Space Forum / Shuttle / June 2005

William Kahan gives a stable formula at this link:
http://www.cs.berkeley.edu/~wkahan/Math128/angle.pdf
Note that he doesn't include the radius of the planet, you have to multiply
his 'v' (in radians) by the radius of the earth.
He's giving the angle between two stars using  alt-azimuth coordinates, but
his 'a' is the longitude difference, 'h' the latitude difference, and 'H'
the latitude of the first point in your application. You should filter out
the trivial cases where the tangents blow up, but the formula seems to work
anyway even if you don't (in Open Office 1.3).

Murray Anderson