This is the description of the namespace http://docs.g-vo.org/rdf/timescale as of 2019-03-15.
This vocabulary is not yet approved by the IVOA. This means that terms can still disappear without prior notice.
A collection of timescales in common use in astronomy.
| Predicate | Label | Description | Parent | Preferred |
|---|---|---|---|---|
| TAI | International Atomic Time TAI | atomic time standard, TT-TAI = 32.184 s. | ||
| TT | Terrestrial Time TT | Time measured by a continuous clock on the surface of an ideal Earth. Defined via TCG as having been idential on 1977-01-01 and since running slower than it by an empirically determined factor L_C. It is continuous with the ephemeris time ET widely used before 1984-01-01. The term TT should therefore be used for times in ET, too. (IAU standard) | ||
| UT | Earth rotation time UT | We do not distinguish between UT0, UT1, and UT2. Applications requiring this level of precision need additional metadata. This should also be used to label GMT times in datasets covering dates between 1925-01-01 and 1972-01-01. GMT in astronomical use before 1925 had a 12 hour offset and would require a new term. | ||
| UTC | Universal Time, Coordinated UTC | This is TAI, with leap seconds inserted occasionally in order to keep UTC within 0.9 s of UT1 (a different convention was in use before 1972-01-01). | ||
| GPS | Global Positioning System time | Runs (approximately) synchronously with TAI | ||
| TCG | Geocentric Coordinate Time TCG | Time measured by a clock moving with the Earth's center but not subject to the gavitational potential of the Earth. | ||
| TCB | Barycentric Coordinate Time TCB | Derived from TCG, but taking into account the relativistic effects of the gravitational potential at the barycenter as well as velocity time dilation variations due to the eccentricity of the Earth’s orbit. See 1999A&A...348..642I for details. | ||
| TDB | Barycentric Dynamical Time TDB | Runs slower than TCB at a constant rate so as to remain approximately in step with TT. Therefore runs quasi-synchronously with TT, except for the relativistic effects introduced by variations in the Earth’s velocity relative to the barycenter. | ||
| UNKNOWN | Unknown or unavailable timescale | This value indicates clients cannot transform the times reliably. This is to be used for simulated data, free-running clocks, or data for which information on the time scale has been lost. |