Hour angle

Hour angle is the angular distance of a celestial object west of the local meridian, measured along the celestial equator or a parallel of declination. It increases as Earth’s rotation carries the object westward across the sky.

Local meridian: The great circle passing through an observer’s zenith and the north and south points on the horizon. Hour angle is measured westward from this observer-dependent reference circle.
Altazimuth coordinate system: A horizon-based system locating celestial objects by altitude and azimuth. Hour angle and declination can be converted into an object’s local altitude and azimuth.
Celestial sphere: An imaginary sphere centered on an observer or Earth, onto which celestial positions are projected. Hour angle is defined as a position on this reference sphere.
Longitude: An angular coordinate specifying east–west position on Earth relative to a reference meridian. Like hour angle, longitude is angular, but it locates places on Earth rather than objects relative to a local meridian.
Sidereal time: Time measured by Earth’s rotation relative to the distant stars, rather than the Sun. Local sidereal time minus right ascension gives hour angle.
Telescope mount: A mechanical support that points a telescope toward selected directions in the sky. Hour angle helps equatorial mounts track objects as Earth rotates.
Declination: An equatorial coordinate measuring angular distance north or south of the celestial equator. Hour angle is measured along a parallel of the object’s declination.
Right ascension: An equatorial coordinate measuring celestial longitude eastward from the vernal equinox. Right ascension is fixed to the celestial reference frame, while hour angle changes with local time.
Equatorial coordinate system: A celestial coordinate system that locates objects using right ascension and declination. Hour angle uses the same equatorial geometry but takes the local meridian as its origin.
Meridian circle: A transit instrument designed to measure celestial positions as objects cross the meridian. Its observations occur at the hour angle’s zero point.

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