles only the size of a sma l l pea . Rarely, very bright meteors called fireballs, or bolides if they explode, can be seen even i n daylight. METEOROI DS a re the particles whi l e they a re stil l i n space . Most a re debris from comets a n d a re made o f stony materia l . Others are a nickle-iron a l l oy, cal led irons. A few rare ones are a mixture of stone and iron .

METEOR I T ES o re meteoroids that have hit the g round . Most o re s ma l l . A very few hove weighed tons, and very rarely, perhaps once every few tens of thousands of years, a huge crater may be formed . Although the most common type of meteorite to fa l l o re stony, the most common meteorites found a re the i ron ones, as they a re more conspicuous on the g round a nd resist eros i o n . METEOR SH OW E RS occur

when t h e Earth posses throug h swarms of meteoroids that o re fol lowi ng in the orbits of comets . At such times, a l a rge number of meteors con be seen, up to a bout 60 o n hour, and occasiona l l y more. Seen from Earth they seem to radiate from a sma l l region of the sky, ca l l ed the radiant, named after the constel lation i n which their radiant is located . The number o f meteors seen g radua l l y rises a few days before and d i m i nishes for a few days after the peak date . TABLE O F MAJOR METEOR SHOWER S Peak Date

Name (rad iant)

Number per Hour

Duration ( i n days)

Jan 4

Quadrantids

2.2

Apr 2 1

lyrids

May 4

Eta Aquarids

Ju l 28

Delta Aquarids

Aug 1 2

Perseids

Oct 2 1

Or ion id s

Nov3

South Taurids

9.2 4

? ?

Nov 1 6

le on ids

Dec 1 3

Gem i n i d s

5.2

Dec 22

U rsids

Note The durati on is the t otal n u m ber of days, centered on the pea k, 1/â&#x20AC;˘ of the pea k rate .

d u r ing wh ich the h ourly rate is at least

Ea rth's Moon

NASA

THE MOON The Moon is the most conspicuous celestial object after the S u n . The Moon goddess was cal led Luna by the Roma ns, Artemis and Selene by the Greeks . "Moon" is a word from the old Germanic la nguages and the G reek "mena , " from an o lder word meaning "to measure . " It shows the ea rly i nterest i n our satellite as a measurer of time . It orbits the Earth once a month i n an orbit that is an average of 2 3 9, 000 m i les from us, movi ng at about 2 , 000 m i les per hour. The word "month" comes from "moneth , " an early word for the Moon . The Moon's c loseness to Earth has caused it to keep one face always toward Ea rth . 68

For thousands of yea rs the Moon has ma rked the pas足 sage of time with its phases . Hebrew, Islamic, and C h i nese lunar ca lendars are sti l l i n use today. Beca use the interva l between fu l l moons is about 2 91/2 days, someti mes there are 1 2 fu l l moons and some years 1 3 fu l l moons i n one calendar yea r of 3651/4 days . There is much fol k lore a bout the Moo n . legends c l a i m you become l u natic sleeping i n t h e light of t h e Moo n . Others c l a i m you should plant a n d harvest particu l a r crops at certa i n phases of the Moon . Some claim crime i ncreases at fu l l moon . Most of this folklore has no truth to i t . is t h e fu l l m o o n closest t o the autumnal equ i nox, rising a round sunset for severa l nig hts i n a row. Its l ight hel ped extend hours for reaping the fields . The fol l owing full moon is often ca l l ed the H unter's Moon , for its l ight a l l owed hunters extra hours to gather suppl ies for the wi nter. The Moon is the only other pla netary body explored in person by manki nd-during the U . S . Apo l l o program from T H E H A R V E S T MO O N

1 96 9-72 .

MOON FACTS Diameter 2, 1 59 mi les or 0.27 X Earth's Mass 0.0 1 23 X Earth's Density 3.34 Gravity 0. 1 6 X Earth's Length of day 27.32 1 7 Earth days Tilt of axis 6.7 degrees Length of year

c om pared t o the stars (siderea l ): 27d07h 43m relati ve t o Earth (phases ): 29" 1 2h 44m

Average distance from Earth 238,820 m i les Orbit eccentricity 0.055 Orbit inclination (varies ) 1 8 . 3 t o 28.6 degrees 69

Lun a r "sea"

NASA

Lun a r h i g h l a n d s

CARN E G I E

T H E MOON i s about one-qua rter t h e s i z e and 1/s 1 t h e mass of the E arth. It has no atmosphere and no water, and probably never had a ny. A 1 00-pound object on Earth woul d weigh 1 6 pounds on the Moon.

is covered by a layer of powder and rubble, ca l l ed the regol ith, produced by meteoroid i mpacts. Ma jor surface featu res a re the darker a reas of smooth lava flows, the "seas" cal led maria (pro足 nounced mar-ee-a h ; singular mare, pronounced mor-ay) ; wa l led craters and plains; mounta i ns and h i g h l a nd a reas; and many, many craters, most produced by i m pact. The Moon's crust is about 40 m iles thick on the side of the Moon facing us and about 60 m i les thick on the far side. Lunar features are named for fa mous scientists and explorers. Beneath the crust i s the mantle, made of denser rock, extend ing down to a bout 5 0 0 mi les below the surface. At this layer many sma l l moonquakes occur that can be mea足 sured by i nstruments left on the surface by the Apo l l o astronauts. We sti l l know l ittle about t h e core o f t h e Moon. Si nce the Moon has no magnetic fie l d , the core probably does not act l i ke a dynamo as does Earth's. T H E SURFACE OF T H E MOON

a re m u c h l i ke those of the Earth, but enriched i n some elements . Moon rocks have no water and have never been exposed to oxygen . Thus the chemical changes and erosion that a l tered all Ea rth rocks have not affected lunar materials. When astronauts establish settlements on the Moon ea rly Lunar mounta ins NASA i n the 2 1 st century, they w i l l u s e lunar rocks to construct their bui l d i ngs . They wi l l use the abundant solar power to decompose the lunar rocks i nto thei r elements, then reassemble those elements i nto water, a i r, and other necessities of l ife . More than 800 pounds of l unar rocks are bei ng studied i n laboratories on Earth for cl ues to the ori g i n and evo l u 足 t i o n o f t h e p l a nets . M O O N R OC KS

FORMAT I O N OF T H E MOON is stil l a puz z l e . There a re three main theories: It was once a part of the E arth that split off; it was formed elsewhere i n the early solar system and captured when it passed near Earth; it was formed by the accretion of smaller bodies where it i s now. More stud ies, and probably visits, wi l l be necessary to decide between these competi ng theories-or to develop a new one!

Lu n a r craters

NASA

takes it around Earth once a month . During a day, the Moon moves from east to west across our skies, caused by the turning of the Ea rth . At the same time, its orbita l motion carries it i n a west to east d i rection against the background of sta rs an amount a bout equal to its own dia meter every hour. This combination of motions mea ns that the time of moonrise is later each day, averag足 ing a bout 54 m i nutes. In fa l l the daily delay i s sma l l er, and i n spring it i s g reater. T H E MO O N 'S O R B I T

P H A S E S O F T H E MOON are the apparent changes i n shape d u e t o the changing i l l u m i nation o f the Moon b y the Sun, as seen from Ea rth . Moonlight is just reflected sun72

light. The far side of the Moon is not a lways the "dark side . " N EW MO O N occurs when the Moon i s approxi mately between Ea rth and Sun . Then the i l l u m i nated side i s away from us, so the Moon a p pears dark . At such times, the Moon rises and sets with the Sun . An ecl i pse of the Sun can occur at new moo n . A s t h e Moon moves east o f the Sun, it appears i n our sky as a crescent (the bright side a lways faces the Sun). Each day it rises later, the crescent is wider, and it i s higher in the sky at sunset . We say the Moon is waxing. FIR ST-Q U A RT E R MO O N (sometimes cal led half moon) occurs about a week after new moon , when the Moon i s a quarter of its way a round its orbit. We see the bright western half of the disk. At this phase the Moon i s hig h in the southern sky at sunset . As days go by, the Moon becomes rounded on both sides, and i s ca l l ed gibbous. Each night it rises later, and is fu l ler. FULL MOON is when the Moon is opposite the Sun i n i ts orbit, about two weeks after new moo n . It is then fu l l y i l l u m i nated a s seen from E a r t h , and opposite t h e Su n , rising about sunset and setting a b out sunrise . A t s uc h times there can be ecl i pses of the Moo n . After fu l l moo n , the moon i s waning. LAST Q U A R T E R (or third quarter), about a week later, is when we see the eastern half of the Moon i l l u m i nated . The Moon then rises about midnight and sets about noon . After that, it i s a wa n i ng crescent, until about a week after third qua rter when it is aga i n new moo n . Then the cycle repeats . 73

THE FIRST-QUARTER MOON The features on the Moon are best seen not at fu l l moo n , when the sunl ight is right overhead for them, b u t a round first and last quarter, when the lig ht comes from the side and shadows emphasize the terra i n. As the Moon waxes and wanes, each night the l i ne separating light and dark足 ca l l ed the terminator-lies across a different part of the Moon. Surface features near the termi nator a re espec i a l l y visi ble. Here are some of the major features visib le on the near side of the Moon using bi nocu lars or a sma l l telescope. MA R I A:

A. Mare Serenitatis B . Mare Tra n q u i litatis C. Mare Nectaris D . Ma re Foecund itatis E. Mare Crisium

F. Mare Frigoris G. Mare Va porum H. Mare Undarum I. Locus Mortis J. Mare Smyth i i

C RAT E R S A N D WALLE D PLA I N S:

1. 2. 3. 4. 5.

Hipparchus A l bateg n i us Stofler Mau rolycus Petavius 6. Langrenus 7. E udoxus 8 . Aristoteles

9. 1 0. 1 1. 1 2. 1 3. 1 4. 1 5. 1 6.

Atlas Al iacencis Werner Man i l i u s Theophi l us Posidonius Hercules J u l i us Caesar

OT H E R FEAT U R ES:

a. Alpine Va l ley b . Pyrenees Mountains c. Taurus Mountains d. A l ps Mounta i ns LICK OBSERVATORY P H OTOGRA P H

e. Caucasus Mounta i n s f. Appeni nes Mounta i ns g. Ariadaeus Rille h. Reita Va l ley 75

THE LAST-QUARTER MOON The third-qua rter moon is less fam i l ia r to many people because it doesn't rise unti l late in the evening . N everthe足 less, it holds some of the more interesting features visible on the Moon . MA R I A:

A . More Imbrium B . Sinus Iridium C . Mo re Frigoris D . Ocea nus P rocellorum E . Ma re Humorum

F. More N ubium G . Sinus Roris H . Sinus Aestuum I. Sinus Med i i J. Mare Va porum

C R AT E RS AND WA L L E D PLAI NS:

1 . Clovius Mog inus 3 . Tycho 4 . Arzochel 5. Al phonsus 6 . Ptolemoeus 7 . A l bategnius 8 . E ratosthenes 9 . Archi medes 1 0 . Auto lycus 2.

11. 1 2. 1 3. 1 4. 1 5. 1 6. 1 7. 1 8. 1 9. 20.

Epigenes Ti mocho ris Copernicus Arista rchus Herodotus Kepler Al petrag ius Herschel Pa l las Grimaldi

OTH E R FEAT U R E S:

o . Alpine Va l ley b. Stra ight Range c . J u ra Mounta ins d . R i phaeus Mountains e. Stra ight Wa l l 76

f. Appeni nes Mounta i n s g . A l p s Mounta i n s h . Piton Mounta i n i . Pica Mounta i n j . Ha rbi nger Mounta in s LICK OBS ERVATORY P H OTOGRAPH

Geometry of a luna r ecli pse

LUN A R ECLI PSES occur at fu l l moon when the Moon passes into the shadow of the Earth. They do not occur each fu l l moon because of the tilt of the Moon's orbit. Usua l l y the Moon passes north or south of E arth's shadow. The Earth's shadow is a cone pointing away from the Sun. You cannot see the shadow except when it fa l l s on something, such as the Moon. At the Moon's d i stance from Earth, the darker i n ner portion of the shadow, the umbra, is a bout 5 , 700 miles across. The lig hter, outer portion where not all of the Sun's light is cut off, the penumbra, is about l0, 000 m i les across. If the Moon passes only through the penu mbra, it is ca l led a penumbral eclipse. Such ecli pses are a l most not noticea ble, because the d i m ming of the Moon is slight. If the Moon passes completely i nto the umbra, it wi l l be a total eclipse. A partial eclipse occurs when the Moon does not fully enter the umbra.

Like solar ecl i pses , l unar ecl ipses take place during times of the year when the S u n appears to cross the Moon's orbit nea r the phases of fu l l a nd new moons . Such ecl ipse seasons ha ppen a bout every SV2 months. Thus the months of the ca lendar yea r i n which ecl i pses occur s l owly change . eclipse seasons,

A TOTAL LUN A R ECLI PSE beg ins when the Moon just touches the edge of the umbra, cal led first contact. From Earth, we see one edge of the Moon grow dim, a nd this wedge of darkness s l owly spreads over the Moo n . At second contact, the Moon i s just with i n the umbra , and tota l ity beg i n s ; it lasts a maximum of not quite two hours . During totality, not a l l light is cut off. The Moon is usua l l y a fa int reddish-orange d isk, i l l u m inated b y light refracted and fi ltered through the Earth's atmosphere . How bright it is during tota lity depends on how many c louds a re a round the sunrise-sunset l i ne of Earth at the time. At third contact the Moon beg ins to leave the u m bra , and at fourth contact, the eclipse is over. The maximum possible duration of a tota l ecli pse, from first to fourth contact, i s 3 hours 40 m inutes . Lunar ecli pses can a lso be used by historians as date markers, a nd have influenced historica l events . It is said that Columbu s used his knowledge of an upcoming lunar ecl ipse to frig hten natives into supplying h i s ships.

is slightly less than that of solar ecli pses . They a re more com monly seen, however, for they can be observed by everyone on the nighttime side of the Earth . At least two , and a maxi m u m o f five, l unar ecl i pses can occu r each yea r. In the table below, "T" i ndicates a tota l ecl ipse, " P " a pa rtia l one . T H E F R EQ U ENCY OF LU N A R E C L I P S E S

TABLE O F LU NAR ECLIPSES Date

Type

Areas of Visibility

1 990

Feb 9

Arctic, Austra l a s i a , E u r ope, Afr ica

1 990

Aug 6

SW Al as ka , Pacific, Antarctica , Austr a l a s i a , Sand E Asia

1 99 1

Dec 2 1

Arctic, NW South America , North America , Pacific, Austra lasia

1 992

Ju n 1 5

Sand W Africa, SW E u r ope, Americas e xcept the NW, Antarctica

1 992

Dec 9*

Asia e xcept SE , Afr i c a , E u r ope, North America e xcept W, Centra l and South America e xcept S

1 993

Jun 4

S of South America, W of North Ame r i c a , Pacific, Austra l a s i a , SE Asia

1 993

Nov 29

E u r ope, W Afri c a , Arc t i c , Americas, N Asia

1 994

May 25

Afric a, E u r ope e xcept NE , Americas e xcept N W and e xtreme N

1 995

Apr 1 5

W of N ort h America , Pac i f i c , Austra lasi a, E and SE As ia

1 996

Apr 3**

W Asi a , Africa , E u r ope, South Amer ic a , West Indies, E of North Amer ica

1 996

Sep 2 7

W Asi a , Africa , E u r ope, Americas e xcept Ala s ka

1 997

Mar 2 4

Africa e xcept E , E u r ope e xcept NE, Amer icas e xcept W Alas ka

1 997

Sep 1 6

Austra l a s i a , As ia e xcept NE , Africa e xcept W, Eur ope

1 999

Ju l 2 8

W and S of South America , Centra l Amer ica, W of N orth America, Pacific, Austra l a s i a , E Asia

2000

Ja n 2 1

Nand NW Asi a , W a nd N Afr i c a , E u r ope, Amer ic as

2000

Ju l 1 6

Pacific, SW Al as ka , Austra lasia , SW As ia

*May occur on Dec 10 i n s ome l ocat ion s . **Ma y occur on Apr 4 in s ome l ocati ons .

