What is sidereal time?

Sidereal time measures Earth's rotation relative to the distant stars (not the Sun). One sidereal day is about 23 hours 56 minutes 4 seconds of solar time. Because of this difference, the stars rise about 4 minutes earlier each night.

What is Local Sidereal Time (LST)?

LST is the right ascension (RA) of stars currently crossing your meridian (the north-south line passing directly overhead). Any star with RA equal to the current LST is at its highest point in the sky, giving the best observing conditions.

Greenwich Mean Sidereal Time (GMST) is LST for the prime meridian (0° longitude). Local Sidereal Time adjusts GMST by your observer longitude: LST = GMST + longitude / 15 (where longitude is in degrees, result in hours).

How do I enter longitude?

Enter your observer's geographic longitude in decimal degrees. Use positive values for East longitude (e.g. London = +0.00, Tokyo = +139.69) and negative values for West longitude (e.g. New York = −74.00, Los Angeles = −118.24).

What Universal Time should I use?

Universal Time (UT or UTC) is the same as the time at the prime meridian in Greenwich, England. Convert your local time to UTC by subtracting your UTC offset. For example, US Eastern Standard Time is UTC − 5.

How accurate is this calculation?

The tool uses the standard IAU polynomial approximation for GMST, which is accurate to within about 0.1 seconds of arc for dates within a few centuries of J2000.0. This is more than sufficient for visual astronomy and most amateur astrophotography.

Sidereal Time Calculator

Calculate Local Sidereal Time (LST) for any observer longitude and date/time to know which stars are on your meridian.

Observer & Date/Time

Observer Longitude (decimal degrees)

Positive = East, negative = West. Range: −180 to +180.

Year

Month

Day

Hour (UT)

Minute

Second

Enter your longitude and date/time, then click Calculate.

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Summary

Sidereal time is an astronomical timekeeping system based on Earth's rotation relative to distant stars rather than the Sun. Local Sidereal Time (LST) tells an astronomer which right ascension is currently on the celestial meridian — any star whose right ascension equals the current LST is ideally placed for observation. This tool computes Greenwich Mean Sidereal Time (GMST) and converts it to Local Sidereal Time for any longitude, using the standard IAU algorithms.

How it works

Enter the observer's longitude in decimal degrees (positive = East, negative = West).
Enter the date and Universal Time (UT) of the observation.
The tool computes the Julian Date for the given date and time.
Greenwich Mean Sidereal Time (GMST) is derived from the Julian Date using the IAU formula.
Local Sidereal Time (LST) is obtained by adding the observer's longitude (converted to hours) to GMST.
Results are displayed in hours, minutes, seconds and as a decimal hours value.

Use cases

Find which deep-sky objects are ideally placed for observation tonight.
Determine when a target with a known right ascension will transit the meridian.
Plan astrophotography sessions by matching LST to object right ascension.
Verify planetarium software or telescope mount sidereal drive accuracy.
Learn the relationship between time, longitude, and celestial coordinates.
Check LST for a past observation to reconstruct which objects were visible.
Convert UT to LST for radio astronomy scheduling.
Cross-reference LST with a star atlas to identify objects on the meridian.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu