Timescales

SLALIB provides for conversion between several timescales, and involves use of one or two others. The full list is as follows:

• TAI: International Atomic Time
• UTC: Coordinated Universal Time
• UT: Universal Time
• GMST: Greenwich Mean Sidereal Time
• LAST: Local Apparent Sidereal Time
• TT: Terrestrial Time
• TDB: Barycentric Dynamical Time.

Three obsolete timescales should be mentioned here to avoid confusion.

• GMT: Greenwich Mean Time - can mean either UTC or UT.
• ET: Ephemeris Time - more or less the same as either TT or TDB.
• TDT: Terrestrial Dynamical Time - former name of TT.