On Fri, 3 Sep 2010, Zefram wrote: > 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.
Yet more interesting stuff, thanks again! Another thing to consider is that the last practical astronomical realisation of GMT was carried out at Herstmonceaux not Greenwich, and some errors were introduced when time was transferred from one to the other which is part of the reason for the discrepancy between WGS84 and the Airy circle. Also I understand that GMT included some corrections similar though not identical to UT2 - see also Steve's little history he sent yesterday which said CCIR rec. 374 required time signals to be within 100ms of UT2 between 1963 and 1970. But this is mostly beside the point, which is that there is no organization maintaining and promulgating an official astronomical GMT. You could create a close approximation to an astronomical GMT but it would have no official force. As we have seen there are a lot of intricate details whose necessity people can legitimately disagree about and no way to determine an official consensus. Which is why I say that astronomical GMT doesn't exist. Tony. -- f.anthony.n.finch <d...@dotat.at> http://dotat.at/ HUMBER THAMES DOVER WIGHT PORTLAND: NORTH BACKING WEST OR NORTHWEST, 5 TO 7, DECREASING 4 OR 5, OCCASIONALLY 6 LATER IN HUMBER AND THAMES. MODERATE OR ROUGH. RAIN THEN FAIR. GOOD. _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
