dynamical timeThe family of timescales that was introduced in 1984 to replace ephemeris time, ET, as the independent variable of dynamical theories and ephemerides. Like ET these timescales are independent of the Earth's rotation.
Terrestrial dynamical time (TDT) is used as the timescale of ephemerides for observations from the Earth's surface. It is used for apparent geocentric ephemerides. For practical purposes
where TAI is the International Atomic Time. This takes advantage of the direct (broadcast) availability of coordinated universal time, UTC, which is an integral number of seconds offset from TAI. Continuity with pre-1984 practices has been achieved by setting the difference between TDT and TAI to the 1984 estimate of the difference between ET and TAI. The increment to be applied to universal time, UT1, to give TDT is called delta T (δT ). The unit of TDT is the day, 86 400 seconds, 24 hours, at mean sea level.
Barycentric dynamical time (TDB) is used for ephemerides and equations of motion that are referred to the barycenter of the Solar System. TDB differs from TDT only by periodic variations.