Hour Angle

Hour angle is a way of measuring how far a celestial object has traveled across the sky since it crossed an observer's local meridian, the imaginary north-south line running directly overhead from due north to due south. It is expressed in units of time — hours, minutes, and seconds — rather than degrees, reflecting the fact that Earth's rotation carries objects across the sky at a steady, clock-like rate of roughly 15 degrees per hour. An hour angle of zero means an object is currently crossing the meridian, at its highest point in the sky for that location; a positive hour angle means the object has already passed that point and is now descending toward the western horizon, while a negative hour angle means it is still rising toward the meridian in the east. Hour angle, combined with an object's declination and an observer's latitude, allows astronomers to calculate exactly where to point a telescope at any given moment, which is why many equatorial telescope mounts have a built-in hour-angle setting circle. It is also central to how sidereal time works, since local sidereal time is defined as the hour angle of the First Point of Aries, the reference point astronomers use to define zero right ascension.