declination

Declination is a term used in astronomy and navigation to describe angular position on the celestial sphere. In astronomy, declination (symbol δ) is the angular distance of a celestial object north or south of the celestial equator. It is measured in degrees, from −90° at the south celestial pole to +90° at the north celestial pole, with 0° lying on the celestial equator. Along with right ascension, declination specifies a location on the sky. The celestial equator is the projection of the Earth's equator onto the celestial sphere, and declination is the analogue of latitude for celestial coordinates. An object's declination helps determine how high the object will appear in the sky and its visibility from a given latitude.

In navigation and observational astronomy, magnetic declination refers to the angle between geographic north (true north) and magnetic north, the direction indicated by a compass. This angle varies by location and slowly changes over time due to fluctuations in Earth's magnetic field. It is typically expressed as east or west deviation and is important for precise compass navigation and map reading.

In linguistics, declination refers to the inflectional system of a word, showing changes to express grammatical case, number, gender, and other categories. The term is often used to describe the set of inflected forms (declensions) in languages with rich case systems; in English, the concept is discussed as inflection and, less commonly, as declination in historical or comparative contexts.

Declination additionally appears in general usage to denote a decline or downward trend, but in technical articles it most often refers to either astronomical coordinates or grammatical inflection.