Geometry of t h e tides

THE T I D E S a re caused when the Moon and Sun pull the water on the near side of the Earth upward away from the Earth, while the Ea rth itself is p u l l ed slightly away from water on the far side. The Sun's effect on the tides is only about a third that of the Moon . High and l ow tides occur 1 21/2 hours a pa r t . Actual tides at coasta l locations a re greatly affected by the body of water and the shape of the bottom of the ocean near the site. Near times of new and full moon, the tides are especially high, ca l l ed spring tides. At times of the qua rter moons, the tides a re lower, cal led neap tides. T i d a l va riation d u r i n g one month

Surface of Mars

MARS The " Red P l a net" has long attracted attentio n . Many ancient cultures associated the red color of Mars with blood, so they gave to this planet the name of the God of War. He was Ares to the Greeks . The symbol for Ma rs is a stylized shield and spear. Mars has been explored by spacecraft from the U . S . and the Soviet U nion . I n 1 976, two U . S . Viking craft soft足 landed on the planet, took a nd analyzed soil samples, measured the atmosphere, a nd sent back photograph s . Despite hopes, no s i g n s o f l ife were found. Mars has the third most e l l i ptica l orbit. Its average distance from the Sun i s 1 42 m i l l i o n mi les, o r 1 . 52 a . u . Its synodic period, or return to Sun-Earth-Mars a l i g nment, is 2 years a nd 50 days. Because of its eccentric orbit, when at opposition Mars can be as much as 60 m i l lion or as l ittle as 35 m i l l ion m i l es from us . These c losest opposi tions occur a bout every 1 7 years. 82

Mars has a day a l most the same as Earth's, 24 hours, 37 minutes . The tilt of its axis is a l so a lmost the same as Earth's , 25 . 2 degrees . This gives Mars day-night cycles a nd seasons much l i ke Ea rth's . But because Mars i s much farther from the Sun, the temperatures there a re much lower and seasons are l onger. About a h undred years ago, astronomers thought they saw stra ight l i nes on the planet, and cal led them "cana ls . " This made some people think there mig ht b e life o n Ma rs, perhaps a n a ncient civi l izatio n . Modern exp lorations have shown the canals were a m i xture of optical i l l usion a nd wishfu l thi n k i n g , for they do not exist. More than any other planet, Mars has been thought a possi ble a bode of life beyond Earth . In 1 924, during a close opposition of Mars, radio stations were s i lenced to enable scientists to l i sten for radio signals from Mars . I n 1 938, a radio drama convi nced many listeners that Earth had been i nvaded by Ma rtians. Mars wi l l certa i n l y be the first planet to be explored i n person by humans, proba bly withi n the first quarter of the 2 1 st century. Ma rs in orbit MARS FACTS Distance to Sun 1 4 1 .6 m illion m iles or 1 .524 o.u.

Length of year 686.98 Earth days Orbit eccentricity 0.093 Orbit inclination 1 .8 degrees Diameter 4,2 1 9 m iles or 0.533 X Earth 's

Mass 0. 1 074 X E a rth 's Density 3.94 Gravity 0.38 X Earth 's Length of day 24"37m Tilt of axis 25.2 degrees

Mars from low orbit

NASA

is more l i ke Earth than any other planet, but there are a lso great difference s . The fourth pla net i s 4 , 2 1 7 m i les i n dia meter, about half the diameter of Ea rth . Its mass is only 1 1 percent that of Earth's . A 1 00 pound weight on Earth would be 38 pounds on Mars .

T H E PLA N ET MARS

is complex and changing . There are huge level areas, deserts, enormous mounta i n s , deep canyons, and craters of a l l sizes. Some of these features were produced by volcanoes, some by meteoroid Oiym pus Mons NASA impacts, some proba bly by ďż˝._....,.flowing water. Mars has the largest vo l Â cano i n t h e s o l a r system . I t is ca l l ed Olympus Mons a nd is more than 1 6 m i les high, three times ta ller than any vo l c a n o o n E a r t h . Ma r s a l so has the la rgest va l l ey system, a complex system T H E S U R FACE OF MARS

of channels cal led Va l les Ma rineri s . There i s no per足 manent surface water now on Ma rs, but perhaps there was i n the past, when the c l i mate was different. The low atmospheric pressure m e a n s f r e e w a t e r wo u l d evaporate i n stantly. Some water i s frozen i nto snow Va l les Ma rineris, the so-ca l led and ice as part of the north足 "Grand Canyon of Mars" NASA ern ice ca p . The southern ice cap is made mostly of dry ice, frozen carbon dioxide. More water may be trapped as ice or water deep i n the soi l . Si nce Ma rs i s farther from the Sun than Earth, i t i s much co lder. At the sites of the two Viking landi ngs, tem pera足 tures range from a l most 200 deg rees below zero to as high as around the freezing point of water. Some of the surface ca nyons a nd other features make scientists think that vol 足 ca nic activity or meteoroid strikes may occasiona l l y melt subsurface ice, causing huge floods . Then the water evaporates or sinks i nto the ground aga i n . Despite its thin atmosphere, Ma rs has wea t h e r t h a t i n c l udes cyclones a nd dust storms that may cover half the planet . N o signs o f life have been found . Martian surface seen byViking NASA

are sma l l , rocky, low-density bodies d iscovered i n 1 877, a lthough they had been "pre足 dicted" more than a century ear l ier i n the story of Gulliver's Travels. Their names a re Phobos and Deimos, the G reek names for the dogs of Ares: Fea r and Panic. Both a re probably asteroids that came too c lose to Ma rs a nd were captured . Both would make idea l space stations from which to study Ma rs itself.

T H E SAT E LL I T ES OF MARS

the i n ner sate l 足 l ite, is 5 , 840 m i les from Mars' center, which i s the usual way astronomers measure distance o f a sat足 el lite, but only 3 , 700 mi les a bove the surface of Mars . It orbits Mars once every 7 . 66 hours, in a west-to足 east d i rection l i ke most of the other sate l lites in the solar system . This orbita l period is much less than the 24. 62-hour rotation period of Mars itself. Thus, if you were on Mars, you wou l d see Phobos as a bright point of lig ht, rising i n the west and setting in the east. It wo u l d r i se u s u a l l y t w i c e each Ma rtian day. P hobos itself is a bout 1 2 by 8 m i les i n size, with a very dark, rocky surface pocked by craters . P H O B OS,

Phobos

NASA

Dei m o s

NASA

Orb its of Ma rs' sate l l ites

orbits Mars every 1 . 26 days at a distance of 1 2 , 500 m i les a bove Mars' surface. Si nce its period of revol ution is c lose to the rotation period of Ma rs, Deimos wou l d rise in the east and s lowly move across the sky, rema i n i ng visible i n the Ma rtian sky for severa l Ma rtian days before setting i n the west . It is around 7 by 6 mi les i n size. D E I MOS

Name

SATELLITES OF MARS D i stance Period of Mass from Revolution (x the Year Mars Moon's) D i sc. ( m iles) (days)

Phobos Deimos

1877

5,840

0.319

1877

14,602

1.263

Density

S ize ( m iles)

0.00000018

1.9

1 2X8

0.000000024

2.1

7X6

+ 10

'OPHIUCHUS

�to

VIRGO

LIBRA • Zuben'u b•

+5 �

"'--

- 5

tt>f

t -._10/t

•

+ 'i

"-.

�··

-1

230

·'

210

AURIGA

�

Spi o

190

SCORPIUS

Hamal �

•

�o \ 90 -5

•

+ 10.

250

Antorese •

._, .

•

PISCES

ARIES

:..· 50

•

30 •

·�·

TAURUS.

OBS E RVIN G MARS The "Red P la net" goes a round the Sun once each 687 days, or 1 . 88 Earth yea rs. Mars is best seen a round the ti mes of oppositi on, the times it is cl osest to Earth . Mars' synod ic period, the time between succes sive oppositions, is 779 days, or about 26 month s . Thus it takes more than two years for Mars to circle the sky as seen from Ea rth . Not a l l oppositions are good for viewing because the eccentricity of Mars' orbit mea ns that at some oppositions Ma rs is c loser to Earth than at others . These favorable oppositions, at which the distance to Earth is a bout 35 m i l l ion mi les, occur about every 1 7 years. The next severa l oppositions and Mars' distance from Earth at the time a re given in the ta ble. The fi nding chart above shows the position of Mars against the background of the conste l l a t i o n s o f t h e zod i a c . When Mars is nea rest, it c a n b e very b r i g h t and distinctly reddish in color. When dista nt, it is only l i ke a med i u m -

•

F i n d i n g c h a rt for Ma rs

bright star, and may not a ppea r red . Sometimes g l o b a l dust storms i n Mars' atmosphere obscure everyth ing for weeks or months. From Earth, Phobos and Deimos a re not visible i n any telescope l i kely to be owned by an amateur, si nce the sate l lites are very s m a l l and do not reflect much sunlight. TABLE O F MARS' OPPOS ITIONS D i stance from E a rt h ( m iles)

Date of Oppos ition

Constellation

1 990

Nov27

Taurus

4 8 m il lion

1 993

Jan 7

Gem in i

5 8 m il lion

1 995

Feb 1 2

le o

63 m il lion

1 997

Mar 1 7

Virg o

6 1 m illion

1 999

Apr 24

Virg o

54 m ill ion

200 1

Ju n 1 3

Oph iu chus

42 m il l ion

2003

Aug 28

Aquar ius

35 m ill ion

2005

Nov7

Ar ies

43 m il l ion

Asteroid orbits

ASTEROIDS: THE MINOR PLANETS The asteroids (meaning "sta r l i ke"), or minor planets, a re a m ultitude of sma l l solid bod ies, most of which a re located i n a huge donutâ&#x20AC;˘shaped reg ion lying between Mars and J upiter, ca l led the asteroid belt, and extending somewhat above and below the plane of the ecl i ptic. They range i n size from invisible pebbles up t o the largest, Ceres, a bout 600 m i les across . Their sma l l size and low reflectivity account for the fact that the first one, Ceres , was not d iscovered until the first day of the 1 9th century, by I ta l i a n astronomer G iuseppe Piazzi . 90

A few asteroids have orbits that take them outside the asteroid belt. If a n asteroid's orbit crosses within the orbit of Mars it i s cal led an Amor-type asteroid . If it crosses with in Earth's orbit, it is a n Apo l l o-type a steroid . These a re named after the first asteroid of each kind to be discovered . As with planets, the fi rst asteroids d iscovered were named for mythological fig ures, and they were numbered i n order of d iscovery. When many more had been foun d , t h e i r discoverers were a l l owed to give t h e m other na mes足 of friends , relatives, pets, pla nts, or a l most a nything else. The fu l l name of a n asteroid consists of the number pl u s a given name, if a ny. Some examples of asteroid names a re 433 Eros, 5 1 1 Davida, 8 F l o ra , 349 Dembowska , and 324 Bamberga . About 1 , 700 asteroids have names, many hundreds more only numbers . There a re probably hundreds of thousands of minor planets of significant s i ze .

Some of the minor planets

Key to minor planets shown above:

Ceres

Fortuna

107

Camille

433

Pallas

Themis

121

Hermione

451

Patientia

Vesta

Daphne

165

Loreley

511

Davida

Hebe

Nysa

250

Bettina

624

Hektor

Iris

Europa

349

Dembowska

702

Alauda

Cymbele

Egeria

Psyche

Eros

375

Ursula

704

lnteramnia

423

Diotima

747

Winchester

I N COMPOSI T I ON the minor pla nets most closely resem ble meteorites that fa l l to E a rth . Probably many meteorites a re chips knocked off asteroids by collisions among them selves . (Others come from comet debris , p. 1 5 1 . ) Also l i ke meteorites, there a re severa l types of aster oids. Some seem to be mostly meta l l i c (nickel-iron), while others resemble basa lt (a volcanic rock found on Earth), and sti l l others conta in carbon compounds and a re very dark . They may be rich i n l i g hter elements . Astronomers think asteroids are debris from the forma tion of the solar system . Because they were too close to Jupiter and its powerful gravitationa l fie l d , they could not form l a rge bodies . Also, some may be frag ments of c o l l i s i o n s between larger bodies. T h e few minor planets that come into the inner solar system a re thought to have had their paths perturbed by co l l isions. There may be other belts of minor pla nets farther out in the solar syste m , but only one asteroid, Chiron, has been d i scovered orbiting beyond Satu r n . Minor pla nets rotate with periods ra nging from three hours to severa l days . Most a re single bod ies and i rreg u l a r in shape, and a re probably pitted and scarred b y col l i sions . Some a re very elongated , and some asteroids may be double bod ies, or even have sma l l er sate l l ites . Most a re sma l l ; o nly 33 asteroids a re larger than 1 25 m i les across . KI R KWOOD G A PS are regions of the asteroid belt where no asteroids orbit, due to complex g ravitational interac tions of the m i nor planets with Mars and J u piter. There a re a lso g roups of asteroids with s imila r properties, probably produced when one large body was broken up by a col l i sion . These are cal led Hirayama families, after the Japanese astronomer who first identified them . Two special g roups travel i n J upiter's orbit-the Trojan asteroids (p. 1 1 1 ) .

