Editing Right ascension (section)

== Effects of precession ==
{{main|Axial precession}}

The Earth's axis traces a small circle (relative to its celestial equator) slowly westward about the [[celestial pole]]s, completing one cycle in about 26,000 years.  This movement, known as [[Axial precession|precession]], causes the coordinates of stationary celestial objects to change continuously, if rather slowly.  Therefore, [[Equatorial coordinate system|equatorial coordinates]] (including right ascension) are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an [[Epoch (astronomy)|epoch]]. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch.<ref>Moulton (1916), pp. 92–95.</ref> Right ascension for "fixed stars" on the equator increases by about 3.1 seconds per year or 5.1 minutes per century, but for fixed stars away from the equator the rate of change can be anything from negative infinity to positive infinity. (To this must be added the [[proper motion]] of a star.) Over a precession cycle of 26,000 years, "fixed stars" that are far from the [[ecliptic pole]]s increase in right ascension by 24h, or about 5.6' per century, whereas stars within 23.5° of an ecliptic pole undergo a net change of{{nbsp}}0h. The right ascension of [[Polaris]] is increasing quickly{{mdash}}in AD 2000 it was 2.5h, but when it gets closest to the north celestial pole in 2100 its right ascension will be 6h. The [[North Ecliptic Pole]] in [[Draco (constellation)|Draco]] and the [[South Ecliptic Pole]] in [[Dorado]] are always at right ascension 18<sup>h</sup> and 6<sup>h</sup> respectively.

The currently used standard epoch is [[J2000.0]], which is January 1, 2000 at 12:00 [[Terrestrial Time|TT]]. The prefix "J" indicates that it is a [[Julian epoch]]. Prior to J2000.0, astronomers used the successive [[Epoch (astronomy)#Besselian epoch|Besselian epochs]] B1875.0, B1900.0, and B1950.0.<ref>see, for instance, {{cite book
 | author1 = U.S. Naval Observatory Nautical Almanac Office
 | author2 = U.K. Hydrographic Office
 | author3 = H.M. Nautical Almanac Office
 | title = The Astronomical Almanac for the Year 2010
 | publisher = U.S. Govt. Printing Office
 | date = 2008
 | page=B2
 | chapter=Time Scales and Coordinate Systems, 2010 }}</ref>