Re: [LEAPSECS] Risks of change to UTC

From: David Malone <>
Date: Mon, 23 Jan 2006 11:08:29 +0000

> Professional and amateur astronomers are not the only ones who need good
> estimates of UT1.

I've been wondering about this for a bit. Do astronomers and
navigators actually want UT1 or do they want GMST? Since UT1 is
based on a mean sun, which I guess no one actually observs, it would
seem that GMST would be much more useful for figuring out your
position or observing something.

As far as I can see from my 1992 edition of the Explanatory Supplement
to the Astronomical Almanac, UT1 and GMST were (defined?) to be
related to one another by a cubic (2.24-1):

GMST1 of 0hUT1 =
        24110.54841s + 8640184.812877s T + 0.093104s T^2 + 0.0000062s T^3

What I don't know is: are the coefficients of this equation constant,
or periodically updated by the IAU? Do astronomers, navigators and
almanacs have to update their calculations when/if the IAU make a

Why do I think they may change? Well, In older explanatory supplements
to the IERS bulletins, such as this one:

they give a reference for the relationship used as a paper by Aoki
et al., 1982. However, in this more recent explanatory supplement:

the relationship seems to have been changed to ones documented in
(Capitaine et al., 2000, Capitaine et al., 2003, McCarthy and Petit,
2004). They say that that "This relationship was developed to
maintain consistency with the previous defining relationship", but
I think this probably means that they were stitched together in a
smooth way, not that they are identical.

If it is the case that the GMST/UT1 relationship is changed regularly
and astronomers/navigators have to keep up with those changes, then
leap seconds could be put into this relationship (amounting to
moving the mean sun when needed).

I'm guessing that this suggestion is only slightly less crazy than
strapping rockets onto the Earth to speed up its rotation ;-)

Received on Mon Jan 23 2006 - 03:08:48 PST

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