Formula for the initial course of a Great Circle Sailing Calculation

[MonkeyIslander]

So, this may seem a bit random, but I've been looking all over the place for the correct formula you use to work out "A" or the initial course of a great circle. The Norries, as usual only provides a somewhat cryptic formula that I can only get to work when both A and B are in the same hemisphere.

From what I remember, the formula is along the lines of:

Cos A = Cos PB - ( Cos PA x Cos AB ) / ( Sin PA x Sin AB )

But when I try it through a Great Circle Calculator (on the Internet) it comes up wrong. I don't suppose anyone could enlighten me? Thanks.

[Silvertop]

That is the correct formula. Maybe your internet calculator is wrong? Try doing it longhand!

[TAL]

Try this formulae always found it easiest

Tan inverse [Sin C/(cos A x tan B) - (sin A x cos C)

A = departure latitude
B = Arrival latitude
C= difference between longitudes