Hi John,

The problem is in Guide's table for Delta-T. It currently looks like
this, with data up to 1 January 2004 coming from actual observations
and the 2006, 2008, and 2010 data coming from USNO predictions made
in 2001:

Year Delta-T

1990 56.8554
1992 58.3093
1994 59.9847
1996 61.6287
1998 62.9659
2000 63.8285268
2002 64.2998152
2004 64.5736400
2006 67.6 +/- 2.
2008 68.9 +/- 3.
2010 70.2 +/- 5.

The problem comes from those predictions, made at a time when
Delta-T was steadily increasing at about a second a year. USNO has
discontinued these predictions, so I can't replace those values with
something more current. For dates near the present, Guide is
interpolating between the (perfectly reasonable) 2004 value and the
(now not-so-reasonable looking) 2006 value. The end result is: Guide
thinks Delta-T is growing pretty briskly, so it's getting out of sync
with UTC.

Fortunately, there is a way around this problem. We can tell Guide
what formula to use for Delta-T, as described at

http://www.projectpluto.com/update7.htm#delta_t_def

What I'd suggest doing is to tell Guide that, for dates from 2004
on, Delta-T is increasing linearly at its current slow rate. That
means hitting Alt-J and entering the following:

DELTA_T=2004,2100:64.574,18.62,0

You will still eventually see discrepancies between UT and UTC that
are greater than .9 seconds. But attempting to evade such discrepancies
does not strike me as especially realistic. It would be more of a
cosmetic fix than anything resembling "what will actually happen".

More specifically, if someone tells you they used, say,
Delta-T=64.9 seconds, and you want to replicate their results,
you could "fix" Guide's Delta-T at a constant 64.9 seconds with

DELTA_T=2004,2100:64.9,0,0

Incidentally, "UT" in Guide means "UT1". (UT0 is time corrected for
the observer location on the earth, with the effects of polar motion
included. UT1 is a "standardized" sort of UT for a non-wandering pole.
UT2 is essentially obsolete. The difference between UT0 and UT1 will
become crucial if Guide is to be accurate at the level of a
few centimeters, instead of at the level of a dozen meters. So far,
the only Guide user with an interest in that level of accuracy has been
its author. I'd like to be able to replicate the laser ranging
observations made of the mirrors left by the Apollo missions.)

-- Bill