Asteroid tra i l among stars

OBSE RVI N G MI N O R PLAN ETS is not easy because they a re sma l l and hence d i m . Only one, 4 Vesta , gets bright enough to see with the una ided eye , and then j ust barely, u nder good cond itions. It i s possible that an asteroid could pass very close to Earth and become visible, but this wou ld be very rare . Asteroids can be seen i n sma l l and medium-sized tele足 scopes, and about 40 brig hter ones a re sometimes visible i n binocu lars with 50 m i l l imeter lenses. Even then, you must have an accurate star chart a nd know where to look, for they a re not conspicuous . They a ppea r just l i ke stars. If you watch over a period of severa l hours, you may notice the motion of an object agai nst the background of stars. This i s the way they are discovered . The motion i s easier to see i n a guided telescopic time-lapse photograph where the asteroid shows up as a streak of lig ht, wh i l e the stars a re points of light. The brightest minor planets a re those that come close to Earth, cal led Ea rth-crossers, or Apo l l o objects, named

Ea rth-cros s i ng a steroid orbits

after one of them . Someday these may be m i ned as sources of m inera l s and i ron .

Name and Number

SELECTED ASTEROIDS Period of Revolution Year (years) Disc.

Average Distance from Sun

1 Ceres

180 1

4.60

2.766 a .u .

2 Pa llas

1802

4.6 1

2.768 2.668

3 Jun o

1804

4.36

4 Ves ta

1807

3.63

2.362

6 Hebe

1847

3.78

2 . 426

16 Psyche

1852

5.00

2.923

433 E r os

1898

1.76

1.458

1566 1carus

1949

1.12

1.078

1620 Ge ogra ph os

195 1

1.39

1.244

View in Ju piter's atmos p here

JUPITER The largest planet in the solar system is some 3 1 8 times the mass of Earth and 0 . 1 percent the mass of the Sun; it conta i n s 71 percent of a l l the non-solar materia l in the sol a r system . Around it a re a thin ring and an extensive system of sate l l ites, a l most a m i niature pla neta ry system . J u piter, known a lso to the Romans as J ove and to the Greeks as Zeus, was king of the gods, ruler of Olympus. The astronomical symbol for this planet i s a sty l i zed thun足 derbolt. The planet is among the brig htest objects in the sky, and so has been known si nce prehistoric ti mes . It circ les the Sun once every 1 1 . 86 Ea rth yea rs at a n average distance of 483 m i l l ion mi les. The prototype of the gas-giant pla nets, J upiter i s mostly gaseous and perhaps partia l l y l i q u i d , with at most a rela足 tive ly sma l l sol i d core . The "surface" we see i n our tele足 scopes i s actua l l y the top of a thick cloud layer. The planet 96

has an average density of only l . 4 times that of water, a bout a quarter the density of rocky planets l i ke Earth . like the other gas-giants, it spins quickly on its axis: a Jovian "day:' a "day" on J upiter, lasts on ly 9 . 8 Ea rth hours . J upiter i s tilted 3 deg rees compared to its orbit. Jupiter's mass and size g ive it a tremendous g ravita足 tiona l fie l d . A l 00-pound object on Earth wou l d wei g h 2 5 4 p o u n d s o n J u piter. T h e pla net has been explored by two Pioneer and two Voyager spacecraft, which d iscovered many exciting deta i l s about this the largest p l a net . J upiter emits a bout twice as much energy as it receives from the Sun . This is probably d ue to its enormous mass and g ravity, which may cause it to contract s l owly. The amount of contraction needed to explain this extra amount of energy i s very sma l l , wel l below an amount that could be detected by telescopes from Earth . This s l ow squeezing a lso means that deep with i n J upiter's atmosphere the tem足 perature i s many tens of thousands of deg rees. A space probe na med Ga l i leo is schedu led to exp l o re J u piter's atmosphere i n the mid- l 990s . Jup iter in orbit

J U PITER FACTS Distance to S u n 4 8 3 .6 m il l ion m iles or 5 . 20 a.u.

Length of year 1 1 . 86 Earth yea rs Orbit eccentricity 0 .048 Orbit i n c l i nation 1 . 3 degrees Dia meter 88,732 m iles or 1 1 . 2 X Earth 's

Mass 3 1 7 . 8 X E a r t h 's Density 1 .3 3 Gravity 2.54 X Earth 's Length of day 9h5 1 m Tilt of axis 3 . 1 deg rees

The G reat Red Spot on Ju p iter, as seen by Voyager 1

N ASA

is its most conspicuous feature . It is about 90 percent hydrogen and 1 0 percent hel ium , very c l ose to the proportions of those gases i n the S u n . Other elements, s u c h as carbon, su lfur, nitrogen , a n d oxygen, make u p l e s s t h a n 1 percent, b u t they a re impor足 tant because their chemica l compounds a re responsible for the colors of the clouds. The cloud layer is thi n , only about 1 00 m i les thick . Methane, ammonia, water vapor, and ice a re major com足 ponents . I n color photographs, red clouds, the coolest, are on top; white clouds are below them , s lightly hotter. Brown c l ouds a re lower and hotter sti l l , with blue clouds bei ng the lowest and hottest layer. Below the cloud layer is a h uge clear atmosphere about 1 0, 000 mi les thick . At the base of that, the temperatures and pressures a re such that the hyd rogen gas is turned into a meta l layer 25 , 000 mi les thick. At the very center is probably a rocky core about 1 3 , 000 m i l es in diameter. JUP I T E R'S ATMOS PH E R E

The J ovian atmosphere s h o w s b a n d s of c l o u d s , each of which is rea l ly a h i g h - s peed jet strea m of gases . There a re some per足 ma nent features . One is the G r e a t Red S p o t , w h i c h seems t o b e a storm that has lasted 300 years or more . Other colored spots come and go, lasting from days to years . In the upper atmosphere re we can observe lightning Structu of Jupiter and a u rora s , the latter caused by atomic particles hitting the gases i n the atmosphere . The planet has a n extremely strong mag netic field, and sends out radio waves that can be detected on Earth . It a l so has very l a rge reg ions of charged particles around it, similar to , but much more intense than , the Van Allen belts a round E a r t h . J upiter has a relationsh i p with i t s sate l l ite l o t h a t is unique among the planets of the solar system . Strong m a g n e t i c fi e l d s f r o m l o i ntersect J u p iter, a nd c h a r g ed a to m i c p a r t i c l e s fa l l i n t o J u p i t e r 's a t m o 足 sphere from lo's orbit . I n addition to causing auro足 ras, these heat the top lay足 ers of the atmosphere to more than 1 , 500 degrees.

C l ose-u p o f atmos p here

NASA

Orbits of Jupiter's satel l i tes

are a fa int ring and a system of sate l lites, some of which are larger than the smaller planets . Ga l i l eo d iscovered the four la rgest sate l l ites i n 1 6 1 0. The l a rge Gali lean sate l l ites , wh ich can be considered planets themselves, orbit J upiter in periods ranging from 1 . 8 to 1 7 days . Their orbits a re tilted only s l ightly to the equator of J upiter, and are a l most circu l a r. The smaller sate l l ites are both closer to and more d i stant from J upiter than the Ga l i lean satellites . Most are j ust a few mi les across, and have orbits that are both eccentric and inclined to J upiter. The fou r outermost satell ites revolve a bout the planet backwards compared to a l l the others, and are O R B I T I N G J U PITER

1 00

p r o ba b l y c a p t u r e d a s t e r o i d s . A l l t h e s a te l l i te s a r e i mmersed i n J upiter's large, powerfu l mag netic fiel d . The r i n g , about 3 5 , 000 m i les above J u p i ter's cloud tops, is only a few thousand m i les broad and less than 20 m i les th ick. It seems to be made of dark particles a bout a micrometer i n size, and hence is very hard to see . It has been photographed from Ea rth with spec i a l i nstruments , but the best views have been taken from spacecraft . SAT E L L I T E S OF J U P I T E R Distance Year

Mass

from

Period of

J u piter

Revo l u t i o n

(days)

Moon's)

( 1 ,000 m i les)

D i a meter

the Density

Name

Disc.

Met i s

1 979

79 . 5

0. 294

1 . 97

Ad rastea

1 979

80. 1

0. 297

1 . 97

Ama lthea

1 89 2

1 1 1 .8

0 . 498

1 . 89

Thebe

1 979

1 37 . 9

0 . 674

1610

262 . 2

1 . 769

1 .2

1 .61

1 . 97

( m i les)

25(?) 1 6(?) 1 05 62(?) 2 , 256

E u ropa

1610

416.9

3.551

0. 66

1 .61

1 , 95 1

Ganymede

1610

664 . 9

7 . 1 55

2 . 02

1 .61

3 , 268

C a l l i sto

1610

1,171.3

1 6 . 689

1 . 46

1 .61

2 , 983

Led a

1 974

6, 903 . 4

240

1 . 97

9(?)

Himalia

1 904

7, 1 27. 1

251

1 . 90

1 1 5 (?)

lysithea

1 938

7 , 2 76 . 2

260

1 . 93

22(?)

E l o ra

1 905

7,294.9

260

1 . 90

47(?)

Ananke

1 95 1

1 3 , 1 73

63 1 (•)

1 . 95

1 9(?)

Carme

1 938

1 3 , 888

692(•)

1 . 93

25(?)

Pasiphae

1 908

1 4 , 497

735(•)

1 . 90

3 1 (?)

S i nope

1914

1 4, 5 2 1

758(•)

1 .91

2 2 (?)

(r) retrograde motion

Ju p iter's ring seen by Voyager

NASA

lo p h otog ra p hed by Voya ger 1

NASA

10, J upiter's i nnermost Ga l i lean sate l l ite, is one of the stra ngest and certa i n l y the most active volca nic object i n the solar system . I nstead o f lava , i t spews out molten sulfur and sulfur d i oxide . Because its surface g ravity is so wea k , the eruptions jet hundreds o f mi les o u t i nto space a n d around i t s g l obe. The heat to power the volcanoes probably comes from the competing gravitationa l tugs of Jupiter a nd of E u ropa , the next satel lite out, which orbits with a period twice that of l o . These forces squeeze and stretch l o's materi a l , heat足 ing much of l o's i nternal rocks to the melting point. Some heat could a l so come from i nteraction with J upiter's mag足 netic field . The core of lo is probably sol i d , s i l i cate mater ia l l i ke Earth's crust. l o i s 2 , 256 mi les across, about 1 00 mi les larger than Earth's Moo n ; it is a bout 20 percent more massive. Its density leads us to think that l o is made mostly of s i l i cate

1 02

C l ose- u p of l o's s u rface

NASA

rocks. It orbits 262 , 000 mi les from J upiter's center, wel l within the strong magnetic field o f the planet. Following lo around its orbit is a huge cloud of gaseou s sod i u m and other elements, tens of thousands of mi les long . l o 's motion through the magnetic field produces huge electrica l cur足 rents that strike lo's surface, and a lso affects the radio waves J upiter emits. The surface of l o is redd ish, a result of the sulfur from its volcanoes . There a re more than 200 volcanic craters on lo, compa red with only about 1 5 on Earth . There a re a l so high mounta ins, the origin of which is a mystery. lo and J upiter's inner sate l l ites are more rocky and hence denser than the outer sate l l ites . This is probably beca use during the early stages of the solar system the proto足 J u piter was hot and heated its inner satellites, causing the volati le lig hter gases to escape. The outer sate l l i tes kept their gasses . 1 03

E u ro pa

NASA

EUROPA The most puzz ling satell ite of J upiter is E u ropa , second of the Ga l i lean moons . It is a smooth ba l l 1 , 95 1 miles in dia meter. In its orbit 4 1 7, 000 m i les from J u piter's center, E u ropa takes 3 . 55 1 Ea rth days to ci rcle the planet, exactly twice the time it takes l o . The lack o f a n y very h i g h su rface features is surprisi n g . T h e photog raphs taken b y t h e Voyager spacecraft show an intricate maze of severa l sets of intersecting dark lines. The best guess we have so fa r is that they are cracks in the surface . We believe the surface is a layer of water ice that origina l ly welled out of the interior of E u ropa after its formatio n . It is probably tens of mi les thick, l i ke a frozen ocea n covering a rocky core . There are few craters, per足 haps because fewer meteoroids struck there or perhaps because the craters get covered over with ice after they for m .

