Re: Who uses DUT1?

From: Steve Allen <sla_at_UCOLICK.ORG>
Date: Sat, 30 Jul 2005 23:31:52 -0700

On Sat 2005-07-30T10:18:42 -0700, Tom Van Baak hath writ:
> So my question is - is the actual value of DUT1,
> as broadcast with single digit precision, still
> used? And if so, from where do they get the
> value?

We do not use them for our telescopes at Lick, but our big telescope
is built out of battleship-era technology. Pointing to within 15 arc
seconds is quite acceptable for a telescope which is controlled by a
human operator.

This will change for the new telescope we are having constructed this
year. It is to operate entirely autonomously. Its software system
expects to download the predictions from the USNO website on a regular
basis. Given that we are not writing the software, we do not know
what sorts of alarm conditions it will set off if it cannot contact
the internet for an extended period.

The reports and proceedings of the IAU GAs in 1970 and 1973 make it
clear what the concerns regarding DUT1 were. I suspect that the
reports and proceedings of the CCIR from those years reveal the same
thing, the differences being in who realized which things first.

Initially the plan was to broadcast the difference from UT2, because
it had been the practice since 1956 for the broadcasts to give UT2.
The astronomers at the IAU GA in 1970 pointed out that for purposes of
navigation it was UT1 which was relevant, because navigators already
had to back out the UT2 correction to make most precise use of the
broadcasts. In practice that detail was moot for the broadcasts
because the value of |UT1 - UT2| never exceeds 0.35 seconds.

The broadcast values of DUT1 were instituted along with leap seconds
such that the radio broadcasts of time signals would remain just as
useful to the typical limits of human-sighted sextant readings as they
had been. That is to say, a skilled navigator might get to 50 meter
accuracy under the best of circumstances. My unpracticed star shots
were 20 times worse than that, so the DUT1 would never have benefitted

Steve Allen                 <>                   WGS-84 (GPS)
UCO/Lick Observatory        Natural Sciences II, Room 165       Lat  +36.99858
University of California    Voice: +1 831 459 3046              Lng -122.06014
Santa Cruz, CA 95064        Hgt +250 m
Received on Sat Jul 30 2005 - 23:32:05 PDT

This archive was generated by hypermail 2.3.0 : Sat Sep 04 2010 - 09:44:55 PDT