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On 2018-08-29 17:30, I wrote:
> Note that JPL's Horizons online calculator won't exactly agree with all
> my numbers, mainly due to a different precession / nutation model.

Since Horizons is a free and authoritative calculator of solar system
body positions, I thought this discrepancy with respect to my SofaJpl
demonstration program ought to be remedied. Therefore, the entire
precession and nutation computation has been revised, including a
Horizons "compatibility switch" in the source code. If turned on, the
program is compiled with the IAU 1976 precession model. Then nutation
angles are applied to put the pole in the same place as the default
2006/00B precession / nutation model.

That yields equatorial and ecliptical coordinates within 1 mas (milli
arc second) of Horizons. Azimuth and elevation are within .0001°, which
is all the precision available from Horizons. (But for that accuracy you
must set the UT1-UTC offset in the place provided in the source code.)

The IAU 2006/00B model remains the default, and that's what I recommend
unless you need compatibility with Horizons. It is true that you get the
same pole in either mode, but the two coordinate systems have a small
rotational difference about the pole. The 2006/00B model should better
match the true direction of the equinox.

For utmost accuracy you can substitute the IAU 2000A nutation model.
Combined with IAU 2006 precession, it predicts the pole with an accuracy
of about .2 mas in the current era. It's available in SofaJpl. In the
demo program, simply substitute 'A' for 'B' in the name of the nutation
routine.

There's a major increase in computational complexity since the 'A' model
evaluates about 1400 trigonometric terms. I can't imagine you'd notice a
difference in a "one shot" run like the demo program, but nevertheless I
see no need for such elaboration. The default 'B' model is accurate to
"only" 1 mas, which is enough for me. And it's even smaller than the old
IAU 1980 nutation model.

Note that there has been NO change to SofaJpl itself. The current
version is still 2.1.2. Only the C#, C++, and Visual Basic source code
for the demonstration program have changed.

http://home.earthlink.net/~s543t-24dst/SofaJpl_NET/index.html

   

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