Longitude
Longitude is distance east and west of the prime 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 reference meridian.[1] Ergo, the longitude for any place on the globe is measured east or west from this line. Longitude can vary from 0 degrees along the prime meridian to 180 degrees, halfway around the globe.
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.