[LEAPSECS] Question about UT1 and the IERS Reference Meridian

Steve Allen sla at ucolick.org
Thu Apr 30 00:57:44 EDT 2015

On Wed 2015-04-29T21:03:09 -0700, Dennis Ferguson hath writ:
> On 29 Apr, 2015, at 18:01 , Mike Lawson <mflawson2 at cox.net> wrote:
>> So, what I take from this is, 1) UT1 (and hence UTC) is based on
>> the International Reference Meridian, not the Prime Meridian
>> (Greenwich).  2) “True” GMT (if the mean sun can accurately be
>> measured and predicted by formulae, which has its own set of problems
>> and error bars) lags UT1 by about 0.36 seconds; but it doesn’t really
>> matter, since the whole world is now based on the IRM.
> That's correct except that I don't think the 0.36 seconds is
> seconds of time, it is angular seconds.  That would be a
> 24 millisecond time difference.

No.  Look at Google Earth, Google Maps, or your favorite digital world
map.  The slide-open roof of the transit building at the Observatory
in Greenwich is 5.36 arc seconds of longitude west of the GPS
meridian, which is for practical purposes the same as the
International Reference Meridian.  That is a difference of 0.36 s,
and that is what the final paragraph before the acknowledgments
is talking about.

The main point of the Seidelmann and Seago paper is not the offset of
the one terrestrial meridian from the other, but the offset of the
time of mean noon from the International Reference Meridian which is
implicitly the origin that goes with UT1.  The mean transit currently
occurs about 0.18 s after noon UT1.  The significance of that is that
the meridian on which one would stand in order to have the mean sun
overhead at noon UT1 is 0.18 s farther east than the International
Reference Meridian.

Steve Allen                 <sla at ucolick.org>               WGS-84 (GPS)
UCO/Lick Observatory--ISB   Natural Sciences II, Room 165   Lat  +36.99855
1156 High Street            Voice: +1 831 459 3046          Lng -122.06015
Santa Cruz, CA 95064        http://www.ucolick.org/~sla/    Hgt +250 m

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