Leap second list

Phil R. Karn (karn@jupiter.bellcore.com)
Mon, 4 Jan 88 17:09:00 est

I just had a most enjoyable chat with Mr. Kerry Kingham of the US Naval
Observatory. Kerry provided me with the following list of leap seconds.
Each took place on the very last minute of the last day of the indicated

June 1972
December 1972
December 1973
December 1974
December 1975
December 1976
December 1977
December 1978
December 1979
June 1981
June 1982
June 1983
June 1985
December 1987

It turns out that there is a very amusing reason for the obvious patterns
you see here. The original BIH policy was to declare leap seconds at the end
of the year whenever possible, using the June date only when necessary to
stay within the +/- 0.8 second UTC-UT1 limit.

However, it seems that the French eventually rebelled at having to come in
to work, thereby missing their New Year's Eve parties year after year, so in
1981 they decided to "anticipate" the need for a year-end leap second by
doing it in June. That one took UTC as far behind UT1 as it has ever been
(-0.78 seconds), just barely within the limit. (Fortunately WWV UT1
corrections (which only go to +/- 0.7 sec) could still handle this because
their absolute values are truncated down to the next lower 100 ms multiple.)

I mentioned the griping we've heard from various parties about what leap
seconds do to radio clocks, and he says "Oh, that's nothing". He then told
me about how several very upset people called him up once to ask why there
was no leap second in that year. It seems that they had systems with a
hardwired "positive leap second every December 31st" rule -- and some of
those systems were in orbit! He then took great pleasure in letting them
know that it is also possible to have NEGATIVE leap seconds...

Kerry also improved my understanding of UT1. It is based on raw observations
of the earth's rotation angle (defined as UT0) corrected for the effects of
polar wandering. The earth's rotational poles wander over a roughly
circular path every 14 months or so. This is caused by changes in the
earth's mass distribution (solar tides, seasonal atmosphere mass shifts,
melting snow caps, etc). The amplitude of the polar variation is roughly the
size of a baseball diamond, and it causes small changes in the observer's true
latitude and longitude -- therefore affecting the observer's astronomical
observations on the order of +/- 30 ms.


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