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> *From: *François Goldite
> *Date: *2026 Sep 2, 14:20 -0700
>
> Good question! Yes, proper motion is in there — each star has its own
> Hipparcos proper motion baked in, applied right at the start of the
> chain before parallax, aberration and precession/nutation. So that
> part's covered.

A tricky point is that right ascension proper motion in the Hipparcos
catalog is great circle measure. It is not the coordinate rate. I.e., RA
and declination proper motions are the same size everywhere on the
celestial sphere.

Some years ago the Swiss Ephemeris software had a bug because RA proper
motion was calculated as a coordinate rate. This was not noticed until a
discussion of an ancient occultation revealed a small but definite error
in the position of Spica.

Another pitfall in the Hipparcos catalog is the epoch of the data:
J1991.25, not J2000.0. I believe the re-calculation by van Leeuwen is
also 1991.25. But if you get your data from the Simbad site, the data
are adjusted to 2000.0. (I have never understood why the original
Hipparcos catalog didn't do that.) At celestial navigation precision I
can't imagine a few years makes a difference, but if you're testing
software at high precision the tiny discrepancy can be puzzling.


I have not installed your software, but Dave Walden has sent me some
screen shots. The date formats are not consistent. The "Preparation"
screen displays 04/05/2025. But the Ephemeris screen says 2025-05-04.
Apparently the date is 2025 May 4, displayed as dd/mm/yyyy and
yyyy-mm-dd. However, in the US a third format, mm/dd/yyyy, is normal. I
am sensitive to this since I once worked in a place where all three
formats were in use. It depended on what form or computer program you
were using.

In my own software I use the ISO format: yyyy-mm-dd.

Another issue in the program I prefer to discuss privately. You can
contact me via my site.

--
Paul Hirose
sofajpl.com

   

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