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., when such a point in the celestial sphere passes through the zenith of some place on Earth, that place is 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.