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Subject: Re: Star proper motion
From: Noyce, Bill (William.Noyce@XXX.XXX)
Date: Tue Nov 20 2001 - 09:42:07 EST
> The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version)
> (Hoffleit+, 1991)
> Position (J2000) RA 6h 45min 8.9sec Dec -16 42' 58"
> Proper motion RA -.553 arcsec/a Dec -1.205 arcsec/a
>
> data from SKY2000 - Master Star Catalog (Myers+ 1997)
> Position (J2000) RA 6h 45 min 8.821sec Dec -16 42' 56.85"
> Proper motion -0.0385 arc/a Dec -1.205 arcsec/a
Since the difference is close to a factor of 15, it looks
to me as if -.553 is measured in arcseconds, but -0.0385
is measured in seconds of time.
There's a further difference to be aware of: I believe the
-.553 measures the speed of the star in the RA (or -SHA)
direction, but that's not quite the same as saying it's the
amount to add to RA each year. Consider it a 'departure'
rather than a 'delta-longitude', and divide it by cos(decl)
first.
I think if you make both these corrections, the data will
start to agree better.
PS- If you're calculating star positions, be sure to consider
precession (about 1'/year) and annual aberration (about 0.3')
which both have a greater effect than proper motion.
-- Bill
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