#gmst

Sidereal-time astronomy as an API, computed locally and deterministically. The gmst endpoint computes the Greenwich Mean Sidereal Time for a UT date and time, GMST = 18.697374558 + 24.06570982441908·(JD − 2451545.0) hours modulo 24, returning it in hours, degrees and hours-minutes-seconds together with the Julian Day — sidereal time tracks the stars rather than the sun and gains about three minutes and fifty-six seconds each day. The lst endpoint adds the observer's longitude to give the Local Sidereal Time, LST = GMST + longitude/15 (east positive), which equals the right ascension of any star currently crossing the local meridian. The hour-angle endpoint computes the hour angle of a celestial object, HA = LST − RA, from its right ascension and the local sidereal time (or a date, time and longitude): an hour angle of zero means the object is on the meridian at its highest point, a positive hour angle means it is west of the meridian and setting, and a negative one means it is east and rising. Dates are YYYY-MM-DD and times HH:MM:SS in UT, longitude in degrees and right ascension in hours. Everything is computed locally and deterministically, so it is instant and private. Ideal for astronomy, telescope-control, planetarium, observatory and astrophotography app developers, star-pointing and transit tools, and astronomy education. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This is sidereal time; for the sun's position use a solar-position API and for sunrise and sunset times a sunrise API.

api.oanor.com/sidereal-api