Use of the keyword TIMESYS is suggested
as an implementation of the time
scale specification. It sets the principal time system for time-related
keywords and data in the HDU (i.e., it does not preclude the addition of
keywords or data columns that provide information for transformations to
other time scales, such as sidereal times or barycenter corrections).
Each HDU shall contain not more than one TIMESYS keyword.
Initially, officially allowed values are:
- Coordinated Universal Time; defined since 1972.
- Universal Time, equal to Greenwich Mean Time (GMT)
since 1925; the UTC equivalent before 1972;
see: Explanatory Supplement, p. 76.
- International Atomic Time; ``UTC without the
leap seconds''; 31 s ahead of UTC on 1997-07-01.
- International Atomic Time; deprecated synonym of TAI.
- Ephemeris Time, the predecessor of TT; valid until
- Terrestrial Time, the IAU standard time scale since 1984;
continuous with ET and synchronous with (but 32.184 s
ahead of) TAI.
- Terrestrial Dynamical Time; = TT.
- Barycentric Dynamical Time.
- Geocentric Coordinate Time; runs ahead of TT since
1977-01-01 at a rate of approximately 22 ms/year.
- Barycentric Coordinate Time; runs ahead of TDB since
1977-01-01 at a rate of approximately 0.5 s/year.
For reference, see:
Explanatory Supplement to the Astronomical Almanac, P. K. Seidelmann,
ed., University Science Books, 1992, ISBN 0-935702-68-7, or
Use of Global Positioning Satellite (GPS) time (19 s behind TAI) is