# Terrestrial Time

## Description

Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth. For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time (TDT or TD), which in turn succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.

The unit of TT is the SI second, the definition of which is currently based on the caesium atomic clock, but TT is not itself defined by atomic clocks. It is a theoretical ideal, and real clocks can only approximate it.

TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC). TT indirectly underlies UTC, via International Atomic Time (TAI). Because of the historical difference between TAI and ET when TT was introduced, TT is approximately 32.184 s ahead of TAI.

TT differs from Geocentric Coordinate Time (TCG) by a constant rate. TT and TCG are linear counts of SI seconds in Terrestrial Time and Geocentric Coordinate Time respectively and LG is the constant difference in the rates of the two time scales. LG is defined as exactly 6.969290134 × 10^(-10).

The equation linking TT and TCG is more commonly seen in this form.

Related formulas## Variables

TT | Terrestrial Time (sec) (dimensionless) |

TCG | Geocentric Coordinate Time (sec) (dimensionless) |

LG | Constant difference in the rates of the two time scales (dimensionless) |

JD_{TCG} | TCG time expressed as a Julian Date (dimensionless) |