Astronomical coordinate systems

Reference frameworks that specify celestial positions using angular coordinates tied to the sky, Earth, or the solar system. Each system defines an origin, a reference plane, and directions for measuring position.

Celestial sphere: An imaginary sphere centered on an observer or Earth, onto which celestial directions are projected. It provides the geometric surface on which angular sky coordinates are conventionally described.
Equatorial coordinate system: A celestial coordinate system using right ascension and declination relative to Earth's equator and equinox. It gives sky positions in axes tied to Earth's equatorial plane rather than a local horizon.
Right ascension: The equatorial angular coordinate measured eastward along the celestial equator from an equinox. It is the longitude-like coordinate used to locate objects in equatorial catalogs.
Astrometry: The measurement of celestial positions, distances, and motions. It produces positions whose interpretation requires an explicitly defined coordinate framework.
Coordinate system: A scheme for assigning numbers to locations relative to a defined origin and set of reference directions. Astronomical systems apply this general scheme to directions and positions in space.
Horizontal coordinate system: A local sky coordinate system using altitude and azimuth relative to an observer's horizon. Its coordinates directly describe where an object appears above the horizon at a site.
Declination: The equatorial angular coordinate measuring a celestial object's north–south distance from the celestial equator. Together with right ascension, it specifies an object's position on the celestial sphere.
Celestial mechanics: The study of the motions of celestial bodies under gravity and related forces. Its calculations distinguish coordinate choices from the physical trajectories being modeled.
Reference frame: A coordinate framework with specified origins, axes, and conventions for describing positions and motion. Astronomical coordinates depend on the frame whose axes and origin anchor their values.
Ecliptic coordinate system: A celestial coordinate system using ecliptic longitude and latitude relative to the plane of Earth's orbit. It aligns its reference plane with the apparent annual path of the Sun.

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