[LEAPSECS] Mean ... Orbits

Steve Allen sla at ucolick.org
Fri Feb 4 13:48:23 EST 2011

On Fri 2011-02-04T09:44:37 -0700, Warner Losh hath writ:

> It looks like it would be possible to define a mean solar time that had

> different averaging characteristics, or tweaked the parameters of the

> model used in either simpler or more complicated ways than Newcomb's

> equations, but nobody has done it (or if they have, it isn't widely

> enough adopted to warrant mentioning in the references people have sent

> to the list).

Yes, they have done it. That was the notion behind UT2 at its
inception, a value of time smoothed as much as models would permit.
The idea was that this was the most uniform kind of UT, and thus the
most suitable form for use in broadcasts. That's why the CCIR
changed their recommendation for broadcast time to specify that
the broadcasts should provide UT2.

But cesium chronometers became available at the very same IAU meeting
which defined UT2, and by the time cesium had been calibrated with ET
and UT2 it was already evident that cesium revealed the earth, and
thus UT2, was badly irregular. So the time bureaus recognized that it
was impractical to follow the CCIR recommendation, that broadcast
signals wanted to be as regular as possible, and much more regular
than UT2. So for the sake of broadcasts the time bureaus created a
cesium-regulated time scale aimed at roughly following UT2.

In the 1960s there was not enough telecommunications bandwidth nor
compute power to divorce broadcasts from UT without confounding all
traditional applications of broadcast time signals.

The difference between now and then is that now we do have enough
telecommunications and compute power to broadcast completely uniform
time signals while still keeping something that tracks UT in a
smoothed sense. If we divorce the broadcast time signals from UTC
then the kernel hackers can be instantly happy with their uniform
underlying time and we can, at our leisure, decide just how
predictably smooth UTC has to be to suit all applications.

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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