On Wed, 13 Feb 2008, Rob Seaman wrote: > > The "day" is a key concept in our civilization. The "mean solar day" is > the natural way to implement this. Sundials have nothing to do with the > mean solar day, but rather the apparent solar day.
How does the mean solar day relate to ephemeris time? Between the period where time signals were based on observing solar transitions and the period where time signals were based on atomic clocks, official time was ephemeris time. ET is more uniform than the Earth's rotation so a second of ET is not 1/86400 day any more than an SI second is - and of course the final definition of the SI second was chosen to match ET. If we can imagine what things might have been like if essens had a different period, can we imagine how astronomers would have handled the discrepancy between ET and UT1 in the absence of atomic clocks? Tony. -- f.a.n.finch <[EMAIL PROTECTED]> http://dotat.at/ WEST VIKING: NORTHEASTERLY VEERING SOUTHERLY 3 OR 4. ROUGH. FAIR. GOOD. _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs