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Right ascension and declination on the celestial sphere. The origin of the system is the vernal equinox, the ascending node of the ecliptic (red) on the celestial equator (blue). Declination is measured in degrees (°) north or south of the celestial equator. Points north of the celestial equator have positive declinations, while those south have negative declinations.

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.

As seen from locations in the Earth's Northern Hemisphere, celestial objects with declinations greater than 90° − φ (where φ = observer's latitude) are always above the horizon. This similarly occurs in the Southern Hemisphere for objects with declinations less (i.e. more negative) than -90° − φ (where φ is always a negative number for southern latitudes). Such stars appear to circle daily around the celestial pole without dipping below the horizon, and are therefore called circumpolar stars. An extreme example is the pole star which has a declination near to +90°, so it is circumpolar as seen from anywhere in the Northern Hemisphere except very close to the equator. At the horizon, declination is always 0° due east and west. Due north, it is 90° − |φ|, and due south, -90° + |φ| (with absolute value always being positive). Therefore, only at the poles is declination uniform along a 360 degree sweep of the horizon.

The Sun's declination varies with the seasons (see below). As seen from arctic or antarctic latitudes, the Sun is circumpolar near the local summer solstice, leading to the phenomenon of it being above the horizon at midnight, which is called midnight sun.

When an object is directly overhead its declination is almost always within 0.01 degree of the observer's latitude; it would be exactly equal except for two complications. The first complication applies to all celestial objects: the object's declination equals the observer's astronomic latitude, but the term "latitude" ordinarily means geodetic latitude, which is the latitude on maps and GPS devices. The difference (the vertical deflection) usually doesn't exceed a few thousandths of a degree. For practical purposes the second complication only applies to solar system objects: "declination" is ordinarily measured at the center of the earth, which isn't quite spherical, so a line from the center of the earth to the object is not quite perpendicular to the Earth's surface. It turns out that when the moon is directly overhead its geocentric declination can differ from the observer's astronomic latitude by up to 0.005 degree. The importance of this complication is inversely proportional to the object's distance from the earth, so for most purposes it's not a concern for the sun and planets.

Stars[link]

A star lies in a nearly constant direction as viewed from Earth, with its declination roughly constant from year to year, but right ascension and declination do both change gradually due to precession of the equinoxes, proper motion, and annual parallax. The declinations of all solar system objects change much more quickly than those of stars.

For the Southern Hemisphere, reverse the signs (+/-). Non-circumpolar stars may be potentially visible only during certain days or seasons of the year.

See also[link]

• Celestial coordinate system
• Ecliptic
• Geographic coordinate system
• Inclination
• Lunar standstill
• Setting circles
• Euler angles
• Equation of time
• Right ascension
• Position of the Sun

References[link]

External links[link]

• How to compute planetary positions by Paul Schlyter.