[time-nuts] Last one - I promise (was Re: UTC)

Rob Seaman seaman at noao.edu
Tue Jul 26 19:26:56 EDT 2005

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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