On Fri 2005-08-05T10:16:51 +1000, Mark Calabretta hath writ:
> The fiction that there are 86400 "seconds" in a solar day is part of
> the problem - noone uses the word "second" in this context. Currently
> there are about 86400.003 seconds in a mean solar day but we use leap
> seconds so that we can carry on pretending that there are only 86400.
I hope Mark will pardon me for getting very pedantic here (but he
knows where I'm hoping to use that someday).
From the web page of the IERS at
http://hpiers.obspm.fr/eop-pc/products/combined/C04.html#plot
a recent plot of the excess length of day
http://hpiers.obspm.fr/eop-pc/products/combined/C04plot.php?date=2&tver=1&graphe=5&year1=2004&month1=1&day1=1&year2=2005&month2=8&day2=1&SUBMIT=Submit+request
demonstrates the following:
There are currently about 86400.0004 SI seconds of the time scale
Terrestrial Time in each "day of UT1", where the "day of UT1" is now
conventionally defined by the new IERS 2000 conventions which have
no relation to the orbital motion of the earth around the sun.
The 0.0004 means that it takes about 7 years of 365 days each to
accumulate a full second, and that's just what Daniel Gambis pronounced
on July 4.
These are not SI seconds of TAI, TCB, TCG, but of TT.
Given the current definition of UT1 it is really somewhat of a
misnomer to refer to "UT1 seconds" as mean solar seconds, but for the
next several centuries the difference is no worse than the original
expressions for mean solar time derived by Simon Newcomb.
--
Steve Allen <sla_at_ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99858
University of California Voice: +1 831 459 3046 Lng -122.06014
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
Received on Thu Aug 04 2005 - 19:36:43 PDT