Celestial coordinate which is equivalent to the longitude of an object on Earth. The starting point for longitude on Earth is at Greenwich England, whereas in the celestial coordinate system it is at a point in the sky called the vernal equinox. The two great circles defined by the celestial equator and the ecliptic, which is defined as the plane of the Solar System, cross each other at 2 points: at the vernal equinox (first day of spring) and the autumnal equinox (first day of autumn). The vernal equinox is the starting point for the RA coordinate system.