Sidereal Time Calculator
Sidereal time measures Earth's rotation relative to the distant stars and is the key coordinate used in pointing telescopes. Local Sidereal Time equals the right ascension of any star that is currently crossing your local meridian, making it the starting point for all observational planning. This calculator computes Greenwich Mean Sidereal Time (GMST) from the Julian date, then adds your observer longitude (in degrees east) to find Local Sidereal Time (LST). Enter a UT date/time and longitude to compute both values.
GMST and LST formulas
JD = Julian Date from UT
T = (JD - 2451545.0) / 36525 (Julian centuries from J2000.0)
GMST (deg) = 280.46061837 + 360.98564736629*(JD-2451545)
LST = GMST + longitude (degrees) converted to hours mod 24
This is the IAU 1982 GMST formula. GMST in degrees is converted to hours by dividing by 15. LST = (GMST_hours + longitude_degrees / 15) mod 24. The result is accurate to about 0.1 seconds for dates within a few centuries of J2000.0.
How astronomers use sidereal time
When your LST equals the right ascension (RA) of a star, that star is on your local meridian and at maximum altitude. Objects best observed are within 2 hours of your LST. The Vernal Equinox (RA = 0h) is on the meridian when LST = 0h. Orion transits near LST = 5h 30m. The galactic center transits near LST = 17h 45m.
Sidereal time: frequently asked questions
What is sidereal time?
Sidereal time is a time scale based on Earth's rotation relative to the fixed stars, rather than the Sun. One sidereal day is about 23 hours 56 minutes 4 seconds, approximately 4 minutes shorter than a solar day, because Earth has moved slightly in its orbit around the Sun.
What is Greenwich Mean Sidereal Time (GMST)?
GMST is sidereal time at the prime meridian (0 degrees longitude). It is the right ascension of stars on the meridian as seen from Greenwich. GMST increases by approximately 24 hours 3 minutes 56.56 seconds per solar day.
What is Local Sidereal Time (LST) used for?
LST equals the right ascension of objects currently on your local meridian (due south in northern hemisphere). A star with right ascension equal to your LST is currently at its highest point in the sky (transit). Astronomers use LST to plan observations.
How is LST calculated from GMST?
LST = GMST + (observer longitude in hours). Since 360 degrees = 24 hours, divide observer longitude in degrees by 15 to get hours. East longitudes add to GMST; west longitudes subtract. The result is taken modulo 24 hours.
Why is a sidereal day shorter than a solar day?
Earth rotates 360 degrees relative to the stars in a sidereal day. But because Earth also orbits the Sun, it must rotate slightly more than 360 degrees for the Sun to return to the same meridian (a solar day). The difference is about 1 degree per day, corresponding to about 4 minutes.
Official sources
- USNO Astronomical Almanac - sidereal time: aa.usno.navy.mil/data/siderealtime.
- USNO Complete Sun and Moon Data: aa.usno.navy.mil.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.