Tuesday, November 27, 2007

In astronomy, declination (abbrev. dec or δ) is one of the two coordinates of the equatorial 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. Therefore, points north of the celestial equator have positive declinations, while those to the south have negative declinations.
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 φ > 0, celestial objects with a declination greater than 90° - φ, 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 (elevation).

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°. Stars
The declinations of all solar system objects change much more quickly than those of stars.

Declination is used in some contexts that rule out astronomical declination, to mean the same as magnetic declination.
Declination is occasionally and erroneously used to refer to the linguistic term declension.

right ascension, celestial coordinate system
geographic coordinates, ecliptic
Setting circles

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