⏱️Sidereal Time Calculator
Calculate Local Sidereal Time (LST) and Greenwich Mean Sidereal Time (GMST) for any location and time. Find what constellation is on your meridian.
Calculate local and Greenwich sidereal time for telescope pointing. Shows which constellation is on your meridian right now. Free.
How to Use Sidereal Time Calculator — Step by Step
- 1Enter the date and UTC time
- 2Enter your longitude in degrees east
- 3Click 'Calculate Sidereal Time'
- 4View Local Sidereal Time and Greenwich Mean Sidereal Time
- 5See which constellation is currently crossing your meridian
Why Use ToolNest for Sidereal Time Calculator?
- ⏱️Accurate GMST calculation using IAU formula (sub-second precision)
- 📍Local Sidereal Time for any longitude on Earth
- 🌌Shows which constellation is currently on your meridian
- 🔭Critical for telescope pointing: RA on meridian = LST
- 🆓Browser-based calculation — no network required
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Frequently Asked Questions
What is sidereal time?
Sidereal time is based on Earth's rotation relative to distant stars (not the Sun). A sidereal day is 23 hours 56 minutes 4 seconds. Local Sidereal Time (LST) tells you which Right Ascension is currently on your meridian.
Why is sidereal time important for telescopes?
If LST = 6h, then any object with RA = 6h is currently crossing your meridian (at its highest point). GoTo telescopes use LST internally to calculate where to point. Knowing your LST helps you judge what's well-placed for observing.
What is the difference between GMST and LST?
Greenwich Mean Sidereal Time (GMST) is sidereal time at the prime meridian (longitude 0°). Local Sidereal Time (LST) adds your longitude correction: LST = GMST + longitude/15 hours.
How does sidereal time differ from solar time?
Solar time is based on the Sun, which takes 24 hours to complete a cycle. Sidereal time is based on distant stars and completes a cycle in 23h 56m. Over a year, this 4-minute difference means stars rise about 4 minutes earlier each night.