How do I calculate the difference between ephemeris time and universal time?

I looked for a general formula in the
most reliable astronmical data set I know, the Astronomical Almanac (a
yearly almanac of astronomical events and data). I am sorry to report
that it is not possible to determine delta T very accurately for future
times.

As you pointed out, delta T is the difference between Terrestial
Dynamical Time (formerly Ephemeris Time, but corrected to agree with the
SI definition of a second and general relativity) and Universal Time.
TDT is well known; it is international atomic time TAI + 32.184 seconds,
and is easily extrapolated into the future by allowing one day to be
86400 seconds. However, Universal Time is not very accurate. Here is a
quotation from a web site describing Universal Time:

"The actual universal time (denoted UT or UT1) is tied to the rotation of
the Earth. Because the Earth's rotation rate is rather irregular and
unpredictable at the 0.1 s level, Universal Time can only be deduced from
observations of star transits. Once known, UT can be compared with
known Ephemeris Time, and the difference can be derived. UT is always
kept within 0.9 seconds of Coordinated Universal Time (what
WWV and other time broadcast services provide) by the insertion or
deletion of Leap Seconds, usually at 23:59:59 UTC on either June 30 or
December 31."

So, only observations can locate future UT accurately; extrapolation is
just not possible. Here is an example from two different astronomical
almanacs on extrapolated delta T:
Year Estimated Actual
1996 61.63 s 61.63 s
1997 63 s 62.29 s
1998 64 s ~63 s.
So, in just two years of extrapolation, differences of greater than one
second are obvious. But, you might be able to estimate delta T: from 1902
to 1992, delta T changed from 0.02 s to 58.31 s.

I hope this answers your question, and sorry I couldn'
t give you a formula!

Sincerely,
Kurtis Williams

On Sun, 14 Feb 1999, ALI ALHADAD wrote:

"A man is like a fraction whose numerator is what he is and
whose denominator is what he thinks of himself. The larger the
denominator the smaller the fraction. " -- Lev Tolstoy

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