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. Declination's angle is measured north or south of 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 toward") and recline ("bend backward").
1 Explanation 2 Effects of precession 3 Stars 4 Sun 5 Relation to latitude 6 See also 7 Notes and references 8 External links
Main article: Equatorial coordinate system
celestial equator has a declination of 0° north celestial pole has a declination of +90° south celestial pole has a declination of −90°
The sign is customarily included whether positive or negative. Effects of precession
Main article: Axial precession
The Earth's axis rotates slowly westward about the poles of the
ecliptic, completing one circuit in about 26,000 years. This effect,
known as precession, 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 an epoch. Coordinates from
different epochs must be mathematically rotated to match each other,
or to match a standard epoch.
The currently used standard epoch is J2000.0, which is January 1, 2000
at 12:00 TT. 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.
A star's direction remains nearly fixed due to its vast distance, but
its right ascension and declination do change gradually due to
precession of the equinoxes and proper motion, and cyclically due to
annual parallax. The declinations of
Stars visible by latitude
Observer's latitude (°) Declination
of circumpolar stars (°) of non-circumpolar stars (°) of stars not visible (°)
+ for north latitude, − for south − for north latitude, + for south
90 (Pole) 90 to 0
0 to 90
45 (midpoint) 90 to 45 +45 to −45 45 to 90
23.5 (Tropic of Cancer/Capricorn) 90 to 66.5 +66.5 to −66.5 66.5 to 90
+90 to −90
Non-circumpolar stars are visible only during certain days or seasons of the year.
The night sky, divided into two halves.
Sun Main article: Position of the Sun The Sun's declination varies with the seasons. 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. Likewise, near the local winter solstice, the Sun remains below the horizon all day, which is called polar night. Relation to latitude 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. In the continental United States and surrounding area, the difference (the vertical deflection) is typically a few arcseconds (1 arcsecond = 1/3600 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 give declinations measured at the center of the Earth. (An ellipsoid is an approximation to sea level that is mathematically manageable). For the moon this discrepancy can reach 0.003 degree; the Sun and planets are hundreds of times more distant and for them the discrepancy is proportionately smaller (and for the stars is unmeasurable). See also
Celestial coordinate system Ecliptic Equatorial coordinate system Geographic coordinates Lunar standstill Right ascension Setting circles Position of the Sun
Notes and references
^ 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. ISBN 0-935702-68-7. ^ Barclay, James (1799). A Complete and Universal English Dictionary. ^ Moulton, Forest Ray (1918). An Introduction to Astronomy. New York: Macmillan Co. p. 125, art. 66. , at Google books ^ Moulton (1918), pp. 92–95. ^ see, for instance, U.S. Naval Observatory Nautical Almanac Office, Nautical Almanac Office; U.K. Hydrographic Office, H.M. Nautical Almanac Office (2008). "Time Scales and Coordinate Systems, 2010". The Astronomical Almanac for the Year 2010. U.S. Govt. Printing Office. p. B2,. ^ "Celestial Coordinates". www.austincc.edu. Retrieved 2017-03-24. ^ "USDOV2009". Silver Spring, Maryland: U.S. National Geodetic Survey. 2011. ^ P. Kenneth Seidelmann, ed. (1992). Explanatory Supplement to the Astronomical Almanac. Sausalito, CA: University Science Books. pp. 200–5.
MEASURING THE SKY A Quick Guide to the Celestial Sphere James B.
Kaler, University of Illinois
Celestial Equatorial Coordinate System University of Nebraska-Lincoln
Celestial Equatorial Coordinate Explorers University of
Merrifield, Michael. "(α,δ) – Right Ascension & Declination".