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. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and south of the celestial equator. 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 declination of 0°.
- An object at the celestial north pole has a declination of +90°.
- An object at the celestial south pole has a declination 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. A pole star therefore has the declination near to +90° or −90°. 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 the Sun not setting is called midnight sun.
Because a star lies in a nearly constant direction as viewed from Earth, its declination is approximately constant from year to year. However, both the right ascension and declination do change gradually due to the effects of precession of the equinoxes, proper motion, and annual parallax.
The declinations of all solar system objects change much more quickly than those of stars.
The declination of the Sun, δ☉, is the angle between the rays of the Sun and the plane of the Earth's equator. The Earth's axial tilt (called the obliquity of the ecliptic by astronomers) is the angle between the Earth's axis and a line perpendicular to the Earth's orbit. The Earth's axial tilt changes gradually over thousands of years, but its current value is about ε = 23°26'. Because this axial tilt is nearly constant, solar declination (δ☉) varies with the seasons and its period is one year.
At the solstices, the angle between the rays of the Sun and the plane of the Earth's equator reaches its maximum value of 23°26'. Therefore δ☉ = +23°26' at the northern solstice and δ☉ = −23°26' at the southern solstice.
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