Time
Sidereal Time Calculator
Greenwich Mean Sidereal Time and Local Mean Sidereal Time for any date and time from 1800 to 2200, with a live ticking sidereal clock.
This calculator needs JavaScript. Enable JavaScript and reload the page to compute sidereal time.
Date, time, and longitude
Any date from 1800 to 2200.
24-hour clock, seconds optional.
Decimal degrees, -180 to 180, east positive. Example: New York City is -74.0060. Leave blank for Greenwich only.
Now starts a live sidereal clock that ticks once per second from your device clock; select it again to stop.
Results
Greenwich Mean Sidereal Time
Local Mean Sidereal Time
24-hour sidereal dial: the short brass hand marks Greenwich mean sidereal time, the long navy hand marks local mean sidereal time.
About sidereal time
Sidereal time tracks the rotation of the Earth against the stars instead of the Sun: it is the hour angle of the vernal equinox, and a star crosses your meridian when the local sidereal time equals its right ascension. Because the Earth also moves along its orbit, the sidereal day is 23h 56m 4.0905s of ordinary clock time, about 3 minutes 56 seconds shorter than the 24-hour mean solar day, so sidereal time gains roughly four minutes on your clock every day.
This calculator converts your date and time to UTC, forms the Julian Date, and evaluates Greenwich Mean Sidereal Time with the full polynomial expression of equation 12.4 in Jean Meeus, Astronomical Algorithms, 2nd edition, chapter 12. Local Mean Sidereal Time is GMST plus your east longitude converted to hours (15 degrees per hour), so a site at longitude -74 lags Greenwich by 4.9333 hours. Sidereal time depends only on longitude, never on latitude. Times you enter in your device's zone are converted with the browser's own zone rules; during a daylight-saving jump the browser resolves skipped or repeated wall-clock times, and the interpreted UTC instant is always shown with the result.
Values are mean sidereal time: the equation of the equinoxes, at most about 1.2 seconds, is neglected, so apparent (true equinox) sidereal time can differ by up to that amount. The input time is treated as UT1, which differs from UTC by less than 0.9 seconds. Within the supported years 1800 to 2200 the result is therefore within a few seconds of the apparent sidereal time an observatory would use, which is ample for pointing, planning, and education, but not for precision astrometry or navigation. The live Now mode simply reads your device clock once per second, so its accuracy matches your device's clock setting. Everything is computed in your browser; the longitude you type or fetch with the location button never leaves your device.