Re: Online calculators for position angle

Bill Owen wrote:

> > angular dist = arccos ( sin(dec1)*sin(dec2) +
> > cos(dec1)*cos(dec2)*cos(ra1-ra2) )
>
> This formula is mathematically correct, but it's computationally
> unreliable for small separations (think double stars). ...


William Kahan, a numerical analyst of some renown, obsesses over such
computational issues. I recently ran across his original solution to
this very problem -- a solution he claims is both accurate for *any*
angular separation and efficient:

http://www.cs.berkeley.edu/~wkahan/Math128/angle.pdf

(In these lecture notes, Kahan mistakenly says that azimuth is
measured from compass north; his formulae are valid nevertheless.)

--
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Mark Gingrich grinch@xxxxxxxxx San Leandro, California
.