Product Code Database
In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or south (negative) of the celestial equator, along the hour circle passing through the point in question.
(1992). 9780935702682, University Science Books, Mill Valley, CA. ISBN 9780935702682
[[File:Ra and dec demo animation small.gif|right|thumb|350px| Right ascension and declination as seen on the inside of the celestial sphere. The primary direction of the system is the 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 root of the word declination (Latin, declinatio) means "a bending away" or "a 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.
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 the degrees (°), minutes (′), and seconds (″) of sexagesimal, 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.
An object at the
The sign is customarily included whether positive or negative.
The currently used standard epoch is J2000.0, which is January 1, 2000 at 12:00 Terrestrial Time. The prefix "J" indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, and B1950.0. see, for instance,
As seen from locations in the Earth's Northern Hemisphere, celestial objects with declinations greater than 90° − (where = observer's latitude) appear to circle daily around the celestial pole without dipping below the horizon, and are therefore called . This similarly occurs in the Southern Hemisphere for objects with declinations less (i.e. more negative) than −90° − (where is always a negative number for southern latitudes). An extreme example is the pole 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 the equator. 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 is is circumpolar for some observer (where is either positive or negative), then a star whose declination is − never rises above the horizon, as seen by the same observer. (This neglects the effect of atmospheric refraction.) Likewise, if a star is circumpolar for an observer at latitude , then it never rises above the horizon as seen by an observer at latitude −.
Neglecting atmospheric refraction, for an observer at the equator, declination is always 0° at east and west points of the horizon. At the north point, it is 90° − ||, and at the south point, −90° + ||. From the poles, declination is uniform around the entire horizon, approximately 0°.
| + visible by latitude | Observer's latitude (°) | Declination | ||
| of (°) | of non-circumpolar stars (°) | of stars not visible (°) | ||
| + for north latitude, − for south | − for north latitude, + for south | |||
| 90 (Pole) | 0 to 90 | |||
| 66.5 (Arctic Circle/Antarctic Circle) | 23.5 to 90 | |||
| 45 (midpoint) | 45 to 90 | |||
| 23.5 (Tropic of Cancer/Capricorn) | 66.5 to 90 | |||
| 0 (Equator) | ||||
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 (the vertical deflection) is typically a few arcseconds (1 arcsecond = of a degree) but can be as great as 41 arcseconds.
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 to sea level that is mathematically manageable).
|
|
