Hour angle

In astronomy, the hour angle is one of the coordinates used in the equatorial coordinate system for describing the position of a point on the celestial sphere. The hour angle of a point is the angle between the half plane determined by the Earth axis and the zenith (half of the meridian plane) and the half plane determined by the Earth axis and the given point. The angle is taken with minus sign if the point is eastward of the meridian plane and with the plus sign if the point is westward of the meridian plane.

The hour angle is usually expressed in time units, with 24 hours corresponding to 360 degrees.

The hour angle must be paired with the declination in order to fully specify the position of a point on the celestial sphere as seen by the observer at a given time.

Relation with the right ascension

The hour angle (HA) of an object is equal to the difference between the current local sidereal time (LST) and the right ascension ($alpha$) of that object:

HA_object = LST - $alpha$_object

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 hours (1 hour = 15 degrees). For example, if an object has an hour angle of 2.5 hours, it transited across the local meridian 2.5 sidereal hours ago (i.e., hours measured using sidereal time), and is currently 37.5 degrees west of the meridian. Negative hour angles indicate the time until the next transit across the local meridian. Of course, an hour angle of zero means the object is currently on the local meridian.