[ntp:questions] Re: GMT: UT1 vs UTC

Terje Mathisen terje.mathisen at hda.hydro.com
Fri Oct 31 09:56:24 UTC 2003

Rob Seaman wrote:
> UT1 is only known after the fact when observations can be consulted to
> derive the precise Earth orientation at a given epoch (and also remove
> the higher order terms that go into UT2).  That said, the precision
> timing community's predictions of UT1 have become exceedingly accurate.
> Take a look at Figure 2 from http://iraf.noao.edu/~seaman/leap.  (In the
> interest of full disclosure, you'll note that I have an axe to grind :-)

No problem, I happen to agree totally: Both that leap seconds is the 
only reasonable way to handle the issue, and that any changes should be 
limited to making the current system better.

I believe going to a monthly schedule would also break quite a bit of 
sw/hw, but even a quarterly schedule for leap seconds would suffice to 
reduce the maximum offsets quite a bit.

>>Why would you want one?
> Good question.  For many purposes raw UTC suffices.  An approximation
> of the Earth's orientation to one second of time is often adequate.
> For some purposes, it is not.  On the other hand, why would anybody

Afaik, not being an astronomer like you, the standard procedure is to 
use the current UTC, or if known, UT1 approximation, to point the 
telescopes in approximately the correct direction, then do any remaining 
fine-tuning by selection of one or more calibration stars.

I.e. effectively generating the local UT time scale, corrected also for 
stuff like temperature or Earthquake-caused twisting of the base of the 
observatory, right?

>>Even in England/Great Britain GMT is just a local name for UTC these days.
> Legally this is not true, as Ian points out:
>>Legally I think GMT is still UT1 --- the bill to redefine it as UTC
>>didn't pass in 1997.


"We are British, we don't care that we really don't have a choice, we're 
still not going to accept any kind of externally mandated rule."

> Imagine reintroducing such bills in countries around the world - with
> the express (or hidden?) intent being to get each country to switch
> from GMT to UTC simply such that UTC can be later stripped of any
> underlying connection to the spinning Earth.  Should make great
> theater on C-SPAN - at 3:00 am :-(
> Terje continues:
>>UT1 is for astronomers, as well as a way to figure out when/if to
>>add/subtract leap seconds.
> UT1, and Universal Time in general, is for everybody - at some level
> of approximation.  There is indeed an initiative among the precision
> timing community to eliminate leap seconds.  Such a move would

I was one of those who wrote comments to that "initiative":

Sometimes it is much better to do nothing, this is definitely one of 
those times.

- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

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