[LEAPSECS] LEAPSECS Digest, Vol 45, Issue 1

[Zefram]

Tony Finch wrote:

>Thanks for the informative explanation, but GMT is not and was not UT1.


Picky, picky.  OK, let's look at the strictest sense of "GMT", taking the
Greenwich meridian to be defined by the Royal Observatory, Greenwich,
rather than by the ITRF.  Specifically, the 1851 meridian defined by
Airy's transit circle, adopted as the prime meridian by the International
Meridian Conference in 1884.

A strict-GMT clock can be constructed by applying a GMT-UT1 correction
on top of the UT1 clock that I have already discussed.  This correction
corresponds to the ITRF-based longitude of the Airy meridian.  Wikipedia
gives a current value for this of -5.31 arcseconds, corresponding to
-354 ms of solar time.  (The Airy meridian is west of the ITRF meridian;
GMT is behind UT1.)

The ITRF-relative location of the ROG is gradually changing,
northeastwards, due to tectonic motion.  This increases the GMT-UT1
correction (decreasing its magnitude, because it is negative) by tens
of microseconds per annum.  This motion is much more regular and more
predictable than the fluctuations in UT1.  Presumably regular measurements
of Europe's motion can be found somewhere online, but I don't know where,
which is why I've referred to Wikipedia.  Even if not, static model
parameters would be good for many years for millisecond precision.

The next wrinkle is polar motion.  So far I've assumed that the direction
of the meridian through ROG is constant.  If you really want mean solar
time as subjectively viewed at ROG, another correction is required,
corresponding to the difference between UT0-at-ROG and UT1.  This could
be determined from Bulletin A, which projects (x, y) in addition to DUT1.
The correction is in the low tens of milliseconds.

Observation: the difference between strict GMT and UT1 is bigger than I
was really expecting, having not looked at the numbers before.  Plenty
big enough to discern on a real-time clock display.  Anyone wanting to
implement mean solar time distinct from UTC does have to decide between
UT1 and ROG-based GMT.

-zefram