[LEAPSECS] LEAPSECS Digest, Vol 51, Issue 22

Steve Allen sla at ucolick.org
Mon Feb 7 12:58:16 EST 2011

On Mon 2011-02-07T12:21:08 -0500, Finkleman, Dave hath writ:

> Neiher Gravity nor the Geoid are "standardized."

The SI definition for TAI says "rotating geoid".

Resolution B1.9 (Re-definition of Terrestrial Time TT) from the XXIVth
IAU GA recognizes both that the geoid changes and that it is hard to
specify the geoid, therefore the rate of TT was changed from the
rotating geoid to a constant potential offset from the coordinate
time of the geocenter.

Bernard Guinot has stated his belief that TAI is supposed to be a
realization of TT, therefore the CCTF should incorporate the IAU
definition for TT into TAI, but that has not happened.

I suspect that CCTF has not acted because as a practical matter it
would make no difference. Until there is an ensemble of cesium
chronometers not on the surface of the earth there is no easy way to
measure the potential depth to 1e-10, so the corrections currently
being used to compensate for the NIST and PTB chronometers being about
a mile high are as good as things can get.

If it turns out that an actual measurement finds the potential of the
geoid to differ then that would require a notable rate change in TAI.
So the fact is that we should all be preparing our children for the
day when the rates of TT and TAI differ. Alternatively, the IAU might
redefine TT again. It's hard to say which would cause less grief.

Steve Allen <sla at ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855
University of California Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m

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