[Spice_discussion] CSPICE gives wrong answers with ITRF93?
spice_discuss at barrycarter.info
Wed Dec 23 16:32:36 PST 2015
If I do the above after loading the appropriate C SPICE kernels
(de431_part-1.bsp, de431_part-2.bsp, pck00010.tpc, naif0011.tls,
libraries, etc, I get the position of Venus at the epoch as:
At the point 0N, 0E on Earth, the ITRF93 coordinates are:
and the positive x axis of ITRF93 points towards the zenith.
Subtracting, I get the ITRF93 coordinates of Venus (as viewed from 0N, 0E) as:
If I compute the angle between these coordinates and the x axis, I get
Thus, Venus is 43.6554 degrees from the zenith at 0N, 0E at the epoch. In
other words, Venus has an elevation of 46.3446 degrees.
Now, if I run HORIZONS with the attached parameters (see image), the
result line for the epoch reads:
2000-Jan-01 12:00 *m 242.8428 46.1041
The difference between the answer I got (46.3446 degrees) and the answer
HORIZONS got (46.1041 degrees) is small, but should be much smaller. Note
that I am using an airless model (no refraction) in HORIZONS, so that
shouldn't be the issue.
I've tried several variations (including using IAU_EARTH instead of
ITRF93), but run into similar problems.
What am I doing wrong?
My longer-term goal is to use CSPICE to compute sun/moon rise/set times to
at least the same precision as
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