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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.