Right ascension

Right ascension is the angular coordinate locating a celestial object eastward along the celestial equator from the vernal equinox. It is usually expressed in hours, minutes, and seconds.

Declination: Declination is the angular coordinate measuring a celestial object's position north or south of the celestial equator. It supplies the north–south coordinate paired with right ascension.
Sidereal time: Sidereal time is a timekeeping system based on Earth's rotation relative to the stars, measured by the hour angle of the vernal equinox. Local sidereal time converts an object's right ascension into its hour angle.
Astronomical catalog: An astronomical catalog is a systematic list of celestial objects with identifying data and measured properties. Catalogs commonly record right ascension alongside declination to specify positions.
Ecliptic coordinate system: The ecliptic coordinate system locates celestial objects by ecliptic longitude and latitude, measured relative to the Sun's apparent path. It replaces the celestial equator with the tilted ecliptic as its reference plane.
Celestial equator: The celestial equator is the projection of Earth's equator onto the celestial sphere. Right ascension is measured eastward along this reference circle.
Sidereal day: A sidereal day is the time Earth takes to rotate once relative to distant stars, about 23 hours 56 minutes. Its duration explains why the stars’ right ascensions cross the meridian earlier each solar day.
Spherical astronomy: Spherical astronomy describes celestial positions and motions using geometry on the celestial sphere. Right ascension is a central coordinate in its positional calculations.
Galactic coordinate system: The galactic coordinate system locates celestial objects by longitude and latitude relative to the plane of the Milky Way. Its origin and reference plane are galactic rather than equatorial.
Celestial sphere: The celestial sphere is an imaginary sphere onto which celestial objects are projected for describing their apparent positions. It provides the model on which right ascension and declination form a coordinate grid.
Equatorial coordinate system: The equatorial coordinate system locates celestial objects using right ascension and declination. Right ascension is one of this system’s two coordinates.

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