Astronomical
Sidereal Time
Greenwich Mean Sidereal Time · itself a running clock
The clock astronomers actually keep — time measured against the distant stars rather than the sun. Because Earth moves along its orbit each day, a sidereal day runs 3m 56s short of a solar one, so star time gains a whole day on the calendar every year and any star returns to the same place at the same sidereal time all year round.
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At a glance
- Type
- A clock — time by the stars, not the sun
- Origin
- Astronomers’ timekeeping, measured at Greenwich
- Day
- 23 hours 56 minutes 4 seconds of ordinary time
- Counts from
- No count — it is a time of day
- Where it’s used
- Observatories, and pointing telescopes
- How it’s worked out here
- The standard formula for Greenwich Mean Sidereal Time, from your device’s clock
Does something here look wrong? Tell us about the Sidereal Time calendar.