Sidereal Time Calculator (GMST, LMST, JD)
Convert any UTC moment to Greenwich and Local Sidereal Time, Julian Date, Modified Julian Date and Earth rotation angle — for telescope alignment.
How to use
- Enter the UTC date and time, or tap 'Use now' to use the current UTC moment.
- Set the observer longitude in decimal degrees (east positive, west negative), or pick an observatory preset.
- Read GMST/GAST/LMST/LAST in HH MM SS format and the Julian Date / Modified Julian Date for use in ephemerides or astro software.
About this tool
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Astronomers don't use civil clock time when pointing telescopes — they use sidereal time, the angle of the vernal equinox above the local meridian. Enter any UTC date/time plus your observer longitude (or pick from observatory presets like Mauna Kea, Paranal, La Palma, Kitt Peak) and the tool computes Julian Date (JD), Modified Julian Date (MJD), Julian Century (T), Greenwich Mean Sidereal Time (GMST), Greenwich Apparent Sidereal Time (GAST, including nutation), Local Mean Sidereal Time (LMST), Local Apparent Sidereal Time (LAST), and the Earth rotation angle in degrees. Algorithms follow Meeus “Astronomical Algorithms” with a simplified IAU 1980 nutation series — accurate to a fraction of a second.
Frequently asked questions
What is sidereal time and why isn't it 24 hours?
Sidereal time tracks Earth's rotation relative to the stars rather than the Sun. A sidereal day is about 23h 56m 4.0905s — about 4 minutes shorter than a solar day — because Earth also moves around the Sun in that time and must rotate an extra ~1° to bring the Sun back overhead. Astronomers use it because the same sidereal time always points the telescope at the same patch of sky.
What's the difference between mean and apparent sidereal time?
Mean sidereal time (GMST/LMST) tracks the mean equinox — a smoothed reference. Apparent sidereal time (GAST/LAST) adds nutation in right ascension, the small wobble caused by the Moon and Sun pulling on Earth's equatorial bulge. The difference is the 'equation of the equinoxes', typically ±1 second of time.
How is LMST computed?
LMST = GMST + longitude / 15, where longitude is in degrees east of Greenwich. Each 15° east adds one hour of sidereal time; each 15° west subtracts one hour. Result is wrapped into [0h, 24h).
What's the Julian Date for?
Julian Date is a continuous count of days (and fractions of days) since noon UTC on January 1, 4713 BC (proleptic Julian calendar). It avoids calendar irregularities and is the standard time tag in astronomy. JD 2451545.0 is J2000.0 (2000 Jan 1.5 TT).
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