Re: ATM alt/az to ra/dec

["James W. Burrows" <burrjaw@halcyon.com>]

On Mon, 16 Sep 1996, Dan Gray wrote:

> I have need of a way to convert from altitude and azimuth to right
> ascension and declination.  I have found formula's for the other way
> around, and am presently using them, but I need to convert from alt/az
> to ra/dec, to take refraction into account, for my computer controlled
> scope.

Notation:

        declination D
        zenith distance Z
        azimuth A
        latitude L
        local hour angle H

(This is a BTW) the best formula for azimuth is to use whole-angle arctan,
or rectangular -> polar on a calculator: 

        Z = arctan( x=> tanDcosL - sinLcosH, y=> -sinH)

that way you don't have to worry about quadrants.  Now for the other way
around,

        D = arcsin( sinLcosZ + cosLsinZcosA)

and

        H = arctan( x=> cosLcotZ - sinLcosA, y=> -sinA)

Somebody else check? 

     Jim Burrows    phone: 206-244-2933, phone/fax: -0294
     E-mail:        burrjaw@halcyon.com
     Seattle:       WGS84 lat 0.828547, long -2.135692 (rad)