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# Coordinating the Heavens Help (page 2)

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By McGraw-Hill Professional
Updated on Sep 15, 2011

## Latitude

In geography classes you were taught that latitude can range from 90 degrees south to 90 degrees north. The north geographic pole is at 90 degrees north, and the south geographic pole is at 90 degrees south. Both the poles lie on the Earth’s axis. The equator is halfway between the poles and is assigned 0 degrees latitude. The northern hemisphere contains all the north-latitude circles, and the southern hemisphere contains all the south-latitude circles.

As the latitude increases toward the north or south, the circumferences of the latitude circles get smaller and smaller. Earth is about 40,000 kilometers (25,000 miles) in circumference, so the equator measures about 40,000 kilometers around. The 45-degree-latitude circle measures about 28,000 kilometers (17,700 miles) in circumference. The 60-degree-latitude circle is half the size of the equator, or 20,000 kilometers (12,500 miles) around. The 90-degree-latitude “circles” are points with zero circumference. Every latitude circle lies in a geometric plane that slices through Earth. All these planes are parallel; this is why latitude circles are called parallels . Every parallel, except for the poles, consists of infinitely many points, all of which lie on a circle and all of which have the same latitude.

There is no such thing as a latitude coordinate greater than 90 degrees, either north or south. If there were such points, the result would be a redundant set of coordinates. The circle representing “100 degrees north latitude” would correspond to the 80-degree north-latitude circle, and the circle representing “120 degrees south latitude” would correspond to the 60-degree south-latitude circle. This would be confusing at best because every point on Earth’s surface could be assigned more than one latitude coordinate. At worst, navigators could end up plotting courses the wrong way around the world; people might mistakenly call 3:00 the “wee hours of the morning”!

An ideal coordinate system is such that there is a one-to-one correspondence between the defined points and the coordinate numbers. Every point on Earth should have one, and only one, ordered pair of latitude-longitude numbers. And every ordered pair of latitude/longitude numbers, within the accepted range of values, should correspond to one and only one point on the surface of Earth. Mathematicians are fond of this sort of neatness and, with the exception of paradox lovers, dislike redundancy and confusion.

Latitude coordinates often are designated by abbreviations. Forty-five degrees north latitude, for example, is written “45 deg N lat” or “45°N.” Sixty-three degrees south latitude is written as “63 deg S lat” or “63°S.” Minutes of arc are abbreviated “min” or symbolized by a prime sign (′). Seconds of arc are abbreviated “sec” or symbolized by a double prime sign (″). So you might see 33 degrees, 12 minutes, 48 seconds north latitude denoted as “33 deg 12 min 48 sec N lat” or as “33°12′48″N.”

As an exercise, try locating the above-described latitude circles on a globe. Then find the town where you live and figure out your approximate latitude. Compare this with other towns around the world. You might be surprised at what you find when you do this. The French Riviera, for example, lies at about the same latitude as Portland, Maine.

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