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In astronomy and celestial navigation, the hour angle is one of the coordinates used in the equatorial coordinate system to give the direction of a point on the celestial sphere. The hour angle of a point is the angle between two planes: one containing the Earth's axis and the zenith (the meridian plane), and the other containing the Earth's axis and the given point (the hour circle passing through the point). 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 24h = 360° exactly. In astronomy, hour angle is defined as the angular distance on the celestial sphere measured westward along the celestial equator from the meridian to the hour circle passing through a point.〔 〕 It may be given in degrees, time, or rotations depending on the application. In celestial navigation, the convention is to measure in degrees westward from the prime meridian (Greenwich hour angle, GHA), the local meridian (local hour angle, LHA) or the first point of Aries (sidereal hour angle, SHA). The hour angle is paired with the declination to fully specify the direction of a point on the celestial sphere in the equatorial coordinate system.〔 ''Explanatory Supplement'' (1992), p. 724.〕 ==Relation with the right ascension== The local hour angle (LHA) of an object in the observer's sky is : where LHAobject is the local hour angle of the object, LST is the local sidereal time, is the observer's longitude (positive west from the prime meridian).〔 〕 These angles can be measured in time (24 hours to a circle) or in degrees (360 degrees to a circle)— one or the other, not both. Negative hour angles indicate the time until the next transit across the meridian; an hour angle of zero means the object is on the meridian. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「hour angle」の詳細全文を読む スポンサード リンク
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