Editing Ecliptic (section)

==Celestial reference plane==
{{main|Celestial equator|Ecliptic coordinate system}}

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| File:Ecliptic vs equator small.gif
 | The apparent motion of the [[Sun]] along the ecliptic (red) as seen on the inside of the [[celestial sphere]]. [[Ecliptic coordinate system|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.
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| File:Ecliptic inclination dziobek.PNG
 | Inclination of the ecliptic over 200,000 years, from Dziobek (1892).<ref>
 {{cite book 
 | last = Dziobek
 | first = Otto
 | title = Mathematical Theories of Planetary Motions
 | publisher = Register Publishing Co., Ann Arbor, Michigan
 | date = 1892
 |url = https://books.google.com/books?id=WTEaAAAAYAAJ&q=dziobek+mathematical}}, p. 294, at Google books</ref> This is the inclination to the ecliptic of 101,800 CE. Note that the ecliptic rotates by only about 7° during this time, whereas the [[celestial equator]] makes several complete cycles around the ecliptic. The ecliptic is a relatively stable reference compared to the celestial equator.
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The ecliptic forms one of the two fundamental [[Plane (geometry)|planes]] used as reference for positions on the celestial sphere, the other being the [[celestial equator]]. Perpendicular to the ecliptic are the [[ecliptic pole]]s, the north ecliptic pole being the pole north of the equator. Of the two fundamental planes, the ecliptic is closer to unmoving against the background stars, its motion due to planetary [[precession]] being roughly 1/100 that of the celestial equator.<ref name="montenbruck">
{{cite book 
 | last = Montenbruck
 | first = Oliver
 | title = Practical Ephemeris Calculations
 | publisher = Springer-Verlag
 | date = 1989
 | isbn = 0-387-50704-3
}}, sec 1.4</ref>

[[Spherical coordinate system|Spherical coordinates]], known as ecliptic longitude and latitude or celestial longitude and latitude, are used to specify positions of bodies on the celestial sphere with respect to the ecliptic. Longitude is measured positively eastward<ref name="celes direc"/> 0° to 360° along the ecliptic from the March equinox, the same direction in which the Sun appears to move. Latitude is measured perpendicular to the ecliptic, to +90° northward or −90° southward to the poles of the ecliptic, the ecliptic itself being 0° latitude. For a complete spherical position, a distance parameter is also necessary. Different distance units are used for different objects. Within the Solar System, [[astronomical unit]]s are used, and for objects near [[Earth]], [[Earth radius|Earth radii]] or [[kilometre|kilometers]] are used. A corresponding right-handed [[Cartesian coordinate system|rectangular coordinate system]] is also used occasionally; the ''x''-axis is directed toward the March equinox, the ''y''-axis 90° to the east, and the ''z''-axis toward the north ecliptic pole; the astronomical unit is the unit of measure. Symbols for ecliptic coordinates are somewhat standardized; see the table.<ref>''Explanatory Supplement'' (1961), sec. 2A</ref>

{| class="wikitable" style="float:right; margin:0em 1em .5em 0em;"
|+ Summary of notation for ecliptic coordinates<ref>''Explanatory Supplement'' (1961), sec. 1G</ref>
| rowspan="2" bgcolor="#89CFF0" |
| colspan="3" align="center" bgcolor="#89CFF0" | '''Spherical'''
| rowspan="2" align="center" bgcolor="#89CFF0" | '''Rectangular''' 
|- bgcolor="#89CFF0" align="center"
| Longitude
| Latitude
| Distance
|- align="center"
| bgcolor="#89CFF0" | '''Geocentric'''
| ''λ''
| ''β''
| ''Δ''
|
|- align="center"
| bgcolor="#89CFF0" | '''Heliocentric'''
| ''l''
| ''b''
| ''r''
| ''x'', ''y'', ''z''<ref group="note">Occasional use; ''x'', ''y'', ''z'' are usually reserved for [[Equatorial coordinate system|equatorial coordinates]].</ref>
|-
| colspan="5" | {{Reflist|group="note"}}
|}
Ecliptic coordinates are convenient for specifying positions of Solar System objects, as most of the planets' orbits have small [[Orbital inclination|inclinations]] to the ecliptic, and therefore always appear relatively close to it on the sky. Because Earth's orbit, and hence the ecliptic, moves very little, it is a relatively fixed reference with respect to the stars.

Because of the [[precession|precessional motion of the equinox]], the ecliptic coordinates of objects on the celestial sphere are continuously changing. Specifying a position in ecliptic coordinates requires specifying a particular equinox, that is, the equinox of a particular date, known as an [[Epoch (astronomy)|epoch]]; the coordinates are referred to the direction of the equinox at that date. For instance, the ''Astronomical Almanac''<ref>''Astronomical Almanac 2010'', p. E14</ref> lists the [[Heliocentric#Modern use of geocentric and heliocentric|heliocentric]] position of [[Mars]] at 0h [[Terrestrial Time]], 4 January 2010 as: longitude 118°09′15.8″, latitude +1°43′16.7″, true heliocentric distance 1.6302454 AU, mean equinox and ecliptic of date. This specifies the [[mean equinox]] of 4 January 2010 0h TT [[ecliptic#Relationship to the equator|as above]], without the addition of nutation.