Planet Earth: Maps, Longitude, Latitude, Equators Part 1

About the planet Earth, how cartography put a globe on a flat plane, the use of longitude, latitude, equators, and parallels.


Nearly 500 years ago, when it was generally agreed that the earth was a sphere, mapmakers established 2 essentials. One was a standard system of meridians of longitude (from the Latin longus, "long") and parallels of latitude (from latus, "wide"), which would make it possible to pinpoint any location on the globe. The 2nd essential was a method of transferring details from a sphere to a flat map--a procedure that modern mapmakers call a "projection." Since the earth is round, it has no corners from which measurements might be made. Therefore, 2 definite measuring points are used--the North Pole and South Pole, which are the ends of the imaginary axis on which earth turns.

In mapping the earth, cartographers lay out the equator (the center line around the earth) and parallels of latitude like slices of the sphere. Each parallel is a huge circle divided into 360 deg. Each degree is equivalent to approximately 60 nautical miles starting at the equator (a nautical mi. measures 6,080.2', a statute mile 5,280'). Degrees are also divided into 60 minutes, each minute equaling one nautical mi. The minute is divided into 60 seconds, making each second equal to about 101'.

The equator is at 0 deg latitude. Northward, the parallels from the equator are numbered from 1 deg to 90 deg (1/4 of a circle) and are called "north latitude." Parallels to the south are also numbered from 1 deg to 90 deg and are referred to as "south latitude."

Parallels are not enough to pinpoint a location on the earth, so mapmakers place curved meridians (also measured in degrees, minutes, and seconds) of longitude that converge at the North and South Poles. Since the meridians cross the parallels, given the latitude and longitude of any place on earth, one can locate it exactly.

While the parallels of latitude begin with 0 deg at the equator, the 0 deg meridian of longitude used since 1884 by some 25 countries is the prime meridian on which the Greenwich Observatory is situated in London, England. (Previously, many countries used the meridians passing through their capital cities as their prime meridian.) Meridians are measured up to 180 deg east of Greenwich and 180 deg west of Greenwich. Longitude and time are inseparable. One hour is equivalent to 15 deg of longitude. One degree is equal to 4 minutes time-wise.

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