The time-scale that is used in calculating orbital motions within the Solar System. The underlying physical law governing such motions is the law of gravitation. In Newtonian gravitation, time plays a fundamental role; it is absolute and universal, and the problem is to work out the positions of bodies for each instant of time. The time-scale that was used until 1984 was called Ephemeris Time. Since then, however, relativistic effects have been included. According to the theory of relativity, time is different for each observer. It has therefore been necessary to introduce two distinct dynamical time-scales, namely Terrestrial Time (originally Terrestrial Dynamical Time) and Barycentric Dynamical Time. There are only very slight periodic differences between the two, of the order of a thousandth of a second.
Subjects: Astronomy and Astrophysics.