Ecliptic coordinate systemThe ecliptic coordinate system is a celestial coordinate system that uses the ecliptic for its fundamental plane. The ecliptic is the path that the sun appears to follow across the sky over the course of a year. It is also the projection of the Earth's orbital plane onto the celestial sphere. The latitudinal angle is called the ecliptic latitude, and the longitudinal angle is called the ecliptic longitude. Like right ascension in the equatorial coordinate system, the zeropoint of the ecliptic longitude is the vernal equinox.
What do you think such a coordinate system would be useful for? If you guessed charting solar system objects, you're right! Each of the planets (except Pluto) orbits the sun in roughly the same plane, so they always appear to be somewhere near the ecliptic (i.e., they always have small ecliptic latitudes).
This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linu/KDE. See http://edu.kde.org/kstars/index.phtml