Implementations of the RA/Dec to Alt/Az conversion algorithms from "Astronomical Algorithms", "The Explanatory Supplement to the Astronomical Almanac", and "Celestial Mechanics (Taff)". This test suit uses Greenwhich Mean Sidereal Time, rather than Greenwhich Apparent Sidereal Time. This generally produces an error less than a few arcseconds, though Azimuth errors can get large near the zenith. Adding the Equation of the Equinoxes to GMST produces GAST, and would reduce the error, but to get the most accurate results you will need to follow the reduction steps as outlined in the Explanatory Suplement.
Test data is from NASA JPL Horizons.
| Source | Lat° | Lon° | Date | Ra (of Date)° | Dec (of Date)° | Az° | Alt° | LST hours | Hour Angle hours |