(time system, time scale)
(method of designating points in time and quantifying time periods)
A time standard (or time system or time scale,
but not "timescale" in the sense of the order-of-magnitude of the
time period in which some physical process happens)
is a system of designating points in time and quantifying
time periods, often in light of relativity and the resulting
effects position, velocity, gravity, and the finite speed of light.
Such complex systems are needed to handle very small
sub-second timing. The systems generally count off SI seconds,
but can't be in sync, often being out of sync by seconds.
- Barycentric dynamical time (TDB, the abbreviation being from the French, temps dynamique barycentrique). It counts off seconds in a frame of reference coincident with the barycenter of the solar system and is used for exact specification of movement within the solar system such as orbits and space navigation.
- Barycentric coordinate time (TCB for the French, temps-coordonnée barycentrique). Similar but does not include the effects of gravity.
- terrestrial time (TT), used for observations from the surface of the Earth.
- Universal time (UT), time based on Earth's rotation, a more precise follow-on to GMT (Greenwich mean time). There are versions, one being coordinated universal time (UTC).
The International Astronomical Union defines and standardizes the time systems for and
used in astronomy.
Before these systems were developed, similar systems, taking
fewer factors into account or otherwise flawed have been in use, including
ephemeris time (Teph) and terrestrial dynamical time
(TDT or TD).
Julian date (JD)
terrestrial time (TT)