1 04

C l ose-up of E u ropa's s u rface

NASA

There are darker and lighter areas on the surface, possibly indicating that there is more than one kind of surface materia l . It is a l so possible that the surface is bomba rded by streams of atomic particles, l ike that of l o , a n d that this m a y affect t h e surface colors. Because the surface seems to be frozen water from the interior of the sate l l ite, there are no h i g h vertica l geo足 graph ica l features on E u ropa . The loftiest features a ppea r to be only a few hundred feet h i g h . There is debate over whether liquid water sti l l exists beneath the icy surface . If so, E u ropa may be completely surrounded by a n ocean with an icy crust. Beneath the crust is proba b ly a rocky core . E u ropa a nd lo a re both a bout the same size and density as Earth's Moon, and hence have about the same g ravita足 tiona l pu l l . An object of 1 00 pounds on Earth would weigh only a bout 1/6 of that on these sate l l ites . 1 05

Ganymede

N ASA

GANYM E D E This is the most massive and the second larg est sate l l i te i n the sol a r system . It i s even la rger than the planets Mercury and P l uto , but because of its low density, it is only half as massive as Mercury. It is probably com posed of a rocky core, covered by a mantle of ice or water, in turn covered by a crust of ice tens of m i les thick. Some ca l l it the "ice giant" of the solar system . I n c l a s s i c a l G r a ec o - R o m a n myt h o l ogy, G a n y m e d e (sometimes spelled Ga nymedes) w a s an extremely hand some youth taken from Ea rth to become the serva nt and cup-bearer to the gods of Olympus . In keeping with astro nomical trad ition, many of the other sate l l i tes a lso were g iven names com memorati ng their mythological connec tions with J u piter. Ganymede is two-thi rds of a m i l l ion m i les from J u piter,

1 06

C l o se-u p of Ganymede's s u rface

NASA

going a round it in 7 . 1 55 Ea rth days, j ust twice the orbita l period of E u ropa , and fou r times that of lo . When one sate l l ite has a period that is a n exact m u ltiple of the period of another sate l l ite, they are sa id to be in reso n a n c e . The surface shows bright and dark a reas, craters, exten足 sive g rooves, and many-sided blocks of dark materia l . Two of the dark a reas are named for Ga l i leo and for Simon Marius, who di scovered the large sate l lites of J u piter about the same time Gali leo did . The "blocks" showing on the surface may be chunks of material brought to the surface by motions with i n the sate l l ite . The g rooves a re reg ions, some thousands of mi les long, in which the crust spread apart and parts dropped downwa rd . B locks of crust may be moving, somewhat l i ke the plate tectonics of Earth . 1 07

C a l listo

NASA

This outermost Ga l i l ean satell ite is a lso the most heavily cratered object we know of. Its l ow density of 1 . 6 implies that it is largely ice with a rocky core . Orbiting more than a m i l lion miles from J upiter about once every 1 7 Earth days, C a l l i sto is the second la rgest of J u piter's moons, a nd the third largest moon in the solar system . Along with Ganymede and Saturn's Tita n , C a l l i sto is one of what some planetary scienti sts describe as a new class of solar system bodies with planetlike si zes but with much lower densities . This moon is so heavi ly cratered that the only uncratered portions of the surface a re the centers of recent large craters . There a re not many high a reas because the icy surface i s not as strong as rock and cannot support the m . Most o f the craters a re flatter and have lower wa l l s and CALLI STO

1 08

Surface features on Ca l l i sto

N ASA

peaks tha n do craters on the Moo n . Severa l enormous impact structures a re visible, with rings of crushed and broken crust extend ing hundreds of miles . They resemble some of the l a rger impact areas on Ea rth's Moo n . Temperatures on Ca l l i sto m a y get as high a s 1 75 degrees below zero Fahrenheit, but mostly a re much colder. At such temperatures , ice behaves l i ke rock , but can flow slowly, as it does in the m uch warmer g l aciers on E a rth . Such ice flows i n the crust and below it caused cracks and g rooves in the surface . In mythology, C a l listo (which mea ns "most beautifu l " in Greek) was a favorite nymph of J u piter. I n a nger, J upiter's wife Juno ( known in Greek as Hera) turned C a l l isto i nto a bear. To save her, J upiter 路placed her in the sky as the constel lation U rsa Major, the Great Bear. 1 09

Ju p iter seen from Amo ltheo

J u piter has more than a dozen sma l l moons that we know about and probably more we have yet to discover. At half the d istance of lo, Ama lthea and Thebe orbit J u piter, taking a bout half a day. Ama lthea , d i scovered in 1 892, is 1 05 m i l es across, while Thebe is only about 62 miles across . C loser sti l l to J upiter a re two very sma l l ( 1 5 to 25-mil e d i a meter) moon lets very near J u piter's ring . These "shepherd" satellites help keep the materi a l i n the ring together. Beyond the most d i stant Gali lean sate l l ite, C a l listo, 1 . 1 7 m i l lion m iles from J u piter, there i s a big gap with no known sate l l i tes. Then, between a bout 7 and 1 5 m i l lion miles out, there i s a g roup of sma l l bod ies that a re proba足 bly captured asteroids . The largest of them , H i ma l i a , is only 1 1 5 m i les acros s . All have quite eccentric and i n c l i ned JUP I T E R'S SMALLE R SAT E LLITES

1 10

orbits, and not much is known about them . The outer four orbit J upiter i n the opposite d i rection from all the others . Because of their sma l l si zes and fa ir ly dark surfaces , very l ittle is known about J u piter's minor satel lites . Only Ama lthea and Thebe have been photog raphed closely; the others have so far been seen only as tiny poi nts of l i g h t . T h e Ga l i lee spaceprobe m a y provide us w i t h c loser views of a few more of these sma l l sate l l i tes in the mid - 1 990s . Two curious g roups of sma l l objects m i g ht be cal led sate l l ites of both Jupiter a nd the Sun . They a re the Trojan asteroids, which orbit the Sun in the same orbit as J u p iter. They a re nea r the Lagrangian Points (na med after the French astronomer who first found that such objects could have sta ble orbits) i n J upiter's orbit-that i s , at the poi nts 60 deg rees a head of and behind the g ia nt p l a net, making them a l most 500 m i l lion m i les from J u piter. They a re ca l l ed Trojans because a l l of these asteroids a re na med for char足 acters from the Trojan Wa r. Most of the ones preced ing J upiter i n orbit are named for Greek wa rriors, and those fol l owing J u piter have names of Trojan wa rriors; however, there is one Trojan name in the Greek group and one G reek i n with the Trojan group. There are sl ightly over a dozen bod ies know n , and there proba bly are severa l dozen more too sma l l to be seen from Ea rth . They may be escaped sate l l ites of J u p i 足 ter, or asteroids captured into this pa rticu lar orbit by Jupiter's strong g ravity.

A m a l thea

NASA

111

OBSE RV I N G J U P I T E R J upiter's huge size a nd reflective cl ouds make it brighter than a nything except the Sun , Moon, or Ven u s . Orbiting th e Sun on c e every 1 1 . 86 Earth yea rs, it repeats its positions relative to Earth every 399 days . Near times of opposition, it can come withi n 360 m i l l ion m i l es of E a rth . The chart a bove shows J upiter's changing position agai nst the background of the stars of the zod iac during this decade. The l a rger sate l l ites seem to perform a cosmic ba l let a round J u piter, changing relative positions night to night. This makes for i nteresting viewing through binoc u l a rs or a sma l l telescope . Positions may be found in the Observer's Handbook and astronomy magazines (see p . 1 57) . A sma l l telescope wi l l a lso show the banded atmosphere of the planet, and its major features, including the fa mous Great Red Spot . Some of the sma l ler sate l l ites can a l so be seen , but the smal lest ones req u i re a l a rge telescope, or even a spacecraft, to be seen . The ring is not visible i n sma l l telescopes .

1 12

Finding chart for Ju piter

J upiter sometimes approaches other pla nets in the sky as seen from Earth . On August 1 2 , 1 990, J u piter and Venus wi l l pass only 0 . 04 degrees apart i n the sky (for comparison, the Moon i s 0 . 5 degrees across) and a bout 21 deg rees west of the Sun. They wi l l be only 0 . 0 1 degrees apart on May 1 7, 2000, but more d ifficult to see since they wi l l be only 7 degrees west of the Sun . BEST TIMES FOR VIEWING J U PITER Date of Opposition

D i stance from Earth (m iles)

Jan 2 9

400 million

1 99 2

1 99 1

Feb 2 9

4 1 0 m i l l ion

1 993

Mor 30

4 1 4 million

1 994

Apr 30

4 1 2 m i l l ion

1 995

Jun 1

402 million

1 996

Jul 4

389 m i l lion

1 997

Aug 9

376 million

1 998

Sep 1 6

368 m i l lion

1 999

Oct 23

368 m i l lion

2000

Nov 2 8

3 7 6 million

Note The best times for viewing Jupiter are the several months around the dotes of opposition.

1 13

A view of Satu rn

SATURN Probably the most spectacular planet, Saturn ma rked the outer boundary of the solar system to the pre-telescopic astronomers . Its stately motion a round the Sun, ta king . 2 9 . 46 Earth yea rs, led to it bei ng associated with time, as its ancient G reek name, C hronos, shows . Saturn was also associated with agricu lture, and its astronomical symbol i s a styl ized scythe . Being fa rther from the Sun, Saturn is colder, less bright, and less colorfu l than J upiter. Yet it sports a magnificent set of rings that were d iscovered by Ga l i lee i n 1 6 1 0 . Saturn is probably unique among the planets in havi ng an average density of only 0 . 7 . This means that if you cou ld find a body of water big enough to hold it, Saturn wou l d float . (This may a l so be true for P l uto . ) Average density i s simply mass divided by volume. A pla net's mass can be measured by studying the pla net's g ravitationa l 1 14

force on its sate l l ites . Its volume is determ i ned by its size and shape . Saturn has a mag netic field-about 550 times stronger than that of Earth, but only about 3 percent that of J up i 足 ter-which holds charged a t o m i c particles i n vast belts around the p l a net, s i m i l a r to Ea rth's Va n A l len belts . This m agnetosphere lies outside the A ring (see p . 1 1 8) . Many of the ato mic pa rticles i n these belts come from Saturn's larger inner sate l l ites , particu larly Tita n . Li ke J u piter's but much weaker, Saturn's magnetosphere sends out fa int rad io waves . The strength of the waves changes s l i g htly over time, a nd seems to be affected by the motion of some of the sate l l ites , particu larly Dione. Three centuries of telescopic exa mination were fol lowed by three space probes, providing us with the first c l ose- u p views o f t h e planet and i t s moons. P ioneer 1 1 i n 1 979, fol lowed by Voyager 1 i n 1 980, and Voyager 2 i n 1 98 1 d iscovered severa l new moons, and found that there a re rea l l y thousands of sma l l " r ing lets" making up the few major rings that we see from Earth . SATURN FACTS Distance to S u n 8 86 . 7 m i l l i o n m i les or 9 . 5 3 9 o . u .

Length o f year 2 9 . 46 Ea rth yea rs Orbit eccentricity 0 . 056 Orbit i n c l i nation 2 . 5 degrees Dia meter 74 , 56 5 m i les or 9 . 4 1 X Ea rth's

Mass 95 . 1 6 X Ea rth's Density 0 . 70 Gravity 1 . 08 X Ea rth's Length of day 1 Oh J 9 m Tilt of axis 2 6 . 7 degrees Saturn in orbit

Satu rn and its rings

NASA

is much l i ke that of J u piter, mostly hydrogen and helium . Saturn's smal ler mass a l l ows a more extended atmosphere than J upiter's , and cooler temperatures make the chemica l reactions i n the atmosphere less colorfu l . The two principal minor gasses found i n Saturn's atmosphere a re metha ne and ammonia . The planet is 95 ti mes as massive as Earth, about 30 percent the mass of J upiter. Because of its low density, a 1 00-pound person on Earth would weigh 1 08 pounds on Satu r n . It spins on its axis rapidly, one day tak i ng 1 0 . 66 Earth hours . Because of this speed , Saturn is flattened at the poles more than any other planet . This oblateness is noticea ble through a sma l l telescope or i n photog ra phs. like J u piter, a l l we see a re the tops of clouds of methane T H E COMPO S I T I O N OF SATURN

1 16

and ammonia . The highest clouds are pinkish in color, composed mostly of ammo nia ice crysta l s at a temper a t u r e of 1 70 deg rees Fah renheit . Below these are brownish c louds made of crysta ls of a chemical cal led ammonium hyd rosulfide, at a temperature of about 1 0 deg rees . The lowest cloud layer, bluish in color, is composed of water vapor and ice, at a temperature of a bout 50 degrees . -

C l ose-u p of atmos p here NASA

Below the c l ouds is a clear atmosphere even deeper tha n J upiter's, with pressure increasing as you descend . When the pressure reaches 3 m i l lion times Earth's atmospheric pressure, the hydrogen becomes meta l l i c . A t t h e center o f t h e pla net is a core o f i c e and s i l i cate rocks about two and a half ti mes the size of Earth . Also l i ke J upiter, Saturn gives off more energy than it gets from the S u n , aga i n due to contraction and a l so per haps to a process not yet o c c u r r i n g o n J u p i te r-the condensa t i o n of atmo spheric hel i u m i nto helium "raindrops" that fa l l down wa rd , relea s i ng heat.

Interna l structure o f Satu rn

Satu rn's rings

Discovered i n 1 6 1 0 by Ga l i leo, the rings of Saturn we.re thought to be unique i n the solar system until recently. While Saturn's rings a re the biggest and most spectacular, we now know that J u piter, U ranus, and Neptune have ring systems, too . Ga l i leo origina l ly thought that Saturn was th ree c l ose objects . The poor i mage in his telescope a l so made h i m t h i n k that t h e pla net h a d "ears" or "handles . " I t was not unti l 1 659 that C h ristian H uygens correctly expl a i ned the rings of Satur n . Even then , it was not known whether the rings were so l i d sheets of material or if they were made up of sma l l particles. Today we know that the rings are i ndeed a mu ltitude of sma l l particles, mostly ice or ice-covered rock, ra nging from a few i nches to tens of feet i n dia meter, spaced feet T H E R I N G S OF SATUR N

1 18

Voyager's view of the rings

N ASA

apart. They occupy a very thin disk only a round 1 00 feet thick, but stretch i ng more than a quarter of a m i l lion m i les from the inner edge of the i nnermost ring to the far edge of the outer ring . The ring material is so thin that often d i stant sta rs and Saturn itself may be seen through the rings. The i nner ring, cal led the D ring or "crepe" ring beca use it is so thin and transparent, i s only 4 , 000 m i les a bove Saturn's equator. N ext outwa rd is the C r i n g , then the B ring . Between this and the A ring is a gap where a l most no particles orbit, cal led Cassini's division, after the Ita l ian足 French astronomer Jean Dom in ique Cassini who d i scovered it in 1 675 . (The gap is s i m i l a r to the Kirkwood gaps, p. 93, i n the asteroid belt, and occurs where a particle wou l d have a n orbita l period o f half that o f t h e major sate l l ite 1 19

Ring s s howing "spo kes"

NASA

Mimas . ) Within the A ring is a sma ller but s i m i l a r gap, Encke's division, named for German astronomer J ohann Encke . Space probes have d iscovered three rings beyond the A ring : the F ring , the G ring, and fi na l ly the E ring . The outermost edge of the E ring may extend as fa r as 300, 000 mi les from Satu rn . Thousands of "ring lets" make up the rings. Some are c l u m py, some seem "braided" together. There are a l so rad i a l features that look l i ke spokes in the r i ngs . Some of the rings a re not perfectly circular. J ust how and what keeps the rings together is a very complex dyna m ica l problem not yet completely solved . The narrowest ring lets are as sma l l as 50 feet across . The widest ring is more than 1 2 , 000 m i les across . Each particle i n the rings obeys the same l aws of motion as planets do. The particles closer to Saturn orbit faster than the ones farther out. The particles' motion is d i stu rbed 1 20

Back-lit view of the ring s

NASA

by sma l l moon lets orbiting with i n and on the edges of some of the rings. These "shepherd" sate l l ites help keep the rings from flying apart. Saturn's thin rings orbit i n the plane of the planet's equator, which is tilted 30 deg rees to the p l a ne of its orbit. Every 1 5 yea rs, half of Saturn's 30-year orbita l period, Earth passes through the plane of the rings. This occurs severa l times spaced a few months apart. At such times, the rings a re edge-on to us, and hence i nvisible for many months around those times (see pp. 1 30- 1 3 1 ) Seven and a half yea rs after such times we see the rings widely tilted , and thus very visi b l e . Saturn's magnetic field extends i n t o t h e rings, and it is possible that charged bits of ice, caused by c o l l isions between ring particles, may be carried down i nto the top of the atmosphere and cause a kind of "cloud seed ing" that ma kes atmospheric haze condense and drop to l ower levels. .

