time standard

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 on position, velocity, gravity, and the finite speed of light. Systems with precise, potentially complex definitions are needed to handle very small sub-second timing and the multiple frames of reference involved in astronomy and spacecraft. The systems generally count off SI seconds, but cannot remain in sync, often being out of sync by seconds. Examples:

• 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) based upon the Greenwich meridian. There are versions, one being coordinated universal time (UTC).

The International Astronomical Union (IAU) defines and standardizes time systems for use in astronomy. Before its current systems were developed, similar systems were in use, generally taking fewer factors into account or otherwise flawed. Among them were ephemeris time (T_eph) and terrestrial dynamical time (TDT or TD).