Right ascension anddeclination as seen on the inside of thecelestial sphere. The primary direction of the system is thevernal equinox, the ascending node of theecliptic (red) on the celestial equator (blue). Declination is measured northward or southward from the celestial equator, along thehour circle passing through the point in question.
The root of the worddeclination (Latin,declinatio) means "a bending away" or "a bending down". It comes from the same root as the wordsincline ("bend forward") andrecline ("bend backward").[2]
In some 18th and 19th century astronomical texts, declination is given asNorth 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.
Declination in astronomy is comparable to geographiclatitude, projected onto thecelestial sphere, and right ascension is likewise comparable to longitude.[3]Points north of the celestial equator have positive declinations, while those south have negative declinations. Any units of angular measure can be used for declination, but it is customarily measured in thedegrees (°),minutes (′), andseconds (″) ofsexagesimal measure, with 90° equivalent to a quarter circle. Declinations with magnitudes greater than 90° do not occur, because the poles are the northernmost and southernmost points of the celestial sphere.
The Earth's axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known asprecession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore,equatorial coordinates (including declination) are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as anepoch. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch.[4]
The currently used standard epoch isJ2000.0, which is January 1, 2000 at 12:00TT. The prefix "J" indicates that it is aJulian epoch. Prior to J2000.0, astronomers used the successiveBesselian Epochs B1875.0, B1900.0, and B1950.0.[5]
As seen from locations in the Earth'sNorthern Hemisphere, celestial objects with declinations greater than 90° − φ (whereφ = observer'slatitude) appear to circle daily around thecelestial pole without dipping below thehorizon, and are therefore calledcircumpolar stars. This similarly occurs in theSouthern Hemisphere for objects with declinations less (i.e. more negative) than −90° − φ (whereφ is always anegative number for southern latitudes). An extreme example is thepole star which has a declination near to +90°, so is circumpolar as seen from anywhere in the Northern Hemisphere except very close to the equator.
Circumpolar stars never dip below the horizon. Conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earth's surface (except extremely close to theequator. Upon flat terrain, the distance has to be within approximately 2 km, although this varies based upon the observer's altitude and surrounding terrain). Generally, if a star whose declination isδ is circumpolar for some observer (whereδ is either positive or negative), then a star whose declination is −δ never rises above the horizon, as seen by the same observer. (This neglects the effect ofatmospheric refraction.) Likewise, if a star is circumpolar for an observer at latitudeφ, then it never rises above the horizon as seen by an observer at latitude −φ.
Neglecting atmospheric refraction, for an observer at the equator, declination is always 0° at east and west points of thehorizon. At the north point, it is 90° − |φ|, and at the south point, −90° + |φ|. From thepoles, declination is uniform around the entire horizon, approximately 0°.
Non-circumpolar stars are visible only during certain days orseasons of the year.
The night sky, divided into two halves.Declination (green) begins at theequator (green) and is positive northward (towards the top), negative southward (towards the bottom). The lines of right ascension (blue) divide the sky intogreat circles, here 1 hour apart.
The Sun's declination varies with theseasons. As seen fromarctic orantarctic latitudes, the Sun is circumpolar near the localsummer solstice, leading to the phenomenon of it being above thehorizon atmidnight, which is calledmidnight sun. Likewise, near the local winter solstice, the Sun remains below the horizon all day, which is calledpolar night.
When an object is directly overhead its declination is almost always within 0.01 degrees of the observer's latitude; it would be exactly equal except for two complications.[6][7]
The first complication applies to all celestial objects: the object's declination equals the observer's astronomical latitude, but the term "latitude" ordinarily means geodetic latitude, which is the latitude on maps and GPS devices. In the continental United States and surrounding area, the difference (thevertical deflection) is typically a fewarcseconds (1 arcsecond =1/3600 of a degree) but can be as great as 41 arcseconds.[8]
The second complication is that, assuming no deflection of the vertical, "overhead" means perpendicular to the ellipsoid at observer's location, but the perpendicular line does not pass through the center of the Earth; almanacs provide declinations measured at the center of the Earth. (An ellipsoid is an approximation tosea level that is mathematically manageable).[9]
^U.S. Naval Observatory, Nautical Almanac Office (1992). P. Kenneth Seidelmann (ed.).Explanatory Supplement to the Astronomical Almanac. University Science Books, Mill Valley, CA. p. 724.ISBN0-935702-68-7.
