An epoch is the time and date at which an astronomical observation is made (epoch of observation), or the date for which orbital elements (epoch of elements) or the positions of celestial objects are calculated. Specifying the epoch is important because the apparent positions of objects in the sky change gradually due to precession and nutation, while orbital elements change due to the gravitational effects of the planets.
Data given in star catalogues and ephemerides are referred to a standard epoch, also known as a fundamental epoch. Prior to 1984, coordinates of star catalogs were commonly referred to Besselian epochs (see Besselian year). However, from 1984 on, Julian epochs have been used, denoted by the prefix J. The current standard epoch is J2000.0. It will be superceded in half a century by J2050.0.