Tuesday, December 11, 2007


In astronomy, declination (abbrev. dec) is one of the two coordinates of the equatoria coordinate system, the other being either right ascension or hour angle. Dec is comparable to latitude, projected onto the celestial sphere, and is measured in degrees north and south of the celestial equator. The points north of the celestial equator have positive declinations, while those to the south have negative declinations.
An object on the celestial equator has a dec of 0°.
An object above the north pole has a dec of +90°.
An object above the south pole has a dec of −90°.
The sign is customarily included even if it is positive. Any unit of angle can be used for declination, but it is often expressed in degrees, minutes, and seconds of arc.

A celestial object that passes over zenith, has a declination equal to the observer's latitude, with northern latitudes yielding positive declinations. A pole star therefore has the declination +90° or -90°. Conversely, at northern latitudes > 0, celestial objects with a declination greater than 90° are always visible. Such stars are called circumpolar stars, while the phenomenon of a sun not setting is called midnight sun.
If instead of measuring from and along the equator the angles are measured from and along the horizon, the angles are called azimuth and altitude.