121

Orbits of Satu rn's sate l l ites

THE SAT E LL I T E S O F SAT U R N can be d ivided i nto three groups: ( 1 ) very sma l l moon lets, a few tens of mi les in size; (2) sate l l i tes i n the 50- to 1 , 000-mile diameter range; (3) Tita n . T H E SMALLEST SAT E L L I T E S orbit nea r the rings, a n d help keep them i n place. Typica l ly, their orbits a re i n the plane of Saturn's equator and are a l most perfectly circular. Most of them are nonspherica l in shape, more l i ke eggs or footba l l s . We know little about thei r properties .

typica l ly have slight orbita l i n c l i nations and eccentricities . The smal lest of them is outermost Phoebe, more than 8 , 000, 000 m i les from Saturn . Discovered in 1 898, it travels backwa rds a round Saturn, the fi rst retrograde moon ever found . It may be a captured asteroid .

T H E M E D I UM-S I Z E D SAT E L L I T E S

1 22

T H E LA R G E ST sate l l ite other than Titan (see p p . 1 2 8 - 1 2 9 ) is R h e a , 4 7 5 miles across, orbiting 327, 000 m i les from Satur n . Rhea shows extensive frac足 tures on its surface, and p o ss i b l y s n ow. H y p e r i o n , o r b i t i n g 9 2 0 , 000 m i l e s from Satur n , i s the largest nonspherica l moon .

Yea r Disc.

1 990 S 1 8

1 990

82.9

Atlas

1 980

85. 1

1 980 P a n d o ra 1 980 E p i m e t h e u s 1 966 1 966 Janus Mimas 1 789 1 789 Enceladus 1 684 Te thys 1 980 Te lesto

86 . 3 88 . 2 93 . 8 93 . 8 1 1 6.2 1 47 . 9 1 83 . 3

0 . 58 0 . 60 1 0.61 3 0 . 628 0. 695 0 . 695 0 . 942 1 . 3 70 1 . 888

1 83 . 3 1 83 . 3 234 . 9 234 . 9 326 . 8 758 . 7 920 . 3 2, 2 1 3 8 , 053

1 . 8 88 1 . 8 88 2 . 737 2 . 737 4.517 1 5 . 945 2 1 . 2 76 79 . 3 3 1 550(r)

Ca lypso

1 980

Dione

1 684 1 980 1 672 1 655 1 84 8 1 67 1 1 898

Helene Rhea Titan Hyperion I a pe t u s Phoebe

NASA

SATELLITES OF SAT U R N D i stance from Period Saturn of Mass ( 1 ,000 Revolution ( X the m i les) (days) Moon's)

Name

P ro m e t h e u s

Rhea

Density

D i a meter ( m i l es) 12 19 62

. 0005 . 00 1 .01

1 .2 1.1 1 .2

56 76 1 18 242 31 1 659 16 16

.014 . 033 1 . 83 . 026

1 .4 1 .3 1 . 88 1 .2

696 19 95 1 3 , 449 1 58 907 1 37

(r) = retrograde motion Several of the smaller satell ites ore irregular i n shape.

1 23

Mimes

N ASA

Saturn's innermost major satell ite, orbits 1 1 6, 000 m i les from the planet, once every 0. 942 Ea rth days . The moon i s spherica l , 242 miles i n diameter, with a density of only 1 . 2 . Like a l most a l l the Saturnian sate l lites , its surface is made mostly of ice and is heavily cratered . At a tempera 足 ture a l most 300 deg rees below zero, i ce is l i ke rock in its properties . The surface resembles the cratered highlands of the Moon, except that the ice ma kes it much more reflective . The l a rgest crater, named Herschel , is 84 m i les across, and the col lision that produced it proba bly a l most destroyed Mimas long ago. Mimas mai nta ins a fixed orientation as it c i rcles Saturn, with one side always facing the planet . The Voyager space足 craft mapped much of Mimas . Other than the la rgest cra足 ter, Hersche l , most of the other featu res on this satell ite are na med for characters from Britain's Arthurian legends, including craters ca l l ed Merl i n , Arthur, and Galahad .

MIMAS,

1 24

Encelad u s

NASA

E N C E LA D U S is a l so a sate l l ite made of ice. It is 3 1 1 m i les in diameter and orbits 1 48 , 000 m i les from Satu r n . It, too, is cratered , but a l so shows what appea rs to be the results of vu lcani sm-not of lava , but of water. There a re a l so long , narrow va l leys . Astronomers deduce conti nuing vu l 足 canism beca use there a r e no craters larger t h a n about 2 0 mi les across, and a l l the craters are younger t h a n those on other sate l l i tes . Someth ing m ust erode them or fi l l them i n . Also, the surface is more reflective tha n that o f the other moons, mea n i ng it must be newer materia l . Probably the interior of E nceladus i s sti l l liquid and sti l l in motio n . The source of this i nternal heat which keeps Enceladus active is unknown . Like many of the other smal ler sate l l ites, E nceladus keeps one face towa rd Saturn at all times. The Voyager 2 space probe fl ew fa irly close to this satel lite and mapped part of its surface in better deta i l than most of the other Saturnian sate l l ites .

1 25

Tethys

NASA

orbits Saturn 1 83 , 000 m i les from it, once every 1 . 888 E arth days . An interesti ng feature is that there are two "co-orbita l " moonlets, named Telesto and Ca lypso, orbiting i n exactly the same orbit as Tethys . Only 1 6 miles in size, they lie 60 degrees ahead of a nd behi nd Tethys , similar to the way the Trojan asteroids precede and fo l l ow J upiter i n its orbit. (The more-distant satellite Dione a l so has an orbital companion, Helene . ) Their existence is not fu l l y understood . This 659-mile dia meter icy ba l l is cratered , but some parts show more craters than others . This leads us to think that i nterna l geo log ica l processes have smoothed some reg i ons of the satel lite. Tethys a lso shows long branching va l l eys, which indicate that this sate l l ite is sti l l geolog i ca l l y active . A few o f t h e features on Tethys , ma pped b y Voyager s p a c e c r a f t , have been n a m e d after fi g u res in G reek mythology. TETHYS

1 26

I a petus

NASA

I A P E T U S , 2 , 2 1 3 , 000 m i l es from Saturn, is one of the most . bizarre objects we know of i n the solar system . It i s 907 m i les i n diameter, and orbits Saturn once every 79 Ea rth days . Its orbit i s the most highly i n c l i ned of all Saturn's satellites except for Phoebe's , which is tilted 1 5 deg rees from the plane of the planet's equator and r i n g s . L i k e most of Saturn's sate l l ites , it a lways keeps o n e face toward the p lanet . The curious feature is that its tra i ling half is covered with bright icy materia l , while its forward half is made of some much da rker materia l . The moon has a low density of only 1 . 2 , which means it i s mostly ice. Somehow the lead ing edge picked up a dark coating of unknown materia l . The lea d i n g , dark hemisphere reflects less than 5 percent of sunlight hitting it. We don't yet know if the dark mater ia l came from outside the sate l l ite or from withi n th e sate l l ite itself.

1 27

Titan

NASA

is the largest sate l l ite in the solar system . It is 3 , 449 m i les across, larger than the pla nets Mercury and P l uto . Once every 1 6 Ea rth days Tita n orbits Saturn at a d i stance of 759, 000 m i l e s . Titan's other u n i q u e feature i s i t s t h i c k atmosphere . Tita n 's is even denser than Earth's at the surface . It i s a l so the only atmosphere in the solar system , other than our own , that is mainly nitrogen-a bout 80 percent. There is no oxygen . This atmosphere g ives Titan a fa int orange color. Despite a surface temperature no warmer than 300 deg rees below zero Fahrenheit, the atmosphere i s active . I n addition to nitrogen, methane and a rgon make up most of the atmosphere . Minor gases incl ude hydrogen cya nide, acetylene, and other hyd roca rbons. They produce a thick chemical smog layer more than 1 20 m i les thick, and hence we have never seen Titan's surface . There are a l so methane clouds and a higher haze layer. If you were on the surface, T I TA N

1 28

you could probably see for only a few tens of miles. The atmospheric pressure there i s 1 . 6 ti mes that at the surface of E a rth . There may be oceans of methane on the surface, perhaps covering it completely. The surface is ice. Titan may conta i n a h u g e reservo i r o f hydrocarbon chem ica l s , t h e richest i n the s o l a r system, produced b y reactions a mong i t s atmo足 spheric gases ra i n i ng down to the surface . Titan is so I o rge that after it formed , it differentiated . That i s , i nternal heat and heat from the i mpacts of mete足 oroids caused the heavier materials to form a rocky core, covered by a liquid mantle and o n icy crust. By now the mantle has proba bly cooled off and solid ified . When Voyager photog raphed Tita n , we noticed that the northern hemi sphere was somewhat darker than the south足 ern hemisphere . This may indicate that Tita n experiences season s . If so, each one wou l d lost 7 . 5 yea rs, si nce it tokes Saturn 30 yea rs to orbit the S u n . S u rface o f Titan

O B S E RV I N G SAT U R N When nea rest E a rth, around ti mes of opposition, Saturn i s among the brig hter objects in the sky. Near conj unction , however, it i s much fa i nter. It often appears s l ightly yel lowish. Saturn's synod ic period is just two weeks longer than a year, and it takes 30 yea rs for it to c i rcle the zodiac. At opposition it comes within about 750 m i l l ion m i les of Earth. The chart apove shows the position of Saturn agai nst the background of sta rs on January 1 of each yea r of this decade. I n the middle of the decade of the 1 990s, Earth passes through the ring plane on May 2 1 , 1 995 , August 1 1 , 1 995 , and aga i n on February 1 1 , 1 996 . Saturn is then tilted i n such a way that i t s r i n g s appear edge-on to us, and a re therefore invisible because they are so th i n . Thus the rings wi l l be d ifficult to view throughout the middle of the decade. Saturn's rings and a few of its larger satel l i tes a re visible

1 30

F i n d i n g c h a rt for Saturn

i n sma l l telescopes, but because it is so fa r away little surface deta i l can be seen on Saturn except for fa int atmospheric bands and the major d ivisions of the rings. BEST TIMES FOR VIEWING SATURN Date of Opposition

1 990 1 99 1 1 992 1 993 1 994 1 995 1 996 1 997 1 998 1 999 2000

Jul 1 4 J u l 27 Aug ? Aug 1 9 Sep 1 Sep 1 4 Sep 26 Oct 1 0 Oct 23 Nov 6 N ov 1 9

Distance from Earth ( m i l es)

835 million 8 3 1 million 825 million 8 1 7 million 807 million 799 million 789 million 779 million 771 million 763 million 756 mil lion

Note The best times for viewing Saturn occur for several months around the dotes of opposition.

131

URANUS U ranus, the third - l a rgest planet, was the first one d i scov足 ered si nce ancient ti mes . English astronomer Sir Willia m Herschel found it by chance wh ile search i ng the sky w ith a telescope on March 1 3 , 1 78 1 . Herschel named it "Geor足 gia n , " after his patron, King George I l l of England . Others ca l l ed it " Herschel . " The German astronomer Johann Bode suggested it should have a mythologica l name l i ke the other pla nets, and proposed the name " U ranus , " after the very ancient G reek deity of the heavens . (The proper modern pronunciation i s "Yoo-ray-nus. ") U ranus is so far from the Sun that its d i scovery doubled the size of the known solar syste m . Its orbit marked the solar system 's outer boundary until 1 846, when N eptune was d iscovered . U ntil the mid- 1 980s, U ranus was thought to have only five sate l lites : two were d iscovered by Herschel i n 1 787, two more were found i n 1 85 1 , and a nother was sighted in

A vie w of U ranus

1 94 8 . I n 1 986 Voyager 2 flew past the planet and found ten sma l ler moons. Voyager a l so confi rmed the Earth based observation that U ranus has a very faint set of rings. U ranus is 1 4 . 5 times the mass of Earth, but has a density only slightly g reater than water. A 1 00-pound person wou ld wei g h 91 pounds there . It takes U ranus 84 Earth yea rs to circle the Sun once, at an average d i stance of 1 . 8 billion miles. Sunlight takes more than two a nd a half hours to reach this p l a net . U ranus is usua l ly g rouped with J upiter and Saturn as a gas-giant, or J ovi a n , planet, as is more d i stant N eptune. Some pla netary scientists now think that U ranus and Nep tune-both about 3 2 , 000 miles in diameter-should be con sidered a separate class of planet on their own . These two pla nets a re o n l y about a third of the size of J upiter and about five percent of its mass. They have s l i g htly higher densities . Being farther from the Sun, they a re colder. Whereas J u piter and Saturn are mostly hyd rogen and hel i u m , calcu lations pred ict that U ranus and N eptune have substantial qua ntities of heavier elements such as oxygen , nitrogen, silicon, and i ron . U ranus i n orbit

U RAN U S FACTS Distance to S u n 1 , 7 8 3 , 000, 000 m i les or 1 9 . 1 82 a . u .

