Editing Hour angle (section)

{{Short description|Coordinates used in the equatorial coordinate system}}
{{More citations needed|date=April 2018}}
[[File:HourAngle_Observer_en.png|thumb|right|300px|The hour angle is indicated by an orange arrow on the [[celestial equator]] plane. The arrow ends at the [[hour circle]] of an orange dot indicating the [[apparent place]] of an [[astronomical object]] on the [[celestial sphere]].]]

In [[astronomy]] and [[celestial navigation]], the '''hour angle''' is the [[dihedral angle]] between the ''[[meridian plane]]'' (containing [[axial tilt|Earth's axis]] and the [[zenith]]) and the ''[[hour circle]]'' (containing Earth's axis and a given point of interest).<ref>{{cite book |author=U.S. Naval Observatory Nautical Almanac Office |editor=P. Kenneth Seidelmann |title=Explanatory Supplement to the Astronomical Almanac |publisher=University Science Books |location=[[Mill Valley, California|Mill Valley, CA]] |date=1992 |page=729 |isbn=0-935702-68-7}}</ref> 

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<sup>h</sup> = 360° exactly.
In [[celestial navigation]], the convention is to measure in degrees westward from the [[prime meridian]] ('''Greenwich hour angle''', '''GHA'''), from the local meridian ('''local hour angle''', '''LHA''') or from the [[First Point of Aries|first point of Aries]] ('''sidereal hour angle''', '''SHA''').

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]].<ref>''Explanatory Supplement'' (1992), p. 724.</ref>