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In astronomy and celestial navigation, the hour angle is the angle between two planes: one containing Earth's axis and the zenith , and the other containing Earth's axis and a given point of interest.
It may be given in degrees, time, or rotations depending on the application.The angle may be expressed as negative east of the meridian plane and positive west of the meridian plane, or as positive westward from 0° to 360°. The angle may be measured in degrees or in time, with 24 = 360° exactly.In celestial navigation, the convention is to measure in degrees westward from the prime meridian , from the local meridian or from the first point of Aries.
The hour angle is paired with the declination to fully specify the location of a point on the celestial sphere in the equatorial coordinate system.