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# Coordinating the Heavens Help (page 3)

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By — McGraw-Hill Professional
Updated on Sep 15, 2011

## Longitude

Longitude coordinates can range from 180 degrees west, down through zero, and then back up to 180 degrees east. The zero-degree longitude line, also called the prime meridian , passes through Greenwich, England, which is near London. (Centuries ago, when geographers, lexicographers, astronomers, priests, and the other “powers that were” decided on the town through which the prime meridian should pass, they almost chose Paris, France.) The prime meridian is also known as the Greenwich meridian . All the other longitude coordinates are measured with respect to the prime meridian. Every half-circle representing a line of longitude is the same length, namely, half the circumference of Earth, or about 20,000 kilometers (12,500 miles), running from pole to pole. The eastern hemisphere contains all the east-longitude half circles, and the western hemisphere contains all the west-longitude half circles.

There is no such thing as a longitude coordinate greater than 180 degrees, either east or west. The reason for this is the same as the reason there are no latitude coordinates larger than 90 degrees. If there were such points, the result would be a redundant set of coordinates. For example, “200 degrees west longitude” would be the same as 160 degrees east longitude, and “270 degrees east longitude” would be the same as 90 degrees west longitude. One longitude coordinate for any point is enough; more than one is too many. The 180-degree west longitude arc, which might also be called the 180-degree east-longitude arc, is simply called “180 degrees longitude.” A crooked line, corresponding approximately to 180 degrees longitude, is designated as the divider between dates on the calendar. This so-called International Date Line meanders through the western Pacific Ocean, avoiding major population centers.

Longitude coordinates, like their latitude counterparts, can be abbreviated. One hundred degrees west longitude, for example, is written “100 deg W long” or “100°W.” Fifteen degrees east longitude is written “15 deg E long” or “15°E.” Minutes and seconds of arc are used for greater precision; you might see a place at 103 degrees, 33 minutes, 7 seconds west longitude described as being at “103 deg 33 min 7 sec W long” or “103°33′07′″W.”

Find the aforementioned longitude half circles on a globe. Then find the town where you live, and figure out your longitude. Compare this with other towns around the world. As with latitude, you might be in for a shock. For example, if you live in Chicago, Illinois, you are further west in longitude than every spot in the whole continent of South America.

Practice problems of this concept can be found at: Coordinating the Heavens Practice Problems

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