Sidereal time

A timekeeping system based on Earth’s rotation relative to distant stars. It locates celestial objects by relating their right ascension to the local meridian.

Hour angle: The angular distance, measured westward along the celestial equator, between an object’s meridian and the local meridian. Sidereal time gives the hour angle of an object when its right ascension is known.
Telescope mount: A mechanical support that points a telescope and controls its orientation. Equatorial mounts can track stars by rotating at the sidereal rate.
Sidereal day: The interval Earth takes to rotate once relative to distant stars, about 23 hours 56 minutes. Sidereal time advances through one full cycle during a sidereal day.
Greenwich mean sidereal time: Mean sidereal time measured at the Greenwich meridian, with nutation effects averaged out. It is the standard global sidereal-time reference before conversion to a local meridian.
Hipparchus: An ancient Greek astronomer who compiled star positions and developed quantitative methods in astronomy. His star catalog exemplifies the stellar references that make sidereal reckoning possible.
Right ascension: A celestial coordinate measuring angular distance eastward along the celestial equator from the vernal equinox. Subtracting an object’s right ascension from local sidereal time yields its hour angle.
Equatorial coordinate system: A celestial coordinate framework using right ascension and declination. Sidereal time supplies the local rotation coordinate needed to use right ascension observationally.
Solar time: Timekeeping based on the apparent motion of the Sun across the sky. Its reference is the Sun, unlike sidereal time’s distant-star reference.
Local sidereal time: Sidereal time measured at a particular observer’s meridian. This is the form directly used to calculate local hour angles.
Precession: The slow change in the orientation of Earth’s rotational axis, caused mainly by lunar and solar gravity. Precession shifts the equinox reference used to define right ascension and sidereal time.

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