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From Citizendium, the Citizens' Compendium

Longitude is a measure of angular distance east and west of a reference meridian, measured in degrees. Because all meridians are identical, the choice of a zero line is arbitrary. The meridian that passes through the Royal Observatory at Greenwich, England, is universally accepted as the prime meridian for Earth.^[1] Conventionally, the longitude for any place on the globe is measured east or west from the prime meridian. Longitude can vary from 0 degrees along the prime meridian to 180 degrees, halfway around the globe. In some uses, longitude is measured as positive angles east of the prime meridian and negative angles west of the prime meridian.

The length of a degree longitude depends on where the measurement is taken. At the equator, which is a great circle, a degree of east–west distance is equal to approximately 111 kilometers (69 miles). This number is found by dividing Earth's circumference (40,075 km or 24,900 mi) by 360.^[2] However, with an increase in latitude, the parallels become smaller, and the length of a degree longitude becomes smaller.

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