Sidereal Zeit

GMST, GAST und local sidereal time für any date und longitude

About this tool

Sidereal time is the right ascension currently on the meridian: objects with that RA are transiting (highest in the sky).

What is reported

GMST — Greenwich Mean Sidereal Time, IAU 1982 polynomial in JD UT.
GAST = GMST + equation of the equinoxes (low-precision nutation correction; arc-second level).
Local MST / AST = G(M/A)ST + observer longitude (east+).

Why it matters

An object with RA α transits the meridian when local sidereal time equals α. Telescope drives are programmed in sidereal time so stars stay still in the eyepiece.

Sanity check

At 2000-01-01 12:00 UT (J2000), GMST ≈ 18:41:50.5 = 18.697 374 6 h. The tool returns this exactly.