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See  and the PDF file
.
The length of the astronomical day varies because of several different
phenomena including tidal friction and the exchange of angular momentum
between the Earth's core, mantle, and atmosphere. During the past 100 years or
so, the length of day based on the Earth's rotation has almost always been
longer than a day containing exactly 86,400 seconds (the UTC day). The shorter
UTC day requires the periodic insertion of leap seconds, the last one at the
end of December 1998. Over the past few years, the Earth's average rate of
rotation has increased slightly and consequently the need for an additional
leap second has not yet been announced.

-- Richard Langley

On Tue, 27 Jul 2004, George Huxtable wrote:

>On 24 July Paul Hirose wrote-
>
>>A few days ago the IERS announced there would be no leap second at the
>>end of this year. We will have a record-setting run of six
>>consecutive years with no leap second. The previous longest break was
>>1986 and 1987.
>>
>>At present UT1 is .46 second behind UTC. The offset is predicted to be
>>.49 second at the end of the year, and .52 second in mid-2005.
>
>Does this imply, then, that delta-T, the difference between universal time
>and ephemeris time, has remained "stuck" at a constant, or nearly-constant
>value, over that period of six years? If so, what vaue has it stuck at?
>
>And if so, does anyone understand the geophysical causes that have resulted
>in the slowing of the Earth's rotation, which has been so consistent over
>the last 80 years or so, to suddenly switch off, 6 years ago?
>
>Or have I misunderstood something completely?
>
>George.
>
>================================================================
>contact George Huxtable by email at george@huxtable.u-net.com, by phone at
>01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
>Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>================================================================
>


===============================================================================
 Richard B. Langley                            E-mail: lang@unb.ca
 Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
 Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
 University of New Brunswick                   Fax:      +1 506 453-4943
 Fredericton, N.B., Canada  E3B 5A3
     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
===============================================================================

   

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