time standard
(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 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
(Teph) and terrestrial dynamical time (TDT
or TD).
(time,physics,relativity)
Further reading:
https://en.wikipedia.org/wiki/Time_standard
https://en.wikipedia.org/wiki/Barycentric_Dynamical_Time
https://en.wikipedia.org/wiki/Terrestrial_Time
https://en.wikipedia.org/wiki/Universal_Time
https://stjarnhimlen.se/comp/time.html
https://www.mssl.ucl.ac.uk/grid/iau/extra/local_copy/systime.html
https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/The-USNO-Master-Clock/Definitions-of-Systems-of-Time/
https://www.bipm.org/en/time-metrology
https://www.iau.org/static/resolutions/IAU2006_Resol3.pdf
Referenced by pages:
astronomical catalog
epoch B1950.0
Julian date (JD)
photographic zenith tube (PZT)
pulsar timing array (PTA)
solar time
Terrestrial Time (TT)
timescale (t)
Index