Donaldson R F Sidereal Time Calculator
Greenwich Mean Sidereal Time (GMST) and Local Sidereal Time (LST) for any UTC date, time and longitude.
How this is worked out
Greenwich Mean Sidereal Time is the hour angle of the mean vernal equinox at the Greenwich meridian — in effect, a clock that tracks the sky's own rotation rather than the Sun's. This tool uses the IAU 1982 expression for GMST (a cubic polynomial in Julian centuries of UT1 since J2000.0), the same formula implemented in ERFA, the IAU's own reference astronomy library. Local Sidereal Time is simply GMST plus your longitude, expressed in hours (15° = 1 hour), with east longitudes added and west longitudes subtracted. Because sidereal time tracks the stars, it gains about 3 minutes 56.6 seconds on clock (solar) time every day — a full sidereal day is about 23h 56m 4.09s of ordinary time.
Frequently asked questions
What is sidereal time used for?
Astronomers use it to know which part of the sky is on the meridian right now: a star's hour angle equals the Local Sidereal Time minus the star's right ascension, so LST tells you, at a glance, which objects are well placed for observing.
Why isn't a sidereal day exactly 24 hours?
A solar day is timed by the Sun returning to the same position in the sky, but the Earth also moves about 1° along its orbit each day, so it has to rotate a little further to catch the Sun up again. A sidereal day, timed by the distant stars instead, doesn't need that extra bit of rotation, so it finishes about 3m56.6s sooner.
Related tools: Julian Date Converter, Timezone Converter, World Clock, Coordinates Converter.
Source / Last verified: IAU 1982 GMST expression, as implemented in ERFA (the IAU's reference C library, successor to SOFA), function eraGmst82 (github.com/liberfa/erfa), whose header cites Transactions of the International Astronomical Union, XVIII B, 67 (1983) and Aoki et al., Astron. Astrophys., 105, 359–361 (1982); read 28 September 2026. Checked against the commonly-quoted reference value GMST = 6h 39m 52.2s at 2000 Jan 1.0 UT (matched to the nearest second) and against the formula's own implied sidereal-gain rate of 236.555 seconds per day (exact match).