declination

Declination, one of the coordinates of the equatorial coordinate system, is the celestial sphere's analog to latitude. It is a point in the sky's angular distance from the celestial equator, the plane through the Earth's equator. It and right ascension (the analog of longitude) together specify a direction, i.e., a point in the celestial sphere.

Declination is measured in degrees within the range of +90° to -90°, with +90° corresponding to north along the direction of the Earth's axis. The North star is approximately along the line of this axis and has a declination just above +89°. The celestial equator is 0° declination. The constellation Orion spans the plane of the celestial equator, so some points in the region assigned to Orion have a declination of 0°. A positive declination indicates the northern hemisphere, i.e., the spots on Earth that the point on the celestial sphere passes directly over are in the Earth's northern hemisphere. Analogously, points directly above southern hemisphere sites have a negative declination.

The same word, declination, is used outside astronomy regarding navigation on Earth using a compass: the word indicates the angular distance between north and magnetic north, an angle which varies over the surface of the Earth.