Sidereal Time Calculator

Summary

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

How it works

1. Enter the observer's longitude in decimal degrees (positive = East, negative = West).
2. Enter the date and Universal Time (UT) of the observation.
3. The tool computes the Julian Date for the given date and time.
4. Greenwich Mean Sidereal Time (GMST) is derived from the Julian Date using the IAU formula.
5. Local Sidereal Time (LST) is obtained by adding the observer's longitude (converted to hours) to GMST.
6. 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