2005/02/18 | 时间[Time]
类别(¤〖天文〗) | 评论(0) | 阅读(33) | 发表于 14:14
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. Only after the observations of star transits can Universal Time be deduced. Once known, UT can be compared with known ephemeris time , and can be derived. UT is always kept within 0.9 seconds of UTC (what WWV and other time broadcast services provide) by the insertion or deletion of leap second,usually at 23:59:59 UTC on either June 30 or December 31.
Differences between time standards are

The predicted value of rounded to 0.1 s and given in some radio time signals called DUT.



used prior to 1984,

currently used

and

the increment to be applied to UTC to give UT.
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