The latitudinal (north-south) coordinate in the astronomical equatorial coordinate system. Declination, denoted by the Greek letter δ, is measured in units of degrees, arcminutes, and arcseconds. There are 60 arcseconds in one arcminute and 60 arcminutes in one degree. It is measured with reference to the projection of the Earth's equator onto the sky. The declination of objects directly overhead corresponds to one's latitude on Earth, i.e. if you are standing on the equator, objects with zero degrees declination are directly overhead. If you are standing at the south pole, objects with -90 degrees declination are directly overhead, and if at the north pole, objects with +90 degrees declination are overhead. Note that Polaris, the north star is almost-but-not-quite at a declination of +90 degrees (according to simbad.harvard.edu, it is at +89d 15' 50.8'').

Objects which never set below the local horizon are called circumpolar. In the northern and southern hemispheres, they correspond to objects with declinations of

δ = +90 - (your latitude)
δ = -90 - (your latitude)

respectively, assuming that southern latitudes are negative as measured from the equator. Thus, if you live at +40 degrees latitude, objects with declinations higher than +50 degrees are circumpolar (ignoring obstructions and increased atmospheric extinction near the horizon).

The longitudinal (east-west) coordinate in equatorial coordinates is the right ascension, α.

Dec`li*na"tion (?), n. [L. declinatio a bending aside, an avoiding: cf. F. déclination a decadence. See Declension.]

1.

The act or state of bending downward; inclination; as, declination of the head.

2.

The act or state of falling off or declining from excellence or perfection; deterioration; decay; decline.

"The declination of monarchy."

Bacon.

Summer . . . is not looked on as a time Of declination or decay. Waller.

3.

The act of deviating or turning aside; oblique motion; obliquity; withdrawal.

The declination of atoms in their descent. Bentley.

Every declination and violation of the rules. South.

4.

The act or state of declining or refusing; withdrawal; refusal; averseness.

The queen's declination from marriage. Stow.

5. Astron.

The angular distance of any object from the celestial equator, either northward or southward.

6. Dialing

The arc of the horizon, contained between the vertical plane and the prime vertical circle, if reckoned from the east or west, or between the meridian and the plane, reckoned from the north or south.

7. Gram.

The act of inflecting a word; declension. See Decline, v. t., 4.

Angle of declination, the angle made by a descending line, or plane, with a horizontal plane. -- Circle of declination, a circle parallel to the celestial equator. -- Declination compass Physics, a compass arranged for finding the declination of the magnetic needle. -- Declination of the compassneedle, the horizontal angle which the magnetic needle makes with the true north-and-south line.

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