Editing Ecliptic (section)

== Relationship to the celestial equator ==
{{Main|Axial tilt}}
[[File:Earths orbit and ecliptic.svg|thumb|350px|The [[plane (geometry)|plane]] of [[Earth]]'s [[orbit]] projected in all directions forms the reference plane known as the ecliptic. Here, it is shown projected outward (gray) to the [[celestial sphere]], along with Earth's [[equator]] and [[Earth's rotation|polar axis]] (green). The plane of the ecliptic intersects the celestial sphere along a [[great circle]] (black), the same circle on which the Sun seems to move as Earth orbits it. The intersections of the ecliptic and the equator on the celestial sphere are the [[equinox]]es (red), where the Sun seems to cross the celestial equator. ]]

Because [[Earth's rotation|Earth's rotational axis]] is not [[perpendicular]] to its [[Orbital plane (astronomy)|orbital plane]], Earth's [[Equator|equatorial plane]] is not [[Coplanarity|coplanar]] with the ecliptic plane, but is inclined to it by an angle of about 23.4°, which is known as the [[axial tilt|obliquity of the ecliptic]].<ref>''Explanatory Supplement'' (1992), p. 733</ref> If the equator is projected outward to the [[celestial sphere]], forming the [[celestial equator]], it crosses the ecliptic at two points known as the [[equinox]]es. The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at these points, one from south to north, the other from north to south.<ref name="celes direc">The directions ''north'' and ''south'' on the celestial sphere are in the sense ''toward the north [[celestial pole]]'' and ''toward the south celestial pole''. ''East'' is ''the direction toward which Earth rotates'', ''west'' is opposite that.</ref> The crossing from south to north is known as the [[March equinox]], also known as the ''first point of Aries'' and the ''[[Orbital node|ascending node]] of the ecliptic'' on the celestial equator.<ref>''Astronomical Almanac 2010'', p. M2 and M6</ref> The crossing from north to south is the [[September equinox]] or [[Orbital node|descending node]].

{{Main|Axial precession}}

The orientation of [[Earth's rotation|Earth's axis]] and equator are not fixed in space, but rotate about the [[Ecliptic pole|poles of the ecliptic]] with a period of about 26,000 years, a process known as ''lunisolar [[precession]]'', as it is due mostly to the gravitational effect of the [[Moon]] and [[Sun]] on [[Figure of the Earth|Earth's equatorial bulge]]. Likewise, the ecliptic itself is not fixed. The gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earth's orbit, and hence of the ecliptic, known as ''planetary precession''. The combined action of these two motions is called ''general precession'', and changes the position of the equinoxes by about 50 [[Minute of arc|arc seconds]] (about 0.014°) per year.<ref>''Explanatory Supplement'' (1992), sec. 1.322 and 3.21</ref>

{{Main|Astronomical nutation}}

Once again, this is a simplification. Periodic motions of the [[Moon]] and apparent periodic motions of the [[Sun]] (actually of Earth in its orbit) cause short-term small-amplitude periodic oscillations of Earth's axis, and hence the celestial equator, known as [[astronomical nutation|nutation]].<ref>
{{cite book 
 | author = U.S. Naval Observatory Nautical Almanac Office
 |author2=H.M. Nautical Almanac Office
 | title = Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac
 | publisher = H.M. Stationery Office, London
 | date = 1961}}
, sec. 2C</ref>
This adds a periodic component to the position of the equinoxes; the positions of the celestial equator and (March) equinox with fully updated precession and nutation are called the ''true equator and equinox''; the positions without nutation are the ''mean equator and equinox''.<ref>''Explanatory Supplement'' (1992), p. 731 and 737</ref>