Sidereal Time Calculator
Calculate the local sidereal time (LST) and Greenwich sidereal time (GST) based on your local date, time, and longitude. Useful for astronomy and satellite tracking.
Published by:
isCalculator Editorial Team
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How to use this tool?
How to Use the Sidereal Time Calculator
- Enter the Date – Use the Date field to select the calendar date (e.g., 2025-03-25).
- Enter Local Time – Input your current local time in 24-hour format (e.g., 12:00 for noon).
- Set Longitude – Enter your location’s longitude in decimal degrees. Use positive values for East, negative for West (e.g., -74.006 for New York). Must be between -180 and 180.
- Set Timezone Offset – Enter your timezone’s offset from UTC in hours (e.g., -5 for EST, +1 for CET). Use half-hour increments if needed (e.g., +5.5). Must be between -12 and 14.
- Calculate – Click the Calculate Sidereal Time button. Results display:
- Greenwich Sidereal Time (GST) – Sidereal time at the Prime Meridian, in hours.
- Local Sidereal Time (LST) – Sidereal time at your longitude, in hours.
All inputs are required. Longitude and timezone are validated in real time. Results update instantly after calculation.
Previous Results
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Frequently Asked Questions
Sidereal time is a timekeeping system based on Earth's rotation relative to distant stars. It is used in astronomy to locate celestial objects.
Greenwich Sidereal Time (GST) is the sidereal time at the Prime Meridian. Local Sidereal Time (LST) adjusts GST for your longitude, so it represents the sidereal time at your location.
Longitudes west of Greenwich are negative (e.g., New York is -74°), and east are positive. This ensures correct calculation of LST.