Re: [LEAPSECS] Precise time over time

From: Mark Calabretta <mcalabre_at_atnf.CSIRO.AU>
Date: Fri, 12 Aug 2005 10:14:03 +1000

On Thu 2005/08/11 12:37:52 MST, Rob Seaman wrote
in a message to: LEAPSECS_at_ROM.USNO.NAVY.MIL

>As far as the maximum permitted size for DUT1 - some think 0.9
>seconds is too small. There appears to be a consensus among quite
>divergent thinkers here that 0.9 hours is much too big. I imagine
>most astronomers would be willing to entertain intermediate values.

But, thinking particularly of the piece by Jan Meeus (on this list
recently via Christian Steyaert), I expect that many astronomers would
be quite irritated by that.

However, allowing DUT1 to grow potentially to tens of seconds would be
more patatable than the current leap hour proposal, so a predefined
long-term extrapolation, as described by Steve Allen (Aug/5), seems
possible as a compromise.

However, the question that naturally arises is the required timescale
of the extrapolation. A figure of 50 years seems first to have been
suggested by Poul-Henning Kamp (Aug/04, "My personal opinion is that 50
years seems right, 20 years might be livable") and since seems to have
become set. However, I question the need for such a long extrapolation.

What systems being constructed now will need to know the time to the
nearest second for 50 years without the possibility of being updated?
Cast your minds back to 1955; the state of technology then; what was
built then that is still running now. If the extrapolation could be
reduced the potential excursion of DUT1 would also be reduced. I think
the idea would be much more saleable with an initial timescale of, say,
20 years, extended by 5 years every 5 years. So at any time the
extrapolation would range between 15 and 20 years.

Mark Calabretta
ATNF
Received on Thu Aug 11 2005 - 17:15:51 PDT