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Re: Star proper motion
From: Bill Noyce
Date: 2001 Nov 20, 7:42 AM
From: Bill Noyce
Date: 2001 Nov 20, 7:42 AM
> 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