I feel obligated to reply lest someone think Poul-Henning Kamp scored
a point off me:
Pop quiz! What is the length of the day? No tricks - no gimmicks.
Launch a tee-shirt to that guy in the last row. Right you are! A
day on Earth is 23h 56m 4s.
Now, for two t-shirts:
Which fraction of the earths population would disagree with the
answer that astronomer gave ?
Right there in front: 99.999% sounds about right.
This was, of course, exactly my point. We wouldn't even be having
this discussion if the length of an SI second hadn't been chosen to
approximate a "solar" second. Four minutes per day would certainly
be too much to tolerate - but so would 1s per day. Some may think
that solar time is a fantasy - but the length of a solar day forms a
very precise limit on civil timekeeping.
One could - and often does at a telescope - use a sidereal time scale
in which 24 hours of 60 minutes of 60 seconds passes in the time that
it takes for the Earth to rotate once with respect to the stars. One
sidereal day of 86400 sidereal seconds. Few other than astronomers
use such days or such seconds. (Although sidereal signals are easily
detected at radio frequencies.)
Similarly, only a handful of engineers, programmers and scientists
use what might be called "atomic" days.
Everybody (100.000%), however, uses solar days. And the cleverness
of UTC is that they are able to use atomic (SI) seconds at the same
time. The best of both worlds often comes with a price. That price
is the leap second.
The ITU proposal isn't a challenge to leap seconds issued every few
hundred days. The proposal is a challenge to the integrity of each
day that dawns. And if such a major change is to be made to a
fundamental international standard, the process used to decide this
new policy should represent the best of international cooperation -
not the sneakiest.
Anybody want to try for three tee-shirts?
What fraction of the Earth's inhabitants believe that a day is
defined to be a solar day?
Rob Seaman
National Optical Astronomy Observatory
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