Sidereal Time Calculator
Enter a UTC date/time and your longitude to determine the sky’s rotation angle—crucial for telescope alignment and star tracking.
Use 24-hour time (00:00–23:59).
Julian Date: 2461004.99306
Greenwich Sidereal Time (GST): 16h 08m 42.69s (16.145192 h)
Local Sidereal Time (LST): 16h 08m 42.69s (16.145192 h)
How to Use This Calculator
Enter the observation moment
Use UTC date and time for the precise second you want to evaluate.
Specify longitude
Use positive degrees east of Greenwich, negative west.
Read GST and LST
GST is global; LST applies to your longitude. Use LST for telescope RA alignment.
Formula
JD = Julian Date(UTC)
GMST (deg) = 280.46061837 + 360.98564736629 × (JD − 2451545.0) + 0.000387933T² − T³ / 38710000
GST (hours) = GMST ÷ 15
LST = GST + Longitude ÷ 15
Example: UTC 2025-11-09 12:00 at −74° (NYC) → LST ≈ 4h 35m.
Why Sidereal Time Matters
Sidereal time tracks Earth’s rotation relative to the stars, not the Sun. Astronomers rely on it to aim telescopes, calibrate planetarium software, and plan observations.
- Position telescopes to specific Right Ascension values.
- Compare the sky’s rotation rate to local clock time.
- Simulate night sky positions in astronomy apps.
- Understand why stars rise ~4 minutes earlier each night.
Frequently Asked Questions
What time zone should I use?
Sidereal calculations require UTC. Convert your local time to UTC before entering it.
Why does sidereal time differ from solar time?
Earth rotates ~361° per solar day, so sidereal time gains about 4 minutes daily.
Can I use this for southern hemisphere?
Yes. Longitude sign handles east/west. Declination affects visibility, not LST.
How precise is the calculation?
Within fractions of a second for typical use. For observatory-grade accuracy, include nutation and polar motion corrections.