Re: UT v.s. UTC in Guide8b

toadatrix May 27, 2004

I agree with Bill A on this, but I made the following change to his
formula for DeltaT:

DELTA_T=2002,2100:63.829,18.62,0;2100,100000:-20.051,70.0,32.5

This modification avoids the large discontinuity in DeltaT after
2100 and also restores the second order deceleration term, suggested
by Stephenson and others, for distant dates more than 100 years
after 2000.

Another good candidate for DeltaT might be the following formula:

DELTA_T=2002,100000:63.829,17.325,32.5

This keeps the 32.5 second order term, is generally within the eror
range of the now outdated USNO predictions through the year 2010,
but still uses a lower linear term to better reproduce the last few
years observations.

Based on the latest USNO IERS Bulletin the value of DeltaT at May
27, 2004 was 64.652577 if anyone cares about that many decimal
places.

--- In guide-user@yahoogroups.com, "wcappwca" <wcapp@h...> wrote:
> --- In guide-user@yahoogroups.com, Bill J Gray <pluto@p...> wrote:
> > 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
> >
>
> I tried this and noticed a small point:
>
> since T=(y-2000)/100 the above is equivalent to
> 64.574 + (y-2000)*0.1862; for 2004 this is 65.3118
>
> since the value for 2004 should be 64.574, I believe the constant
> should be changed to 64.574 - (4*0.1862) = 63.8292
> 63.8292 + (y-2000)*0.1862; for 2004 this is 64.574
>
> Also, my copy of Guide apparently doesn't have the table entry for
> 2004 yet so I changed the 2004 to 2002 and my final formula was
>
> DELTA_T=2002,2100:63.829,18.62,0
>
> Bill A.