Length o f yea r 8 4 . 0 1 Earth years Orbit ecce ntricity 0 . 04 7 Orbit i n c l i nation 0 . 8 deg rees Dia meter 3 1 , 8 1 4 m i les or 4 X Earth's

Mass 1 4 . 500 X E a rth's Density 1 . 30 Gravity 0 . 9 1 X E a rth's Length of day 1 7h 1 4• Tilt of axis 98 degrees

U r a n u s photographed by Voyager 2

NASA

U RA N U S seen from Ea rth shows a bland surface, rea l l y the tops o f c l ouds i n i t s hydrogen-helium atmosphere. Much colder than J u piter or Saturn, there is very little color and few deta i l s visible i n the atmosphere . The planet appea rs sl ightly bl ue-g reen . Though it may look bland, U ranus is a pecu liar planet i n severa l ways . The other pla nets (with the exception of P l uto) have their north and south poles (the axis of their spi n , or rotation) tilted over less than 30 deg rees measured from a l i ne perpend icular to their orbita l planes. ( E a rth is

1 34

tilted 23 . 4 degrees, and this causes the seasons we expe rience; see p . 64 . ) U ranus i s tilted over a l most 98 degrees ! This means that its north pole poi nts a l most along the plane of its orbit; actua l l y it points slightly to the south side of the solar system . Thus there a re times during U ra nus' orbit when its north pole poi nts a l most di rectly at the Sun; one quarter orbit (2 1 years) later the Sun wou l d lie over U ranus' equator; and 2 1 years after that the south pole is poi nted at the Sun. This must make for very stra nge seasons on U ranus. Because its sate l l ites orbit i n the plane of U ranus' equa tor, this tilt a lso mea ns that the sate l lites move a l most perpendicular to the plane of U ranus' orbit. The only other pla net i n the solar system with a tilt l i ke this is P l uto . U ranus a lso has a mag netic fiel d , l i ke many of the other planets . But u n l i ke the other pla nets-which have their north magnetic poles nea r their north geog raphic poles-the poles of U ranus' mag netic field are tilted over more than 60 deg rees compa red to its axis of spi n . Furthermore, the center of the magnetic field i s not at the center of the planet . After hyd rog e n a nd Struc tu re of U r a n u s hel i u m , the main compo nent of the atmosphere is methane. The only atmo spheric features that are visible are bands of polar haze prod u ced by the action of sunlight o n meth ane, a lthough Voyager did observe a u roras high i n the atmosphere .

THE

S AT E L L I T E S

tota l 1 5 i n num ber but fa l l into two differ ent-sized groups. The five o u t e r o n e s a re severa l hund red m i les in size, and a re major worlds l i ke the satel lites of J upiter and Sat urn. Ten smal ler moons, a l l orbiting closer, were dis covered by Voya g e r i n 1 98 6 . A l l the sate l l ites and the rings are i n the highly tilted equatorial plane of A r i e l NASA U ranus. Each of the major moons has its own peculiarities . Some are very complex geo logically and show evidence of active geo logy. The i n ner sma l l satel lites, some of which act as "shep herds" for some of the rings, are all very dark, as dark as charcoa l , l i ke the ring material itse lf. Before Voyager we knew The r i n g s of U r a n u s NASA of the existence of severa l rings of sma l l dark parti c l e s , o r b i t i n g n e a r e r to U ra n u s than Miranda . Voyager found 2 more, for a tota l of 1 1 , plus unex p l a i ned seg ments, or "arcs , " of rings. There a re now 1 1 rings known , but their origin and what keeps them i n p l ace a r e st i l l debated . U RA N U S

1 36

Orbits of U ra n u s' sate l l ites

SAT E L L IT E S OF U RA N U S Distance from Uranus

Name

Period o f

Mass

Year

( 1 ,000

Revo l u t i o n

Disc.

m i les)

(days)

Moon's)

the

Dia meter Density

( m i les)

Corde l i a

1 986

30. 944

0 . 333

Ophelia

1 986

3 3 . 430

0 . 375

B i anca

1 986

3 6 . 785

0 . 433

Cressida

1 986

38.401

0 . 463

D e s d e m o n a 1 98 6

3 8 . 960

0 . 475

J u l i et

1 986

40. 0 1 6

0 . 492

Portio

1 986

4 1 . 07 2

0.51 3

Rosa l i n d

1 986

4 3 . 496

0 . 558

Belinda

1 986

46. 789

0.62 1

Puck

1 986

5 3 . 438

0. 763

M i randa

1 948

80 . 7 1 6

1 .41

96 .001

1 . 26

301

Ariel

1 85 1

1 1 8 . 620

2.521

.018

1 . 65

72 1

Umbriel

1 85 1

1 65 . 285

4 . 1 46

.01 7

1 . 44

739

Tita n i a

1 787

2 7 1 . 1 04

8 . 704

. 047

1 . 59

1 , 000

Oberon

1 78 7

3 6 2 . 508

1 3 . 463

. 040

1 . 50

963

1 37

O B S E R V I N G T H E O U T E R PLAN ETS is d i fficult at best . The chart a bove shows the positions of the outer pla nets against the background of the zod iac for January 1 of each year of this decad e . U ranus is only visible with the unaided eye under perfect cond itions, but it is visible through binocu l a rs or a sma l l telescope if you know j ust where to look. Neptune can be seen with a sma l l telescope, and, l i ke U ranus, a ppea rs only as a sma l l greenish d i s k . P luto requ i res a large telescope to b e seen . U ranus' sate l l ites Titania and Oberon a re visible in sma l l i nstruments; the other satell ites are too fa int. T h e ri ngs a re not visible even in the largest telescopes, a nd can be detected from Earth only by special instruments . Neptune's sate l lite Triton is also visible in amateur telescopes . P luto's C haron req u i res a l a rge telescope . Because they are so far away to beg i n with , these planets vary less in brig htness as seen from E a rth than do closer pla nets . Thei r synodic periods are a l l j ust a few days more than one year. 1 38

F i n d i n g c h a rt far the outer p l a nets: U r a n u s ( ma g e nta ) , N ept u n e (ye l low), a n d P l uto ( b l u e)

B EST TIMES FOR V I E W I N G U RA N U S Date o f Oppos i t i o n

D i s t a n c e f r o m E a r t h ( m i l es)

1 99 1

Jul 4

1 . 7 1 6 b i l l ion

1 992

Jul 7

1 . 7 2 1 billion

1 993

Jul 1 2

1 994 1 995 1 996 1 997 1 998 1 999 2000

Jul 1 7 Jul 2 1 J u l 25 Jul 29 Aug 3 Aug 7 Aug 22

1 . 727 bill ion 1 . 733 bill ion 1 . 738 bill ion 1 . 743 billion 1 . 748 billion 1 . 752 billion 1 . 756 billion 1 . 759 bill ion

BEST T I M E S FOR V I EW I N G N E PT U N E All oppositions i n the 1 990s occur i n J u ly, with a typical E a r t h - N eptune d i stance at opposition of 2. 7 1 billion miles.

B EST T I M E S FOR V I EW I N G PLUTO A l l opposi tions i n the 1 990s occur in mid-May, with a typical Earth - P l uto di stance at opposition of 2 . 68 b i l l i o n m i le s . Because of the eccentricity of its orbit, P l uto w i l l be c l oser to Earth than N eptune unti l 1 99 9 .

1 39

Neptu n e as seen from the s u rface of Triton

NEPTU NE Fol lowing the d iscovery of U ranus, astronomers noted it did not fol l ow exactly the path Newton's and Kepler's Laws pred icted . The Engl ish astronomer J . C. Adams a nd the French scientist J . J . Leverrier independently proposed that a sti l l more d i stant planet wa s a ltering its motion, and they pred icted where that new pla net wou l d be see n . In 1 84 6 , the German J ohann Bode found the pla net where they predicted , aga in a l most doubl i ng the size of the solar syste m . This pla net in the depths of space wa s na med for the Roman god of the depths of the sea . Its symbol is a trident. 1 40

The fourth - l a rgest planet, Neptune takes 1 64 . 79 Earth yea rs to c i rcle the Sun, at a n average distance of 2 . 793 billion miles. Its orbit i s a l most circular, incli ned less than 2 deg rees . We know that Neptune is 1 7 . 2 times as massive as Earth, with an average density of 1 . 76 . A 1 00-pound person wou ld weigh 1 1 9 pounds on Neptune. The combination of Neptu ne's a l most circ u l a r orbit and P l uto's highly eccentric orbit (see p . 1 48) means that P l uto is for a few yea rs closer to the Sun than N eptune. This unusua l situation i s occu rring now, and will last until 1 999. It won't occur again for more than two centu ries . U ntil recently not much was known about Neptune si nce it could be studied only by large telescopes from Earth . In August, 1 989, the Voyager 2 spacecraft made its fi nal pla neta ry encounter, revo l utionizing our knowledge of this outermost of the gas-giant planets . Voyager found six new sate l lites, add i n g to the two discovered from Earth, and a fa int series of rings a round the planet.

Neptune in orbit N EPTU N E FACTS Distance to S u n 2 . 793 b i l lion m i les or 3 0 . 058 a . u .

Length of year 1 64 . 79 Earth years Orbit ecce ntricity 0 . 009 Orbit i n c l i nation 1 . 8 deg rees Diameter 30, 1 98 m i les or 3 . 8 1 X E a rth's

Mass 1 7 . 204 X E a rth's Density 1 . 76 Gravity 1 . 1 9 X E a rth's Length of day 1 6 . 03 hours Tilt of axis 2 9 . 6 degrees

Neptu n e as seen by Voyager 2

NASA

N E PT U N E'S ATMO S P H E R E i s at least as a ctive as U ranus' atmosphere, and much more colorful-which is surprising, since Neptune i s farther from the Sun tharr U ranus, and hence colder. Sunlight at Neptune is less than five percent as strong a s at J upiter's orbit. Voyager detected very high wind speeds , over 700 m i les an hour, blowing westward足 even though Neptune turns eastwa rd . This had led astron足 omers earlier to think that the pla net rotated more slowly than the 1 6 hours and 3 minutes it takes to turn once . The ea rlier estimate of a little over 1 7 hours i s the period for the clouds at the top of the atmosphere . Severa l dark a reas a re visible floating h ig h i n N eptune's atmosphere, and they change over a period of days . The largest feature, ca l led the "Great Dark Spot, " is moving at about 760 m i les a n hour. Auroras were detected high in 1 42

the atmosphere, not j ust in the polar reg ions as expected , but a l l over the pla net-for reasons not yet understood . like the other gas-giant pla nets , the atmosphere is mostly hydrogen and helium, but most of the visible features are a result of methane clouds. It a ppears that the Sun's u l travi足 olet light converts methane high i n the atmosphere i nto hydrocarbons that then drop toward the surface. When they reach lower, hotter layers they evaporate back i nto methane aga i n , which then rises to the top of the atmosphere . T h e r e i s e v i d e n c e of g l o b a l a t m o s p h e r i c m ot i o n , whereby heat from some latitudes i s carried to pol a r regions . Neptune has a mag netic field inclined to its rotation axis by 50 degrees . The center of the magnetic field i s offset b y a l most half N eptune's rad ius from t h e center of the planet. Why this is so i s not yet understood . This is similar to the situation for U ranus, and means that the magnetic field probably arose during the planet's orig ins, not by some unique accident. The presence of a mag netic fie l d means that Neptune must conta i n a Neptune's structure reg i o n of e l e c t r i c a l c o n 足 d uct i ng flu i d t hat i s i n motio n. L i k e the o t h e r J o v i a n p l a nets , N e ptune e m i t s more energy than i t gets from the Sun . In Neptune's case, it emits 2 . 7 times the so l a r e n e r g y . T h i s m a y account for the fact that its atmosphere is more active and colorfu l than that of U ranus.

Triton

NASA

N E PT U N E'S SAT E LL I T E S AND R I N G S When N eptune was d iscovered in 1 846, its largest sate l lite, Trito n , was a l so fou n d . A century later, in 1 94 9 , Nereid was d iscovered orbiting at five and a half m i l l i o n m i les from Neptune, taking a yea r to orbit once. Because of N eptune's great d i sta nce, no more satellites were found by terrestrial observers. I n 1 98 9 , during Voyager's last planetary encounter, six new sate l l ites were seen . One of them is larger than Nereid but had not previously been seen because it i s cl ose to Neptune and thus lost i n the pla net's Neptun e's r i n g s

NASA

g l a re as viewed from Earth . Voyager a l so found sev足 era l fa int rings of d ust and sma l l rocks c i rc l i ng the planet . The "Ma i n Ring" i s c l u m py, a n d l i e s 3 9 , 0 0 0 mi les from Neptune's cen足 ter. At 3 3 , 000 miles from Neptune i s the "I nner R i ng , " and c l oser i n at 2 6 , 000 mi les i s the "I nside Diffuse Ring . " J ust outside

the i n ner ring is a flat sheet of materia l cal led the "pla足 teau . " These are all much l e s s d e n s e t h a n S a t u r n 's rings, and not visible from Earth . The g reatest surprise of Voyager's visit was Trito n . It has a very thin, mostly nitrogen atmosphere, The sate l l ite 1 989 N 1 N ASA about one-thousandth the pressure on E a rth . The surface shows extensive cratering and scarring . N o other body i n the solar system com足 bines all the types of features Triton shows . Its density is about twice that of water, mea n i ng that it conta i n s a h i g h e r proportion o f r o c k t h a n other i c y sate l l ites . Dark and light surface materia l indicates that some sort of vul足 ca nism may sti l l be going o n , but the "lava" i s not molten rock; it i s probably liquid nitroge n . P la neta ry scientists think Triton, though actua lly s l ightly larger than P l uto, may i n fact be a lot l i ke that planet . SAT E L LITES OF N E PTU N E Distance Year

from

Period of

Mass

Neptune

Revo l ution

(

(days)

Moon's)

( 1 ,000 m i les)

Diameter

the

Name

Disc.

1 989 N6'

1 989

29.9

0. 296

1 989 N 5 '

1 989

31 . 1

0. 3 1 3

1 989 N3'

1 989

32.6

0. 333

1 989 N4'

1 989

38.5

0. 396

1 00

1 989 N2'

1 989

45 . 7

0. 554

1 24

1 989 N 1 '

1 989

73.0

1 . 121

261

Triton

1 846

220

Nereid

1 949

3 , 478

5 . 877

0 . 29

365 . 2 1

Density

( m i les)

1 . 689 1 86

"'These ore provisional names until permanent names are adopted .

