Relation With The Right Ascension
The local hour angle (LHA) of an object in the observer's sky can be calculated
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where LHAobject is the local hour angle of the object, LST is the local sidereal time, is the object's right ascension, GST is Greenwich sidereal time and is the observer's longitude (positive west from the prime meridian).
Thus, the object's hour angle indicates how much sidereal time has passed since the object was on the local meridian. It is also the angular distance between the object and the meridian, measured in sidereal hours (1h = 15°). For example, if an object has an hour angle of 2.5h, it crossed the local meridian 2.5 sidereal hours ago (i.e., hours measured using sidereal time), and is currently 37.5° west of the meridian. Negative hour angles indicate the time until the next transit across the local meridian. An hour angle of zero means the object is currently on the local meridian.
Read more about this topic: Hour Angle
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