Knowra Equatorial coordinate system Equatorial coordinate system A celestial coordinate system that locates objects by declination and right ascension relative to Earth’s equator and celestial poles. Its coordinates are independent of an observer’s local horizon.
Declination : The angular distance of a celestial object north or south of the celestial equator. It gives the north–south coordinate in the equatorial system.
Epoch (astronomy) : A specified date used as a reference for astronomical coordinates or orbital elements. A coordinate listing’s epoch indicates the orientation of its reference frame.
Star catalog : A structured listing of stars with measured properties such as position, brightness, and motion. Catalogs commonly report stellar positions as right ascension and declination.
Horizontal coordinate system : A local sky-coordinate system that locates objects by altitude and azimuth relative to an observer’s horizon. Unlike equatorial coordinates, its values depend on location and observation time.
Right ascension : The angular coordinate measured eastward along the celestial equator from the vernal equinox. It gives the east–west coordinate, paired with declination.
Precession : The slow change in the direction of Earth’s rotation axis, mainly caused by lunar and solar gravity. It gradually shifts the celestial equator and equinox, changing equatorial coordinates over time.
Messier object : One of 110 deep-sky objects cataloged by Charles Messier, including nebulae, clusters, and galaxies. Messier entries are commonly identified by their equatorial coordinates.
Ecliptic coordinate system : A celestial coordinate system that uses the ecliptic as its fundamental plane. It is convenient for solar-system motion, while the equatorial system uses Earth’s equator.
Celestial equator : The projection of Earth’s equator onto the celestial sphere. It is the zero line from which declination is measured.
Proper motion : The apparent angular motion of a star across the sky, excluding effects from Earth’s orbit and precession. A star’s position can change even after coordinates are corrected for frame precession.
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