the origins is the difference between the Earth Rotation Angle and the apparent sidereal time at Greenwich. Diminishing role of the equinox in astronomy In modern astronomy the ecliptic and the equinox are diminishing in importance as required, or even convenient, reference concepts. (The equinox remains important in ordinary civil use, in defining the seasons, however.) This is for several reasons. One important reason is that it is difficult to be precise what the ecliptic is, and there is even some confusion in the literature about it. Should it be centered on the Earth's center of mass, or on the Earth-Moon barycenter? Also with the introduction of the International Celestial Reference Frame, all objects near and far are put fundamentally in relationship to a large frame based on very distant fixed radio sources, and the choice of the origin is arbitrary and defined for the convenience of the problem at hand. There are no significant problems in astronomy where the ecliptic and the equinox need to be defined. References External links Celestial Coordinate System, UTK Astronomical coordinate systems