Astronomy Answers
From the Astronomical Dictionary

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the longitude

The longitude is a coordinate that is used to measure lccations on a sphere or directions in the sky. The longitude of a location is equal to the distance of the meridian of that location from the prime meridian of the coordinate system, measured in degrees along the equator.

Places on the prime meridian have a longitude equal to zero. On almost all celestial bodies in the Solar System and also in the sky, the longitude is measured from 0° to 360° such that it increases towards the west. On Earth and the Moon, the longitude is measured eastward and westward to 180° from the prime meridian.

In the sky, longitude is used in the ecliptic and galactic coordinate systems. The corresponding second coordinate is the latitude.