Celestial Coordinates

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 23⁰.5 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°.