Terrestrial Time and Other Time Systems
Astronomers use other measures of time, such as Terrestrial Time (TT), which is 32.184 seconds ahead of International Atomic Time (TAI).
Terrestrial Time (TT)
Terrestrial Time (TT) used to be known as Terrestrial Dynamical Time (TDT).
The unit for TT is the SI second, one second in International Atomic Time (TAI).
TT 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 (TT). TT has been in use 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
- 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.