nometry, marked off the earth's surface into 360 parts. These are the "degrees" of modern geography . Ptolemy , fol lowing this plan of Hipparchus, further s ubdivided each of these degrees into partes minutae primae ("m inutes") and partes minutae secundae ("second s"). Each meridianal line of longitude was spaced 15 degrees apart, one for each of 24 hours (one full revolution of the earth), making 360 degrees. Initial attempts of determining longitude followed the same approach as for latitude, namely, using astronomical observations. Hipparchus was the first to suggest using ecl ipses of the moon for finding longitude. This was to be accomplished by a comparison of time at two places during an ecl ipse. The problem with this method was the infrequency of eclipse observations and inaccurate pre-
Th e wreck of Sir Clowdisley Shovel's fleet, 1707. This disaster- lour ships were lost, with nearly 2000 men- was a profound shock to the British public. Th ough not actually caused by th e lack of a method ofjlnding longitude, the magnitude of the disaster lent impetus to th e search for steps that might make navigation safer.
dictions of the time of eclipse. Columbus tried thi s method during hi s 1494 voyage, but was unable to achieve an accuracy any closer than 18 degrees. William Baffin, most noted for his exploration of far northern waters, attempted to fix his positions of longitude by meridi an passage of the moon . That he was unable to acco mpli sh thi s is not surprising , for not unti I the later part of the 18th century were lunar tables published of sufficient exactness for reliable use. Another astronomical method utilized the satellites of Jupiter. Galileo, first to observe the four principal satellites ( 16 in
The Lunar Distance Method Th e theory behind many of the solutions to the longitude problem was to compare the local time of the seaman wilh that of a standard known place. The time difference between the two places could then be recalculated into a longitudinal difference since 4 minules in time would mean 1째 difference in longitude, 1 hour would equal 15째, 4 hours 60째, and so on . The seaman could find his local time by the Sun or the stars: but , what of a standard time? A clock had not yet been built that could carry standard time at sea. This is where the lunar distance method came in . Th e moon moves fairly quickly against the background of the stars , rather like a hand moving over the dial of a clock. The seaman could measure the angular distance between the moon and a given star and by looking up this distan ce in an almanac he would get the slandard time that the Moon and the star would have this separation. The seaman could then calculate his longitude, by comparing that standard time with local observed time. Th e Nautical Almanac,firsl published in 1767, contained the necessary information. However, the observation required extreme accuracy and an elaborate calculation beyond 1he abilities of most seamen.
SEA HISTORY 66, SUMMER 1993
total) of Jupiter, quickly realized that using occultations and eclipses of these satellites could be a means of finding longitude at sea. This method had the advantage over using lun ar ecl ipses by the greater numberof celestial bodies and the frequency of occurrences; but similar to the moon , as a method it suffered from lack of an accurate publi shed almanac and difficulties of observation while at sea. Finding long itude by occultations of stars or planets by the Moon' s di sc was employed. Thi s required observations made when the declination and right ascension of the moon and celestia l body are identical. The comp lex ity of the mathematica l calcu lations was far beyond that of most navi gators. Another lunar method for findin g longitude involved finding the local time of the Moon 's transit and compari ng it with the time of transit at a prime meridian . Without a natural point of origin for measurement of meridians of longitude, such as the equator is to parallels of latitude, there was no predetermined position for a prime meridian. Earliest maps used Alexandria, that great seat of ancient learning, as the prime meridian, although Rhodes and other important geographic points such as Carthage, the Pillars of Hercules and Rome, were also used. Arabian geographers naturally used a prime meridi an in keeping with their known world. They selected a line midway between the farthest east and farthest west, through a mythical city called Arin situated on the equator. Thi s meridian , subsequently assumed to be I 0째 East of Baghdad , appears to be of Hindu origin. The voyages of Columbus brought many new and exciting discoveries to the world . Reflecting this new knowledge, other prime meridians began to be 19