Other Time Systems
Astronomers use other measures of time, such as Terrestrial Dynamical Time (TDT), which is 32.184 seconds ahead of TAI. TDT, also known as Terrestrial Time (TT), with a unit of duration 86400 SI seconds on the geoid, is the independent argument of apparent geocentric ephemerides. Therefore:
TDT = TAI + 32.184 seconds.
TDT is used for calculating planetary positions in relation to the Earth’s center. Delta T is the difference between Earth’s rotational time (UT1) and dynamical time (TDT). TDT has been used since 1984. Prior to this, astronomers used a time measure known as Ephemeris Time (ET). Before atomic clocks, Ephemeris Time (ET) was the closest available approximation to a uniform time for planetary motion calculations.
Other time systems include:
◾Barycentric Dynamical Time (TDB), the independent argument of ephemerides and dynamical theories that are referred to the solar system barycenter. TDB varies from TT only by periodic variations.
◾Barycentric Coordinate Time (TCB), which is a coordinate time having its spatial origin at the solar system barycenter. TCB differs from TDB in rate.
◾Sidereal Time, with unit of duration the period of the Earth's rotation with respect to a point nearly fixed with respect to the stars, is the hour angle of the vernal equinox.
◾Geocentric Coordinate Time (TCG), a coordinate time having its spatial origin at the center of mass of the Earth.
Note: timeanddate.com wishes to acknowledge some of the information in this article is courtesy of the UK’s National Maritime Museum and the United States Naval Observatory. It is also important to note that this article does not mention every time system but simply provides an overview of time systems.