# Celestial Coordinates - Horizon Coordinates, Celestial Latitude B, Galactic Longitude - Equatorial coordinates, Right ascension a, Declination d, Hour angle, Ecliptic coordinate, Celestial longitude l

### circle object arc distance

Celestial coordinates locate objects on the sky, which is considered to be an infinitely large (celestial) **sphere**. The four conventional celestial coordinate systems are defined.

These are based on the **earth's rotation**, which produces an apparent westward **rotation** of the celestial sphere around the NCP and SCP.

Measured eastward along the celestial equator from the vernal **equinox** to where an object's hour circle meets the celestial equator, usually measured full circle in **time** units from 0^{h} to 24^{h}.

An object's arc distance along its hour circle from the celestial equator, positive north of the equator, negative south of it. *d*=+90° for the NCP, and *d*=+90° for the SCP.

An object's hour angle t is the arc distance westward along the celestial equator from its intersection with the celestial meridian above the horizon to where the celestial equator meets the object's hour circle; its value increases with time from 0^{h }to 24^{h}.

Right ascension and declination on the celestial sphere are analogous to geographic longitude and latitude, respectively, on **Earth**, but their measurement differs somewhat.

The ecliptic is the basic circle and the vernal equinox is the **zero** point for these coordinates. The north (NEP) and south (SEP) ecliptic poles are 23^{0}.5 from the NCP and SCP, respectively, and are everywhere 90° from the ecliptic.

The arc distance eastward along the ecliptic from the vernal equinox to where an object's secondary to the ecliptic meets the ecliptic; it is expressed in arc units from 0° to 360°.

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