The angle between the observer’s meridian and the hour circle of a celestial object, measured clockwise along the celestial equator. The observer’s meridian (the great circle joining the celestial pole to the zenith) is therefore the meridian of zero hour angle. The hour angle of any object increases as the Earth rotates, and it can be expressed as a time by equating 360° with 24 hours. For example, an hour after an object crosses the meridian its hour angle is 1 h, and so on. It is sometimes used as a coordinate in place of right ascension. The sum of the hour angle and right ascension is equal to the observer’s sidereal time. See also greenwich hour angle.