1 45

Pluto's motion among the sta rs

PALOMAR

PLUTO When it turned out that Neptune did not account for a l l the d i sturba nces in U ranus' orbit (p. 1 40), astronomers W. H . Pickering and Perciva l lowe l l predicted that a nother more d i stant plonet must exist beyond N eptune. lowe l l hi mself founded a n observatory and funded a search he d i d not l ive to see completed . PLUTO WAS D I S C OV E R E D in 1 930 . However, it i s so sma l l that it ca nnot account for the motions o f U ranus' orbit . The pred ictions were spurious, and P l uto's d iscovery was a fortunate accident due to the carefu l sky survey at the Lowel l Observatory by American a strooomer C l yde Tom足 bo u g h . The new planet was given the name P l uto, after the Roman god of the underworl d . The first two letters of the

1 46

name are a lso lowe l l 's initials, and its symbol is a combi足 nation of P and l . More tha n 3 . 6 b i l l i o n m i les out, P l uto i s sma l l a n d icy, and some astronomers have even proposed "demoti ng" it from the status of a planet to that of an asteroid . It may be an esca ped satel lite of N eptune . The Sun seen from P l uto is j ust a very bright star, a bout 1 , 000 times brig hter tha n the fu l l moon is on Ea rth . Sunl ight takes 5 1/2 hours to reach P l uto . Because it is so far away and d i m , not much is known with certa i nty about P l uto, and no space p robes are now schedu led to visit this the most remote of the planets . PLUTO FACTS Distance to S u n 3 . 666 b i l l i o n m i les or 3 9 . 4 3 9 a . u .

Length o f yea r 247 . 69 Earth years Orbit eccentricity 0 . 250 Orbit i nc l i nation 1 7 . 2 degrees Mass 0 . 0026 (?) X E a rth's

P l uto and Cha ron in orbit

Dia meter 1 , 42 9 m i les or 1 8 X Earth's

Density 1 . 1 (?) Gravity 0 . 05 (?) X E arth's Length of day 6. 3867 Earth days Tilt of axis 94 degrees

PLUTO A N D C H A R O N P l uto is thought to be a bout 1 , 429 m i les i n d i ameter, mostly ice, and covered with frozen methane. Its density i s thought to be j ust sl ightly g reater than that of water. Its orbit is highly eccentric, ta king it closer to the Sun than N eptune at times . Its 248-year orbit took it closest to the Sun i n 1 989, and until 1 999 P l uto wi l l b e closer t o the S u n than Neptune. P l uto is a ppropriately named for the god of the underworld because it orbits far from the Sun i n a perpetual twi light. P l uto appears only as a speck of lig ht even i n the la rgest telescopes . Much of the information about it i s uncerta i n . Before 1 978 w e thought P l uto was larger than we now know it to be, and it was thought to be the second-sm a l l est pla net, after Mercury. This was because our photographs b l urred together the i mage of P l uto itself and the image of its satel lite. Si nce we could see no surface features, it was i m possible to know how long it took to rotate on its axis. I n 1 978 a sate l l ite was d iscovered and na med Charon . It is so close to P l uto that we believe they are g ravitation足 ally l ocked together. Thus P l uto rotates i n the same period of time as Charon revo lves about it, a l ittle over six days . P l uto's axis seems tilted over (by 94 deg rees), much l i ke that of U ra n u s . P l uto and Charon both keep the same face toward each other. Charon , 808 miles across, is the la rgest satel l ite compared to its primary planet in the solar system . It is over half the size of P l uto . Charon is named for the mythological figure who ferried the dead across the River Styx i nto the underworld . Ca refu l measurements by telescopes on Ea rth have revea led signs of what appears to be frozen metha ne on the surface of P l uto and its sate l l ite. Their surfaces are probably entirely covered with ice, and a re only a bout 1 0 , 000 m i les apart where they face each other.

1 48

P l uto's mass is thought to be only 0 . 003 that of Earth's, so that a 1 00-pound person on Earth wou l d weigh only 5 pounds on P l uto . The temperature on the surface is 350 degrees below zero Fahrenheit, and it proba bly has a very thin atmosphere of methane, 1 /60 the density of E arth's . P l uto's sma l l mass mea ns that it cannot be responsible for all the d i screpancies i n the motions of U ranus and Neptune . This has led some to conjecture that there i s another pla net even farther o u t than P l uto .

SATELLITE OF PLUTO Di stance from P l uto Name

Year Disc.

( 1 ,000 m i l es)

Period of Revol ution (days)

Mass ( x the Moon's)

Density

Diameter (mi les)

Charon

1 978

1 1 . 68

6. 387

0 . 02 (?)

0 . 9 (?)

808

A view from P l uto's s u rface

A c:omet i n a 1 30 1 pa inting by G i allo

ESA

COMETS Of a l l the objects in the sky, probably none has ca used as much fea r as comets . The name mea ns "hairy star" in Greek, for bright comets often appea r as d i ffuse blobs of l ight with wispy ta i l s . H i storica l ly, they have been seen as the finger of an angry god or a sword pointing to Ea rth . Today, comets are na med for their d i scoverers . Comets are remnants of materi a l left over from the formation of the solar system . We believe hundreds of b i l l i ons of them exist in a vast reg ion surrounding the solar system at distances of bill ions to tri l l ions of m i les from the S u n . Once i n a while, the orbit of one of these comets is perturbed and it fa l l s toward the i nner solar system . If it comes close enough to Earth and the Sun , it may become visi ble in our skies . It may then return to the depths of space, or the gravity of J upiter and Saturn may capture it into a permanent orbit a mong the pla nets , becom i ng a periodic comet . If a comet passes very cl ose to the Sun 1 50

Comet Halley in 1 9 1 0 CARN EG I E

(ca l l ed a "sun-grazer"), it may be bro k en up , produc ing two comets, or dispers ing completely. Comets' orbits are mostly highly e l l i ptica l , and many a re seen only once . A few b r i g h t c o m et s a r e ca l l ed periodic and return p red i c ta b l y i n p e r i o d s ranging from 3 . 3 yea rs (the shortest k nown) to about 1 50 yea rs . The orbits of comets are usua l l y highly inclined to the plane of the ecliptic, and many go around the Sun i n a retro grade manner. Strewn a l ong many com ets' e l l i pt i c a l orbits a re swa rms of dust and roc k lost by the comet, ca l l ed mete oroid strea m s . When the orbit of one of these i nter sects Earth's orbit, we wi l l see it as a meteor shower each yea r. We believe most m e t e o r s a r e b i t s of d i s persed comets . T h e Eta Aquarid a n d Orionid show ers ( p . 67) res u l t from meteoroids i n Comet H a l ley's orbit.

have been explored cl osely by spacecraft-Ha l l ey and Giacobini-Zinner. The latter was the fi rst comet ever visited by spacecraft, and much was lea rned that later a ided the fleet of J a pa nese , Soviet, a nd E u ropea n space probes that flew by Comet H a l ley during its passage i n 1 98 6 .

O N LY TWO C OMETS

is the centra l part of a comet . It is a small object; in the case of H a l ley, the nucleus is pea nut-shaped and a bout ten miles long and six mi les wide . It is a m i xture of ice, mostly water ice, with dust and rocks, and it is often cal led a "di rty snowba l l . " When the comet heats up due to sunlight, the water ice turns to vapor and esca pes, jetting through cracks in the surface, leaving a porous interior. The surface is sl ightly bumpy and is as black as cha rcoa l . H a l l ey's nucleus rotates once every 5 3 hours . T H E N U C LE U S

Comet structu re

1 52

su rrounds the nucleus. These two parts make up the head of the comet . The coma is a m i xture of gases and dust boi led out of the nucleus by the Sun's heat. ( H a l ley boiled away about 25 tons of water per second . ) The coma may extend 60, 000 mi les into space , surrounded by a hydrogen cl oud m i l l i ons of m i les across . T H E COMA

can stretch to hund reds of m i l l ions of m i les i n length , yet they conta i n very little materia l . There a re rea lly two ta i l s to a comet . The dust tail is stra ight, com足 posed of m i n ute particles freed from the nucleus . It is slightly yel l owish i n color, from reflected sunlight. The gas, or plasma, tail is made of ion ized gases, often water va por or carbon monoxide. It is usua l l y curved , and it i s usua l l y b l u i s h , t h e color emitted b y these gases . (These c o l o r s a re fa int and can be seen only in photog raphs . ) Si nce the molecules in the gas ta i l a re electrica lly cha rged , they i nteract with the solar wind and the Sun's magnetic field . Comet ta i l s a lways point away from the S u n , due to the solar wind, so that when a comet is movi ng away from the Sun, the comet's ta i l s are actua l l y leading the head . Every comet loses some of its material with each passage near the Sun . Some of the l ost material forms long dust tra i l s a head of and behind each comet in its orbit. A comet can only make a few hund red such passes nea r the Sun b e f o r e it is l a r g e l y d i s Comet ta i l s poi nt away from the p e r s e d ; t h is t a k e s t h o u Sun sands to b i l l ions of years. D u r i n g C o m e t H a l l e y 's appea rance i n 1 9 1 0 , Earth actua l l y passed through the ta i l of the comet, with no noticeable effect other than a spectacular view of the comet . COMET TA I LS

Some period i c comet orbits

is easy when they a re bright. While astronomers may view more than a dozen comets each yea r through telescopes, only once or twice a decade does a comet appear that is visible to the unaided eye . Most of these a re not period ic comets, and cannot be pred i cted very fa r in advance . H a l ley is the only bright period ic comet, a nd it returns to the i nner solar system only once every 76 years (next i n 206 1 ) . When fa r from the Sun, comets are very d i m . Pred icted comets may be spotted through telescopes when they a re beyond the orbit of J upiter, but new comets a re usua lly not spotted unti l they a re closer to the Sun and thus brig hter. Even the brightest CC?mets usua lly must be with in the orbit of Mars to be visible to the unaided eye from Earth . Even OBS E RV I N G COMETS

1 54

Comet H a l ley seen by G i otto space probe in 1 986

ESA

then, if the comet i s on the far side of the Sun, it may be d i m and difficult to see from Earth (as was the case with Comet Hal ley i n 1 986) . Contrary to popu l a r opinion, comets do not whiz across the sky. I nstead , they hang a l most motio n l ess agai nst the background of stars, moving slowly among the stars from night to night. Often only the head is seen a s a fa i nt, fuzzy ba l l . The ta i l , when visible, is a fi lmy streak of light, brightest near the hea d . O n l y on rare occasions, perhaps a few times a centu ry, such as i n 1 9 1 0 when Earth actually passed through the ta i l of Comet H a l ley, will a comet be a truly spectacular object i n the sky. The best i nstruments for viewing a comet a re bi noculars because of their wide field and ease of use . Those with telescopes may view severa l fa int comets each yea r. Be cautioned , thoug h , that comets seen with sma l l i n struments wi l l not resem ble the professional photog raphs taken with large telescopes . The excitement of comet viewi ng i s that you a re looking at materia l that is as old as the solar system itself, kept bil lions of yea rs in a deep-space deep足 freeze, holding c l ues to the origins of ourselves and our pla neta ry neighborhood . 1 55

An i m a g i nary p l a n etary system

PLANETS BEYOND PLUTO? Astronomers have sea rched , without success, for major planets beyond P l uto . If a ny exist, they must be very dark, very sma l l , or very d istant-perhaps all of these . They p r o b a b l y r e s e m b l e N e p t u n e o r s o m e of t h e J o v i a n sate l l ites . We have a lso exam i ned nearby sta rs for evidence of planetary systems. Even the nearest star system is a l most 7 , 000 times farther from the Sun than P l uto . Any such p l anets wou l d be too far away to see d i rectly, but we might be able to detect their presence by their gravitational force on the sta r, making its path through space wavy. There is some evidence that a few pla nets do exist around nearby sta r s . The pla nets we think we detect a re huge, many ti mes the mass of J u piter. There could be other smaller p l a nets in such systems. Possibly Earth-sized planets orbit these sta rs i n a reg ion where the tem perature is suitable for liquid water, a nd many other conditions a re j ust right. If so, life may have evolved . And "they" may be looking at sta rs near to them , as we are, wonderi ng if they a re a lone in the u niverse. 1 56

BIBLIOGRAPHY For more i nformation, and to keep up with the latest discoveries, consu l t these and other references : B O O KS

Beatty, J . Kelly, Brian O'Lea ry, and Andrew C ha i k i n , eds . , The New Solar System, 2nd ed . , Sky P u b l ishing Corp . , Cambridge, MA, 1 982 Chartra n d , Mark R . , Skyguide, Golden Press, New York, N Y, 1 982 Roya l Astronomical Society of Canad a , Observer's Hand足 book (yearly), 1 24 Merton Street, Toronto, Ontario M4S 2Z2 C a nada Zim, Herbert S . , Robert H. Baker, and Mark R. Chartra n d , Stars, rev. ed . , G o l d e n Press, N ew York, NY, 1 985 MAGAZ I N E S

AstroMedia Corp . , P. O . Box 92788, Milwau足 kee, Wl 53202 Mercury, Astronomical Society of the Pacific, 1 290 24th Avenue, Sa n Fra ncisco, CA 94 1 22 Sky and Telescope, Sky Pub l ishing Corp . , 49 Bay State Road , Cam bridge, MA 02 1 38 Astronomy,

Also see occasional articles on astronomy i n such maga足 zines as Science News, Scientific American, N e w Scientist, Discover, and Omni.

