Barycentric Dynamical Time

Barycentric Dynamical Time (TDB) is a relativistic coordinate time scale, intended for astronomical use as a time standard to take account of time dilation when calculating orbits and astronomical ephemerides of planets, asteroids, comets and interplanetary spacecraft in the Solar System. TDB is now (since 2006) defined as a linear scaling of Barycentric Coordinate Time (TCB), and a feature that distinguishes TDB from TCB is that TDB, when observed from the Earth's surface, has a difference from Terrestrial Time (TT) that is about as small as can be practically arranged with consistent definition: the differences are mainly periodic, and overall will remain at less than 2 milliseconds for several millennia.

TDB applies to the Solar-System-barycentric reference frame, and was first defined in 1976 as a successor to the (non-relativistic) former standard of ephemeris time (adopted by the IAU in 1952 and superseded 1976). In 2006, after a history of multiple time-scale definitions and deprecation since the 1970s, a redefinition of TDB was approved by the IAU. TDB according to the 2006 redefinition can now be treated as equivalent, for practical astronomical purposes, to the long-established JPL ephemeris time argument T_eph as implemented in JPL Development Ephemeris DE405 (in use as the official basis for planetary and lunar ephemerides in the Astronomical Almanac, editions for 2003 and succeedng years).