Declination

In astronomy, declination (abbreviated dec; symbol δ) is the angular distance from the celestial equator on the celestial sphere; when paired with right ascension, it forms the equatorial coordinate system. The declination angle is measured north (positive) or south (negative) of the celestial equator, lining up with the hour circle passing through the point in question.

Right ascension and declination as seen on the inside of the celestial sphere. The primary direction of the system is the vernal equinox, the ascending node of the ecliptic (red) on the celestial equator (blue). Declination is measured northward or southward from the celestial equator, along the hour circle passing through the point in question.

The word declination traces roots from (Latin, declinatio), meaning "bending away" or "bending down". It comes from the same root as the words incline ("bend forward") and recline ("bend backward").

In some 18th and 19th century astronomical texts, declination is given as North Pole Distance (N.P.D.), which is equivalent to 90 – (declination). For instance an object marked as declination −5 would have an N.P.D. of 95, and a declination of −90 (the south celestial pole) would have an N.P.D. of 180.

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