NavList:

   

Reply

In the discussion on Henning Umland's almanac, Dave Walden posted
geocentric apparent coordinates (true equator and equinox) for Mercury
at Feb 1 1100 UT from several sources:

19h14m15.421s   -20°38′03.89″  USNO
19 14 15.4172   -20 38 03.896  HORIZONS
19 14 15.42394  -20 38 03.8918 IMCCE

The discrepancies are insignificant for navigation. Mainly they're due
to different precession and nutation models. However, if you need high
accuracy it's possible to adjust the coordinates to a common basis.

HORIZONS: first apply the IAU 1980 nutation model in reverse (i.e.,
convert from true to mean equator and equinox of date). Include the
celestial pole offsets published by the IERS, by adding them to the
nutation angles from the 1980 model. Then apply the IAU 1976 precession
model in reverse to obtain the apparent place of the body in the GCRS.
 From the GCRS, apply your precession and nutation model of choice.

IMCCE: follow the same procedure, except DO NOT apply the celestial pole
offset corrections. In other words, simply apply IAU 1976/80 precession
and nutation in reverse.

I have no procedure for USNO coordinates, since I have not coded their
precession - nutation model. (It has been published, however.)

Here are Dave's coordinates after conversion to the GCRS. For comparison
I include my own coordinates.

19h13m18.3980s -20°39'54.627 HORIZONS
19h13m18.3980s -20°39'54.625 IMCCE
19h13m18.3979s -20°39'54.626 me

The IAU SOFA library and my SofaJpl DLL for Windows have all the
necessary tools for the transformations. But note that SofaJpl's
implementation of the 1976 precession model includes frame bias. You
must remove it from the precession matrix. Fortunately that's easy.

   

Reply