Calculate local sidereal time (LST) and Greenwich sidereal time (GST) for astronomy, telescope alignment, and star observation planning.
The Sidereal Time Calculator computes both Greenwich Sidereal Time (GST) and Local Sidereal Time (LST) for any date, time, and longitude. Sidereal time is the time system used in astronomy to determine which stars are currently overhead at a given location, essential for telescope pointing and celestial observation planning.
A sidereal day is approximately 23 hours, 56 minutes, and 4.0905 seconds — about 3 minutes 56 seconds shorter than a solar day. This difference occurs because Earth must rotate slightly more than 360° to bring the Sun back to the same position (since Earth has moved along its orbit), but exactly 360° to bring distant stars back to the same position.
Local Sidereal Time directly indicates which right ascension is currently on your meridian. If your LST reads 5h 30m, objects at right ascension 5h 30m are crossing your meridian right now — the best time to observe them. This calculator also shows the Julian Date, the time difference between sidereal and solar time, and a sidereal clock.
Sidereal time is essential for astronomy and telescope operation. This calculator provides instant LST and GST for planning observation sessions and aligning telescopes. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation. Align this note with review checkpoints.
Julian Date = 367Y − INT(7(Y+INT((M+9)/12))/4) + INT(275M/9) + D + 1721013.5 + UT/24. T = (JD − 2451545.0)/36525. GST₀ = 6.697374558 + 2400.0513369T + 0.0000258622T² − 1.7222e−9T³. LST = GST + longitude/15.
Result: LST ≈ 16h 42m
On June 15, 2025 at 10 PM UTC at longitude -105° (Denver), the local sidereal time is approximately 16h 42m, meaning objects at RA 16h 42m are on the meridian.
Sidereal time measures Earth's rotation relative to the "fixed" stars rather than the Sun. Because Earth orbits the Sun, the sidereal day is about 3 minutes 56 seconds shorter than the solar day. Over one year, this adds up to exactly one extra sidereal day (366.25 sidereal days per 365.25 solar days).
Amateur astronomers use LST to plan observing sessions. If you want to observe the Orion Nebula (RA ~5h 35m), the best time is when your LST is around 5h 35m, as Orion will be crossing your meridian. Professional observatories schedule observations based on sidereal time windows when targets are optimally positioned.
The Julian Date (JD) is a continuous count of days since January 1, 4713 BCE. Astronomers use it to avoid calendar complexity. The sidereal time formula uses JD internally to calculate the celestial position of the vernal equinox, from which sidereal time is measured.
Astronomers use it to know which celestial objects are observable. If an object's right ascension matches your LST, it's on your meridian (highest in the sky).
Earth orbits the Sun, so after one 360° rotation relative to stars, it must rotate ~1° more to face the Sun again. This extra rotation takes ~4 minutes.
GST is sidereal time at the prime meridian (0° longitude). LST adjusts for your longitude: LST = GST + (longitude/15) hours.
Set your telescope's hour angle to (LST - object's RA). When LST equals the object's RA, the hour angle is 0 and the object is on the meridian.
The formula uses UTC internally. If you enter local time, make sure to specify your UTC offset so the calculator can convert.
This uses the standard IAU formula, accurate to within a few seconds for dates within a century of J2000.0 (year 2000).