On Wed 2015-04-29T21:03:09 -0700, Dennis Ferguson hath writ: > On 29 Apr, 2015, at 18:01 , Mike Lawson <[email protected]> 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 <[email protected]> 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 _______________________________________________ LEAPSECS mailing list [email protected] https://pairlist6.pair.net/mailman/listinfo/leapsecs
