Mike Williams wrote:

Are you certain that your values for "proper motion" and "parallax"
have the correct units for the equation you're using? I use a more
direct method and get a vastly different answer.

There are two errors in your calculation, both of which inflate the
component of motion in right ascension.

-7.54775 arcsecs/year of RA is -0.000549399 radians/year
(Note a complete circle is 24h of RA but 360d of Dec)

It isn't necessary to convert -7.54775 from hours:minutes:seconds to
degrees:minutes:seconds. It's already expressed as arcseconds in the
d:m:s system.

But you do have to multiply it by the cosine of Alpha Centauri's
declination. To see why, consider the surface of the Earth. Degrees
latitude (north-south) always correspond to a surface distance of about
110 km, but the surface distance for a degree of longitude depends on
the latitude. It's 110 km at the equator, where cos(lat) = 1, but
smaller than 110 km by the factor cos(lat) at other latitudes.

The declination of Alpha Centauri is -60° 50', and cos(-60° 50') is
about 0.487.

So your figure for radians/year in RA is too big by a factor of about
30: (360 / 24) * (1 / cos(-60° 50')).

The formula Abdul used is pretty standard, and simpler to apply. You
can divide by the parallax (in arcseconds) or multiply by the distance
(in parsecs).

4.74 is just a constant of proportionality that converts between AU/year
and km/s. An object at a distance of 1 parsec with a proper motion of 1
arcsecond/year has a transverse motion of 1 AU/year, or 150 million
km/year, or 4.74 km/s.

- Ernie http://home.comcast.net/~erniew