epoch

A precise moment in time for which given celestial coordinates or orbital elements are strictly correct. It is necessary to define the epoch for such measurements because of the effects of precession on celestial coordinate systems and gravitational perturbations on the orbits of bodies such as comets and planets. It has been the practice to change the standard epoch used for star charts and catalogues every 50 years. Those prepared since the 1970s have typically been for the epoch 2000.0; the previous standard epoch was 1950.0.