Enter your longitude, date, and UTC time to calculate your Local Sidereal Time (LST). The calculator returns both Greenwich Mean Sidereal Time (GMST) and Local Mean Sidereal Time in hours, minutes, and seconds — essential for pointing telescopes and planning astronomical observations.
Local Mean Sidereal Time (LST)
--
Greenwich Mean Sidereal Time (GMST)
--
LST in Decimal Hours
--
GMST in Decimal Hours
--
Julian Date
--
Equation of Equinoxes
--
Sidereal time is a timekeeping system astronomers use to track the position of stars. It measures the hour angle of the vernal equinox and completes one full cycle (one sidereal day) in approximately 23 hours, 56 minutes, and 4 seconds — about 4 minutes shorter than a solar day. At any given moment, stars with a right ascension equal to your local sidereal time are crossing your meridian (due south).
Greenwich Mean Sidereal Time (GMST) is the sidereal time at the prime meridian (0° longitude). Local Mean Sidereal Time (LST) adjusts GMST for your specific longitude — for every degree east you add 4 minutes, and for every degree west you subtract 4 minutes. LST tells you which right ascension is currently on your local meridian.
Mean sidereal time uses the mean equinox, ignoring short-term nutation in the Earth's polar axis. Apparent sidereal time uses the true equinox, accounting for nutation. The difference between them is called the Equation of the Equinoxes, which is typically less than one second of time. For most practical observing purposes, mean sidereal time is sufficient.
LST directly tells you which objects are optimally placed for observation right now. If a deep-sky object has a right ascension of 05h 34m (like the Orion Nebula), it transits your meridian when your LST equals 05:34. Objects near this time are at their highest in the sky with minimum atmospheric distortion, giving you the best viewing conditions.
Sidereal time is computed using the Julian Date. The number of Julian centuries elapsed since J2000.0 (January 1, 2000, 12:00 TT) is used in a polynomial formula defined by the IAU to derive GMST. Local sidereal time is then found by adding the observer's east longitude (converted to hours) to GMST.
Enter positive values for East longitude and negative values for West longitude. For example, New York City is at approximately -74.0° (74° West), while Berlin is at +13.4° (13.4° East). Most mapping applications display longitude this way as well.
No. Sidereal time is the same for all locations sharing the same longitude, regardless of latitude. Latitude affects which objects are visible above your horizon and how high they rise, but it plays no role in the sidereal time calculation itself.
The Julian Date (JD) is a continuous count of days and fractions of a day since noon on January 1, 4713 BC. Astronomers use it because it avoids the complexities of calendar systems (leap years, months of varying length, etc.) and makes time-difference calculations straightforward. All sidereal time formulas reference JD as the fundamental time input.