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n the 21st century, most people wo uld agree that we live The earth's North and South Poles are close ro, bur no t in an age of technology, bur this is actually nothing new. perfectly aligned with, its m agnetic poles. The difference Technology has always been around- it's the applica- between them is expressed as an angle called declinatio n of science and invention to deal with practical real-life tion (o n nautical charts, this is call ed variation). Why pro blems. Early humans who used sto nes fo r cutting were are there two N orth Poles and two South Poles? The using techn ology, and by the 15th geograph ic poles, called "True North" and "True century, technology solved the South," are aligned problem of how people on with the earth's roshi ps and boars could fi gure out what directational axis, the tion they were sailing imagi nary line in and, perhaps even running through the earth around more importantly, what d irection they which it spins. To needed ro go ro get make things even more back home. Before complicated, the earth's that time, sailors rarely magnetic field shifts a littl e bi t ventured our of sight of every year, making it impossible land and instead relied on ro mark its poles permanently on visible landmarks ro get from place earth's magnetic field maps, globes, and charts. Cartographers (mapmakers) take care of ro place. This limited traders and navies ro routes close ro shore, bur the invention of the compass changed all that. No this pro blem by printing a "Compass Rose" on charts and one is exactly sure who invented it (probably the Chinese, maps, which shows the difference between true and magnetic bur the Greeks also undersrood about magnetism in ancient north and notes the change in declination each year. times), bur the magnetic compass as a shipboard navigatio nal The magnetic compass has come a long way since the instrument fi rst shows up in rhe hisrorical record in rhe early sailing days of Zheng H e, and some people might even argue 1400s, when the Chinese explorer Zheng H e (1371-1435) that we don't need them anymore now that we have satelused them ro lead a fleet of shi ps on seven major ocean voy- lites in space sending sig nals ro GPS units. G PS is a great ages thro ughout Asia, eastern Africa, and the Mi dd le East. advance in navigational technology, bur 1 h e magnetic compass contains a magnetic element only a foo l wo uld go ro sea with the .,. , ',~ ...i....L,,.,.f,, ,, . h b . ·11 • ,..~·""" t ......,,.,,""·"··.. that aligns itself with the earth's magnetic field . Think of the expectation t at atten es w1 ~-s:"''~'>"" '4,,,,/" ea rth as a big magnet. It has two magnetic poles, oriented never run down, signals will ,•..,/'''' "''\r,"' \,,.~ north and south, and close ro the geographi c No rth and always make it ro their GPS #../ \,,., So uth Poles. W hen a magnetic element (such as the mineral un it, and their electronic ~/ lodesrone or a fe rro us metal that has been magnetized) is al- instruments will always R j •..-··""'"", \_. lowed ro float freely, it will effectively point north and south, work perfectly. A mag+ ---= •thus showing you which direction you are goi ng in relation ro neric compass is si mple ~l -~ t~ that axis. and relatively inexpen- .~ ·-::;,., \ , ?--::: sive, never needs batteries, . \. ''·,; v·' ,L;: .. :r.; 1,; \ ,, an d easy (an cl fiun!) ro use. ~":>,,,, "i.:..' 1•1.,,f,,, ,,,\~;.,.>:~, ,/·.: "6 ;-,,,,, ~~...,....ii~·r"''"'r... · ,,. . . . , Yo u can even make one your.:·..... ~\· ,,,•"".!> lf . h . £ . l J';;•11,1,,,, \ 1\11\''''~ se wit JUSt a ew matena s... ,Z"""·:~·"'"';r'•"",~"''" l.. I

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Compass Rose. The outer circle points

to True North; the inner circle shows degrees in magnetic. At the center, the cartographer has noted the difference between them and how much it changes annually. (left) Before US Navy officers learn how to use high-tech electronic navigation systems, they must first learn how to use a compass to navigate at sea. In this photo, Ensign Jason Waddell takes a compass bearing aboard the amphibious assault ship USS Makin Island off the coast ofSouthern California in September, 2 011 .

SE A HISTORY 137, WINTER 20 11 - 12

Sea History 137 - Winter 2011-2012  

10 The War of 1812: Year Three-1814, by William H. White • 18 Measure of the Earth: Navigation, Science, and the War of Jenkins's Ear, by L...

Sea History 137 - Winter 2011-2012  

10 The War of 1812: Year Three-1814, by William H. White • 18 Measure of the Earth: Navigation, Science, and the War of Jenkins's Ear, by L...

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