Because most planets (except Mercury) and many small Solar System bodies have orbits with slight inclinations to the ecliptic, using the ecliptic coordinate system as the fundamental plane is convenie...Because most planets (except Mercury) and many small Solar System bodies have orbits with slight inclinations to the ecliptic, using the ecliptic coordinate system as the fundamental plane is convenient. The system's origin can be the center of either the Sun or Earth, its primary direction is towards the vernal (northward) equinox, and it has a right-hand convention. It may be implemented in spherical or rectangular coordinates.
