On Mon 2006-01-09T08:20:40 +0100, Poul-Henning Kamp hath writ: > beginning ("SI seconds are constant length").

Yes, SI seconds are constant length, but the ghost of my general relativity teacher prompts me to assert that my SI seconds are not equal to your SI seconds because we are in different reference frames. The rate at which TAI ticks has been modified several times to meet improved notions of whose SI second it should really try to match. The current notion is that of a coordinate time scale at a depth in the geopotential field which is close to mean sea level. Such a coordinate time scale cannot be extended very far from the surface of the earth without requiring some fascinating corrections to the rates measured by different observers. Tom Van Baak can show you how measuring this is now child's play. Why should my lab use TAI when the proper time experienced by my real-time control processes runs at a different, and continually varying, rate? The answer is the same as for UT defined by Newcomb's expression used from 1901 through 1983 and implemented via astronomy: it is the most practical uniform time scale that we all can agree upon. For current purposes with stationary clocks the varying terms in the rate differences are immeasurable. In the limit of very precise lab timekeeping eventually the question arises as to whether TAI really is the most suitable scale for some applications. (This has nothing to do with leap seconds, but does raise the question of the limits at which it becomes much more difficult to agree on time.) -- Steve Allen <[EMAIL PROTECTED]> WGS-84 (GPS) UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99858 University of California Voice: +1 831 459 3046 Lng -122.06014 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m