Dynamical time scale
This article provides insufficient context for those unfamiliar with the subject.(December 2015) |
In
In the late 19th century it was suspected, and in the early 20th century it was established, that the rotation of the Earth (i.e. the length of the
Using data from
- the fraction 1/31,556,925.9747 of the January 0at 12 hours ephemeris time.
In 1976, however, the
During the period 1991–2006, the TDB and TDT time scales were both redefined and replaced, owing to difficulties or inconsistencies[further explanation needed] in their original definitions.[citation needed] The current fundamental relativistic time scales are Geocentric Coordinate Time (TCG) and Barycentric Coordinate Time (TCB); both of these have rates that are based on the SI second in respective reference frames (and hypothetically outside the relevant gravity well), but on account of relativistic effects, their rates would appear slightly faster when observed at the Earth's surface, and therefore diverge from local earth-based time scales based on the SI second at the Earth's surface.[4] Therefore, the currently defined IAU time scales also include Terrestrial Time (TT) (replacing TDT, and now defined as a re-scaling of TCG, chosen to give TT a rate that matches the SI second when observed at the Earth's surface),[5] and a redefined Barycentric Dynamical Time (TDB), a re-scaling of TCB to give TDB a rate that matches the SI second at the Earth's surface.
See also
References
- ^ Bibcode:1988A&A...194..304G. at p.304
- ISBN 0-935702-68-7.
- ^ W Markowitz, 'Variations in the Rotation of the Earth, Results Obtained with the Dual-Rate Moon Camera and Photographic Zenith Tubes', Astron J v64 (1959) 106-113.
- ^ See S Klioner et al., "Units of relativistic time scales and associated quantities", IAU Symposium 261 (2009).
- ^ IAU 2000 resolutions, at Resolution B1.9.