15.5.78.13 Time Scales

Various time scales are in use. International Atomic Time (TAI) is a uniform time scale with a unit of one si second. It was implemented around 1960. Each TAI minute is 60 si seconds, each TAI hour equals 60 TAI minutes, and each TAI day equals 24 TAI hours. Coordinated Universal Time (UTC) is the basis of most administrative times on Earth. Its units are the si second, the minute, the hour, and the day, and it differs from TAI by an integral number of seconds. By infrequent and irregular insertion of leap seconds, the UTC day start is tied to the Earth’s rotation. The length of specific UTC minutes, hours, and days may therefore differ from TAI minutes, hours, and days. Universal Time (UT, UT1) is a non-uniform time scale tied to the Earth’s rotation. It has no leap seconds, so the length of its unit (the UT second) depends on the Earth’s rotation. UT differs from UTC by less than 0.9 seconds through the judicious insertion of leap seconds in UTC. Terrestrial Dynamical Time (TDT) – or Terrestrial Time (TT) for short – was implemented in 1984 as the dynamical time scale for geocetric phenomena. Barycentric Dynamical Time (TDB) was implemented in 1984 as the dynamical time scale for solar-system barycentric phenomena. The difference between TDT and TDB is always less than 0.002 seconds. The difference between TDT and TAI is fixed at about 32 seconds. Before 1984, Ephemeris Time (et) was used as a uniform time scale for ephemerides, instead of TDT and TDB.