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Celestial reference plane::Ecliptic

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Celestial reference plane {{#invoke:main|main}}

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 coordinates (blue), being inclined to the ecliptic, appear to wobble as the Sun advances.

The ecliptic forms one of the two fundamental planes used as reference for positions on the celestial sphere, the other being the celestial equator. Perpendicular to the ecliptic are the ecliptic poles, 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"> {{#invoke:citation/CS1|citation |CitationClass=book }} , sec 1.4</ref>

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 vernal 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 units are used, and for objects near the Earth, Earth radii or kilometers are used. A corresponding right-handed rectangular coordinate system is also used occasionally; the x-axis is directed toward the vernal 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>


Summary of notation for ecliptic coordinates<ref>Explanatory Supplement (1961), sec. 1G</ref>
  spherical rectangular
longitude latitude distance
geocentric λ β Δ  
heliocentric l b r x, y, z<ref group="note">Occasional use; x, y, z are usually reserved for equatorial coordinates.</ref>
Unknown extension tag "references"

Ecliptic coordinates are convenient for specifying positions of Solar System objects, as most of the planets' orbits have small inclinations to the ecliptic, and therefore always appear relatively close to it on the sky. Because the Earth's orbit, and hence the ecliptic, moves very little, it is a relatively fixed reference with respect to the stars.

CitationClass=book }} , 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, while the celestial equator makes several complete cycles around the ecliptic. The ecliptic is a relatively stable reference compared to the celestial equator.
Because of the 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; 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 position of Mars at 0h Terrestrial Time, 4 Jan 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 Jan 2010 0h TT as above, without the addition of nutation.

Ecliptic sections
Intro  Sun's apparent motion   Relationship to the celestial equator    Obliquity of the ecliptic   Plane of the Solar System  Celestial reference plane  Eclipses   Equinoxes and solstices   In the constellations  Astrology  See also  Notes and references   External links   

Celestial reference plane
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