[LEAPSECS] Question about UT1 and the IERS Reference Meridian

[Mike Lawson]

And later, I found a reference which further answered my own question -  Seago, John H.; Seidelmann, P. Kenneth, The mean-solar-time origin of Universal Time and UTC -  I see this conclusion:

“[...] UT1 still appears synonymous with "mean solar time at the prime meridian" to within a fraction of a second, despite the definition and maintenance of UT1 having evolved significantly over the past century.  The "prime meridian" in this case implies the International Reference Meridian that includes the terrestrial origin from which Universal Time is measured, and which has advanced approximately 0.36 [seconds] to the east of the internationally recommended origin of longitude -- the Airy transit instrument at Greenwich.  This translation resulted from changing conventions and methods by which the relationship between Universal Time and the terrestrial reference were maintained over time by the Bureau International de l'Heure.”

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.
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