On 2019-12-16 19:03, Brian Inglis wrote:
Just to muddy things further, in an article in "Polar Motion: Historical and Scientific Problems", ASP Conference Series, Vol. 208, also IAU Colloquium #178. Edited by Steven Dick, Dennis McCarthy, and Brian Luzum. (San Francisco: ASP) ISBN: 1-58381-039-0, 2000., "History of the Bureau International de l'Heure", Guinot, B., p.181:
http://articles.adsabs.harvard.edu/pdf/2000ASPC..208..175G#page=7
Thanks for the reference!
...so all should bear in mind that TAI is a synthetic timescale calculated and adjusted in arrears, so shares some of the same problems as leap seconds but occurring at higher precisions.
"adjusted in arrears": The definitive values of TAI are fixed about a month after the fact (fast in astronomical terms!) by Circular T. A more uniform time scale can be obtained when more than a month's future data are used; such evaluations are done by the BIPM and published yearly as estimates TT(BIPM xx) of TT. "same problems as leap seconds": I do not think so. That is, no astronomical observations (eg, satellite orbits, pulsar timings) can currently reach or exceed the accuracy of the determination of TT with earth-bound clocks, and the upcoming use of optical clocks will lead to a significant increase in accuracy. Nor is there any sign of a systematic difference in rate between TAI and TT -- it would be "new physics", I guess. Michael Deckers.