# ecliptic coordinate system

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## ecliptic coordinate system,

an astronomical coordinate systemastronomical coordinate systems.
A coordinate system is a method of indicating positions. Each coordinate is a quantity measured from some starting point along some line or curve, called a coordinate axis.
in which the principal coordinate axis is the eclipticecliptic
, the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic). Because of the earth's yearly revolution around the sun, the sun appears to move in an annual journey through the heavens with the ecliptic as its
, the apparent path of the sun through the heavens. The ecliptic poles are the two points at which a line perpendicular to the plane of the ecliptic through the center of the earth strikes the surface of the celestial spherecelestial sphere,
imaginary sphere of infinite radius with the earth at its center. It is used for describing the positions and motions of stars and other objects. For these purposes, any astronomical object can be thought of as being located at the point where the line of sight
. The north ecliptic pole lies in the constellation Draco.
ecliptic coordinate system

## ecliptic coordinate system

A coordinate system in which the fundamental reference circle is the ecliptic and the zero point is the vernal (or dynamic) equinox (γ). The coordinates are celestial (or ecliptic) latitude and celestial (or ecliptic) longitude (see illustration).

The celestial latitude (β) of a star, etc., is its angular distance (from 0° to 90°) north (counted positive) or south (counted negative) of the ecliptic; it is measured along the great circle through the body and the poles of the ecliptic. The celestial longitude (λ) of a body is its angular distance (from 0° to 360°) from the vernal equinox, measured eastward along the ecliptic to the intersection of the body's circle of longitude; it is measured in the same direction as the Sun's apparent annual motion. Although observations are taken from the Earth's surface the coordinates should strictly be geocentric and, tabulated as such, are universally applicable. A slight correction is therefore applied to convert surface (topocentric) observations to geocentric values.

The ecliptic system is older but less used than the equatorial and horizontal coordinate systems. It is sometimes used to give the positions of the Sun, Moon, and planets. See also heliocentric coordinate system.

## ecliptic coordinate system

[i′klip·tik kō′ȯrd·ən·ət ‚sis·təm]
(astronomy)
A celestial coordinate system in which the ecliptic is taken as the primary and the great circles perpendicular to it are then taken as secondaries.
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