Calendars and time
Sidereal Time
Direct answer
Sidereal Time measures Earth's rotation relative to the stars and indicates which right ascension is crossing the local meridian.
Sidereal Time is star-based rotational time used to orient the sky.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Calendar, Solar Calendar, Lunar Calendar, Lunisolar Calendar
Sidereal Time is star-based rotational time used to orient the sky.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Sidereal Time
Observatories used repeated stellar transits and precise clocks to establish sidereal time and test timekeeping.
Why Sidereal Time matters
Sidereal Time lets observers and telescopes connect clock-like time with the part of the celestial sphere currently on the meridian.
Sidereal Time in practice: an example
When local sidereal time is six hours, objects near right ascension six hours are crossing the local meridian.
Where readings of Sidereal Time go wrong
A common error is to confuse a sidereal day with a 24-hour civil solar day.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Computing Approximate Sidereal Time - Official explanation and method for Greenwich and local sidereal time.
- U.S. Naval Observatory: Astronomical Almanac Glossary - Reference definitions for astronomical coordinates, events, calendars, and time scales.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Find the sky on your meridian
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More in Calendars & Timekeeping
- Universal Time
- Apparent Solar Day
- Calendar Year
- Double Dating
- Geocentric Coordinate Time
- Intercalation
- Local Apparent Solar Time
- Old Style Date
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Common questions
What is sidereal time?
Sidereal Time measures Earth's rotation relative to the stars and indicates which right ascension is crossing the local meridian.
What do people get wrong about sidereal time?
A common error is to confuse a sidereal day with a 24-hour civil solar day.
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