Hour angle

Hour angle is the angular distance of a body’s meridian measured westward from a reference meridian, expressed in degrees (0 to 360) or in time at 15 degrees per hour. Measured from Greenwich it is the Greenwich Hour Angle; from the observer’s meridian it is the local hour angle; from the first point of Aries to a star it is the sidereal hour angle. Hour angle increases as the celestial sphere rotates westward, gaining 15 degrees per hour of apparent time. In the astronomical triangle the local hour angle is the angle at the pole, one of the inputs that yields the computed altitude and azimuth.