1 57

I N D EX Boldface type indicates pages w i t h more extensive information. Adrostea, 1 00 , 1 0 1

Chromosphere, 36, 37

Fireballs, 66

Air. See Atmosphere

Columbus, Christopher, 79

F u l l moan, 7 2 , 7 3 , 78

Amalthea, 1 00 , 1 0 1 , 1 1 0 Ananke, 1 00, 1 0 1 Angular size, 40 Annular eclipse, 41 Apo l l o progra m , 20, 2 2 , 69 Apollo-type asteroids, 9 1 , 95

Comet Giacob i n i � Z inner, 2 3 , 1 52 Comet Hol ley, 23, 1 5 1 , 1 52 , 1 53 , 1 54- 1 55 Comets, 8 , 1 1 , 2 3 , 1 501 55 formation, 26

Ar;el, 1 36, 1 3 7

observing, 1 54- 1 55

Asteroid belt, 90, 9 1 , 93

orbits, 29

Asteroids, 1 1 , 90-95 , 1 0 1 ,

properties, 1 50- 1 53

1 22

Compass, 1 6

Amor-type, 9 1

Conjunction, 1 4- 1 5

Apo l lo-type, 9 1 , 95

Constel lations, 5 , 1 2- 1 3

observing, 94-95

Copern icus, Nicolaus, 8

properties, 90-93

Cordel i a , 1 3 7

Trojan, 1 1 0- 1 1 1

Corona, 36, 37

viewing, 94-95

Cressida, 1 37

Astrology, 4 Astronomical names, 1 1 Astronom ical unit, 2 8 Astronomy, 5 Astronomy, Greek, 6-7 Atlas, 1 1 8 , 1 23 Atmosphere, 60-61 effects of, 1 8 A . u . , 28 Auroras, 3 7 , 6 1 . 1 35 , 1 42

Day, 30 Deimos, 86, 87

Earth, 8, 2 1 , 3 2 , 56-67 atmosphere, 60-61 equator, 1 2 , 65 formation, 27 interi or, 62-63

Bailey's Beads, 43

properties, 56-57

Belinda, 1 37

seasons, 64-65

Bianca, 1 37

size, 28, 29

orbit, 1 2 , 28-29

Big Bang, 25

Eorth�crossers, 94-95

B i noculars, 5, 1 38

Eccentrici ty, 28

Bode, Johann, 1 3 2 , 1 40

Ecl ipse seasons, 4 3 , 79

Bolides, 66

Eclipses. See lunar ecl ipses, Solar ecli pses Ecliptic, 1 3 Elora, 1 00, 1 0 1 E l evation, 1 7

Calypso, 1 2 3 , 1 26

E l l ipse, 3 1

Carme, 1 00 , 1 0 1

E longation, 1 4

1 19

Enceladus, 1 2 3 , 1 2 5 Encke, Johann, 1 20

Cossini's Division, 1 1 9

Encke's Division, 1 20

Celestia l equator, 1 2 , 1 7

Epicycles, 7

Celestia l sphere, 1 2

Epi metheus, 1 1 8, 1 23

Ceres, 29, 90, 92

Equinox, 64, 65

Charon, 1 38 , 1 47 , 1 4 8-

E u ropa, 1 00, 1 0 1 , 1 04-

1 49 C h i ron, 93

1 58

1 06- 1 07 , 1 08 Gas�giant p l a nets, 33 Giotto space probe, 2 3 , 1 55 Greatest elongation, 1 4 Great Red Spot, 1 1 2 Harvest moon, 69 Helene, 1 23 Herschel, Sir W i l l i a m , 1 3 2 H i m a l i a , 1 00 , 1 0 1 , 1 1 0 Horizon, 1 7 Hubble Space Telescope, 2 1 , 23 Hunter's moon, 69

motion, 1 6

Cass i n i , Jean Dominique, 9 ,

Ganymede, 29, 1 00 , 1 0 1 ,

Huygens, Christian, 9, 1 1 8

ocea ns, 58-59

1 09 , 1 1 0

1 07 , 1 1 4 , 1 1 8 G a l i l eo space probe, 97

Dione, 1 1 5 , 1 2 3

Azimuth, 1 7

C a l l i sto, 1 00 , 1 0 1 , 1 08-

G a l i l e i , G a l i leo, 9 , 1 00,

Desdemona, 1 37

Autumnal equinox, 64, 65

Brahe, Tycho, 7 , 8, 3 1

Galaxies, 1 0 G a l i l ea n sate l l ites, 1 00- 1 1 1

1 05 , 1 07 Evening star, 1 4

Hyperion, 1 23 Iapetus, 1 2 3 , 1 2 7 I n c l i nation, 28 Inferior conjunction, 1 4 Inferior planets, 1 4- 1 5 International Cometary Expl orer, 23 Interplanetary Monitoring Platform, 2 0 l o , 1 00 , 1 0 1 , 1 02 - 1 0 3 , 1 05 , 1 07 , 1 1 0 Ionosphere, 6 1 Janus, 1 1 8 , 1 23 Jovian planets, 33 J u l iet, 1 37 J u p i ter, 1 5 , 2 2 , 3 2 , 33, 93, 96- 1 1 3 , 1 1 5 , 1 1 6 , 1 33 , 1 50 observing, 1 1 2- 1 1 3 orbit, 2 9 properties, 96-99 ring, 1 0 1 sate l l i tes, 1 00-1 1 1 size, 2 8 , 2 9 view i n g , 1 1 2- 1 1 3 Kepler, J ohannes, 7 , 9 , 3 1 Kepler's lows, 3 1 , 1 40 K i rkwood gaps, 93, 1 1 9

lagrang ian points, 1 1 1

Names, 1 1

landsat, 2 1

Nebula(s), 1 0 , 24

inner, 2 6

low of Gravity, 3 1

Neptune, 1 5 , 23, 33, 1 3 3 ,

m o t i o n , 3 G- - 3 1

leda , 1 00, 1 0 1

1 38-1 39, 1 40- 1 45

P l a nets (cont . ) :

orbits, 28-29, 3 1

leverrier, J . J . , 1 40

observing, 1 3 8- 1 39

orbits, resonance, 1 07

light-year, 1 0

orbit, 29

outer, 26

lowe l l , Perciva l , 1 46- 1 47

properties, 1 40- 1 43

lunar eclipses, 73, 7 8-80

rotation, 29

rings, 1 44-1 45

lysitheo, 1 00, 1 0 1

sate l l ites, 1 44- 1 45

superior, 1 5

size, 28, 29

types, 32-33

viewing, 1 38-1 39

viewing, 1 3 , 1 5

Mariner 2 , 2 1 Mariner 4 , 2 2

Nereid, 1 44 , 1 45

Mariner 5 , 2 1

Neutron stars, 1 0

Mariner 1 0, 2 1 , 47

New moon, 72, 73

sizes, 28, 29

Pluto, 1 1 , 1 3, 1 5 , 32, 33, 44, 1 06 , 1 2 8 , 1 35 , 1 38-1 39, 1 4 1 , 1 46-

1 49 , 1 56

Morius, Simon, 1 07

Newton, Sir Isaac, 3 1

Mo.,, 1 5 , 2 2 , 3 2 , 82-89,

North celestial pole, 1 7

observing, 1 3 8- 1 3 9

North Star, 1 6

orbit, 4 , 29

93 observing, 88-89

Nova, 8

oppositions, 88, 89

Nuclear fusion, 1 0, 25, 26,

orbit, 2 9

34, 37

satel l ites, 86-87 size, 2 8 , 29 Mercury, 1 4 , 1 7 , 20, 3 2 ,

44-4 9 , 1 06 , 1 28 , 1 48 observing, 48-49 orbit, 29 properties, 44-49 size, 28, 29 transits, 48 viewing, 48-49 Mesosphere, 60

Oberon, 1 37 , 1 38 Observatories, 5 Ophe l i a , 1 37 Opposition, 1 5 Orbiting Astronomical

20 Outer planets, observing, 1 38- 1 39

Meteor showers, 67

Posiphae, 1 00 , 1 0 1

Metis, 1 00, 1 0 1

Path of totality, 4 1

1 24 Minor planets. See Asteroids

Ptolemy, C l a u d i u s , 7

Quadrature, 1 5 Radiant, 66, 67 Reddening, 1 8 " Red Planet , " 8 2 Refraction, 1 8 Revo l ution, period of, 30

Pandora, 1 1 8 , 1 23 Partial e c l i pse, 4 1 , 78, 80

Mimes, 1 1 8 , 1 1 9- 1 20, 1 23,

Ptolemaic system, 7 Puck, 1 3 7

Observatory, 2 3 O r b i t i n g Solar Observatory,

Meteors, 6 , 66-6 7 , 1 5 1

M i l ky Way gal axy, 1 0 , 25

Prominences, 36, 37

Orbital period, 30

Meteorites, 67, 93

Meteoroids, 66, 1 5 1

size, 28, 29 Prometheus, 1 1 8 , 1 2 3

properties, 82-85

view i n g , 88-89

properties, 1 46- 1 49 Portia, 1 37

Penumbra, 3 9 , 4 1 , 7 8 Period o f revolution, 3 0 Phases, 1 5 , 5 4 , 72-73

Rhea , 1 2 3 Rosa l ind, 1 37 Rotation period, 30

Sate l l i tes, 1 1 , 33 formation, 26 Satu r n , 1 5 , 20, 2 2 , 3 3 , 1 1 4- 1 3 1 , 1 3 3 , 1 50

Phobos, 86, 87 P h oebe, 1 2 2 , 1 2 3, 1 27

observing, 1 3Q- 1 3 1

Mirando, 1 36- 1 37

Photosphere, 36, 40

orbit, 29

Month, 68

P i a z z i , G u i seppe, 90

Moon, 8, 1 2 , 1 6 , 22, 40,

Pickering, W. H . , 1 46

rings, 9, 1 1 8- 1 2 1 , 1 45

Pinhole camera , 38

sate l lites, 1 2 2- 1 29

68-8 1 , 92 , 1 1 3 eclipses. See lunar ecli pses

Pioneer 1 0, 22 P ioneer 1 1 , 22, 1 1 5

formation, 7 1

P ioneer Venus, 2 1

properties, 1 1 4 - 1 1 7

s i z e , 28, 29 viewing, 1 30-1 3 1 Scintillation, 1 8

illusion, 1 9

P laneta r i u m s , 5

Seasons, 64-65

orbit, 72

P l a netary phenomena,

Shepherd satellites, 1 2 1

phases, 72-73

1 4, 1 5 Planets, 1 1 , 1 8

Shooting stars. See Meteors

properties, 68-7 1 viewing, 72-79

Sinope, 1 00 , 1 0 1

atmospheres, 3 2 , 3 3

Sky, motion, 1 6- 1 7

Moons. See Sate l l i tes

beyond P l u t o , 1 56

Solar e c l i pses, 40-4 3 , 7 3 ,

Morning star, 1 4

early theories, 6

Mythology, 4

i nferior, 1 4

80 Solar filter, 38

1 59

Solar Max i m u m Mission, 20 Solar system, 4 , l G- 1 1 , 30, 34, 37, 9 3 exploration, 2o-23 formation, 24-25

TerrestriaHype planets, 3 2

size, 2 8 , 2 9 viewing, 1 38- 1 39

T ; t o n , 3 3 , 1 08 , 1 1 5 , 1 23 ,

Solar wind, 37

1 28- 1 29

Solstice, 64, 65

Uronus (cont . ) :

Tethys, 1 2 3 , 1 2 6 Thebe, 1 00 , 1 0 1 , 1 1 0 Thermosphere, 60 Tides, 8 1

Von Allen belts, 57, 99 Venera, 2 1 Ven us, 1 4, 1 7, 2 1 , 3 2 , so-

55, 1 1 2 , 1 1 3

Spicules, 36, 3 7

T i ta n i a , 1 3 7 , 1 3 8

Stars, 1 0, 1 8 , 34

Tom b o u g h , C l yde, 1 46

observing, 54-55

Total eclipse, 40, 78, 80 Tota lity {of e c l i pse), 4 1 , 79 Triton, 1 38 , 1 44 , 1 45

properties, so-53

Trojan astero i d s , 93, 1 1 1

transits, 55

formation, 25 Stratosphere, 60 Summer solstice, 64, 65 Sun, 8 , 1 2 , 20, 29, 32, 344 3 , 8 1 , 1 1 1 , 1 56 age, 34 formation, 26-27 observing, 38

ďż˝

Troposphere, 6 0 Twi n k l i n g , 1 8 Tych o . S e e B r o h e , Tycho Tyc h o n i c syste m , 7

viewi

Sunspots,

, 38 de, 39 , 38-39

Superior conjunction, 1 4 Superior planets, 1 5

viewing, 54-55 Vernal equinox, 64, 65 Vesta, 92, 94 Voyoger, 20, 22, 23, 1 02 , 1 04 , 1 1 5 , 1 1 9, 1 24 ,

pro erties, 35 Sunspot

orbit, 29 size, 28, 29

U mbro, 39, 4 1 , 78, 79 Umbriel, 1 37 U n iverse, 6, 8, 1 0, 25, 1 56 Uranus, 1 5, 23, 33, 1 3 21 3 9 , 1 43

1 25 , 1 26, 1 29, 1 33 , 1 36 , 1 4 1 , 1 42 Vulcan, 45 White dwarf star, 34 Winter solstice, 64, 65

Supernova, 2 4 , 25

observing, 1 38- 1 39

Synod ic period, 1 5

orbit, 29

Telescope, 5 , 9 , 1 38

rings, 1 33 , 1 36

Zenith, 1 7

Te1esto, 1 23 , 1 26

sate l l ites, 1 36- 1 37

Zodiac, 1 2- 1 3

properties, 1 32-1 3 5

PH OTO C R E DITS

Year, 30

We o r e i ndebted t o t h e fo l lowing i n s t i t u t i o n s and

i n d i v i d u a l s for the photog r a phs used i n t h i s boo k . E u ropean Space Agency ( E SA ) : 2 3 botto m , 1 50, 1 5 5 ; l i c k Observatory Photog r a p h s : 74, 7 7 ; N a t i o n a l Aeronautics a n d S p a c e A d m i n i strat i o n ( N AS A ) : 20, 2 1 , 2 2 , 23 top, 3 4 , 3 5 , 40, 46, 4 7 , 5 2 , 56, 6 8 , 70 left, 7 1 , 84, 8 5 , 8 6 , 9 8 , 9 9 ,

1 0 1 , 1 0 2 , 1 03 , 1 04, 1 05 , 1 06 , 1 07 , 1 08 , 1 09 , 1 1 1 , 1 1 6 , 1 1 7 , 1 1 9 , 1 20, 1 2 1 , 1 2 3 , 1 2 4 , 1 2 5 , 1 2 6 , 1 2 7 , 1 2 8 , 1 34 , 1 3 6 , 1 4 2 , 1 44 , 1 45 ; The O bservato r i e s of the C a r n e g i e Institution of Wa s h i ng t o n : 1 9 , 70 r i g h t ,

1 5 1 ; P a l o m a r Observatory P h otog ra p h : 1 4 6 ; U n i ted States Geo l o g i c a l Service ( U S G S ) : 6 3 ; J e r o m e Wyckoff: 79

1 60

PLANETS A GOLDEN GUIDE® MARK R. CHARTRAND, Ph .D., is an astronomer and a science writer and lecturer. He wrote the Golden Field Guide Skygu ide, and revised and updated the Golden Guide Stars. He was for many years Chairman of New York City's Hayden Planetar ium, and has been Executive Director of the National Space Society. In addition to teaching at several uni versities, he has been widely published in popular science magazines, and appears frequently on radio and television. RON MILLER graduated from the Columbus Col lege of Art and Design and has spent 15 of the 20 years since then specializing in scientific and astro nomical subjects . His work has appeared in numer ous publications worldwide, in motion pictures, and in a series of best-selling books he has co-authored. His original space art has been exhibited internation ally and hangs in many private and public collec tions . He is considered one of the most prolific and influential space artists now living. He lives with his wife, dau ghter, and six cats in Frede ricksburg, Virginia . GOLDEN PRESS • NEW YORK

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A GOLDEN GU I DE速