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- barycentric dynamical time (TDB): dynamical time for barycentric phenomena which replaced Ephemeris Time
when the IAU 1976 System of Astronomical Constants was implemented in the Astronomical Almanac in 1984. The
difference between TDT and TDB is due to variations in the gravitational potential around earths orbit, and is always
less than 2 milliseconds. It is used as a time scale of ephemerides referred to the barycenter of the solar system.
- ephemeris time (ET): a uniform time measure now kept by atomic clocks. Ephemeris Time was used in the Astronomical Almanac from 1960-1983, but was replaced by Barycentric Dynamical Time when the IAU 1976 System of
Astronomical Constants was implemented in the Astronomical Almanac in 1984.
- Greenwich apparent sidereal time (GAST): Greenwich hour angle of the true equinox of date
- Greenwich mean sidereal time (GMST): Greenwich hour angle (GWA) of the mean equinox of date
- international atomic time (TAI): measured in the SI second
- mean solar time: a time based on a hypothetical sun moving uniformly along the celestial equator. The fluctuations have
ranged between ± 30 seconds over the last 300 years.
- terrestrial dynamical time (TDT): dynamical time for geocentric phenomena which replaced Ephemeris Time when the IAU 1976
System of Astronomical Constants was implemented in the Astronomical Almanac in 1984. It is used as the time scale
of ephemerides for observations from Earth's surface.
- coordinated universal time (UTC): differs from TAI by an integral number of seconds and is the basis of most radio time
systems and legal time systems. The step adjustments are usually inserted after the 60th second of the last minute of
December 31 or June 30.
- universal time UT0: a local approximation to universal time not corrected to polar motion.
- universal time (UT, UT1): the time since
 in the Greenwich time zone. 24 universal hours are a
mean solar day. UT can only be deduced from observations. Universal Time is tied to the rotation of the Earth, and the
Earth's rotation rate is rather irregular, thus unpredictable.
 Barycentric Dynamical Time, Calendar, Coordinated Universal Time, Ephemeris Time, Greenwich Apparent Sidereal Time, Greenwich Mean Sidereal Time, International Atomic Time,
Local Time, Mean Solar Time, Terrestrial Dynamical Time, Time,  UNIX Time,
Time Zone
Duffett-Smith, P. Practical Astronomy with Your Calculator, 3rd ed.
Cambridge, England: Cambridge University Press, p. 14, 1992.
HiLink Communications. "Local Times Around the World." http://www.hilink.com.au/times/.
NIST and USNO. "The Official U.S. Time." http://www.time.gov/.
Seidelmann, P. K.; Guinot, B.; and Doggett, L. E. "Time." Ch. 2 in Explanatory Supplement to the Astronomical Almanac (Ed. P. K. Seidelmann).
Mill Valley, CA: University Science Books, pp. 39-93, 1992.
United States Government Printing Office. The Astronomical Almanac for the Year 2000.
Washington, D. C.: U. S. Government Printing Office, 2000.
U.S. Naval Observatory. "U.S. Naval Observatory Master Clock." http://tycho.usno.navy.mil/what.html.
© 1996-2007 Eric W. Weisstein
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