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On 2019-03-11 20:01, Bill Bertrand wrote:
>    -- The j1991.25 Dec value is listed (it differs ~3x10^-5) degrees for the 9 year difference

Here are Bellatrix data from the original (1997) Hipparcos catalog and
the 2007 re-reduction. Values are RA, dec, parallax (mas), RA PM
(mas/yr), dec PM at J1991.25. The RA and dec I have converted to
sexagesimal at sufficient precision. Note that RA proper motion is great
circle units, not coordinate units. I.e., the unit is the same size as
the declination proper motion.

081.28278416 +06.34973451 13.42 -8.75 -13.28 (1997)
(05 25 07.868 +06 20 59.05)

081.28278339 +06.34973457 12.92 -8.11 -12.88 (2007)
(05 25 07.868 +06 20 59.05)

I don't see those coordinates in the supposed Hipparcos data you quoted.
Where are they?

In addition, the "RA/Dec (on date)" aren't meaningful without knowledge
of the basis of the coordinates — the stuff I mentioned in the previous
message, such as astrometric place vs. apparent place and ICRS vs. true
equator and equinox. Until you resolve that question, it's futile to
compare those values to yours.

Roger thinks they may be apparent place, but if so, they aren't very
accurate — about 7 arc seconds from my computation with respect to the
true equator and equinox.

   

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