Editing Ecliptic coordinate system (section)

== Primary direction ==

[[File:Ecliptic vs equator small.gif|thumb|The apparent motion of the [[Sun]] along the ecliptic (red) as seen on the inside of the [[celestial sphere]]. Ecliptic coordinates appear in (red). The [[celestial equator]] (blue) and the [[Equatorial coordinate system|equatorial coordinates]] (blue), being inclined to the ecliptic, appear to wobble as the Sun advances.]]

{{see also|Axial precession|Astronomical nutation}}

The [[celestial equator]] and the [[ecliptic]] are slowly moving due to [[Perturbation (astronomy)|perturbing forces]] on the [[Earth]], therefore the [[Orientation (geometry)|orientation]] of the primary direction, their intersection at the [[March equinox]], is not quite fixed. A slow motion of Earth's axis, [[Axial precession|precession]], causes a slow, continuous turning of the coordinate system westward about the poles of the [[ecliptic]], completing one circuit in about 26,000 years. Superimposed on this is a smaller motion of the [[ecliptic]], and a small oscillation of the Earth's axis, [[astronomical nutation|nutation]].<ref>
''Explanatory Supplement'' (1961), pp. 20, 28</ref><ref>
{{cite book
 | url = https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA11
 | last1 = U.S. Naval Observatory
 | first1=Nautical Almanac Office
 | editor = P. Kenneth Seidelmann
 | title = Explanatory Supplement to the Astronomical Almanac
 | publisher = University Science Books, Mill Valley, CA (reprint 2005)
 | year = 1992 
 | isbn = 1-891389-45-9
 |pages=11–13}}
</ref>

In order to reference a coordinate system which can be considered as fixed in space, these motions require specification of the [[equinox]] of a particular date, known as an [[Epoch (astronomy)|epoch]], when giving a position in ecliptic coordinates. The three most commonly used are:
;Mean equinox of a standard epoch: (usually the [[Epoch (astronomy)|J2000.0 epoch]], but may include B1950.0, B1900.0, etc.) is a fixed standard direction, allowing positions established at various dates to be compared directly.
;Mean equinox of date: is the intersection of the [[ecliptic]] of "date" (that is, the ecliptic in its position at "date") with the ''mean'' equator (that is, the equator rotated by [[Axial precession|precession]] to its position at "date", but free from the small periodic oscillations of [[astronomical nutation|nutation]]). Commonly used in planetary [[orbit]] calculation.
;True equinox of date: is the intersection of the [[ecliptic]] of "date" with the ''true'' equator (that is, the mean equator plus [[astronomical nutation|nutation]]). This is the actual intersection of the two planes at any particular moment, with all motions accounted for.
A position in the ecliptic coordinate system is thus typically specified ''true equinox and ecliptic of date'', ''mean equinox and ecliptic of J2000.0'', or similar. Note that there is no "mean ecliptic", as the ecliptic is not subject to small periodic oscillations.<ref>
{{cite book
 | last = Meeus
 | first = Jean
 | title = Astronomical Algorithms
 | publisher = Willmann-Bell, Inc., Richmond, VA
 | year = 1991
 |page=137
 |isbn=0-943396-35-2 }}</ref>