Calendars and time
Sidereal Day
Direct answer
Sidereal Day: A Sidereal Day is one rotation of Earth measured relative to the equinox-based sidereal reference, lasting about 23 hours 56 minutes.
A Sidereal Day measures Earth rotation relative to the celestial reference frame.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Apparent Solar Day, Civil Day, Calendar, Julian Date
A Sidereal Day measures Earth rotation relative to the celestial reference frame.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Sidereal Day
Sidereal clocks became essential in observatories because star transits provide precise rotational references.
Why Sidereal Day matters
Sidereal Day explains why stars return to the meridian about four minutes earlier on successive civil dates.
How to read Sidereal Day
Sidereal Day speaks to telescope scheduling, hour angle, and the daily recurrence of stellar positions.
Sidereal Day in practice: an example
A star crossing the meridian at 10:00 tonight will cross near 9:56 by mean solar time tomorrow.
Where readings of Sidereal Day go wrong
It is a mistake to call the sidereal day simply a shorter solar day; it uses a different reference direction.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory Circular 179: The IAU Resolutions on Astronomical Reference Systems, Time Scales, and Earth Rotation Models - Technical reference for TAI, TT, TCG, TCB, TDB, UT1, sidereal time, epochs, and Earth rotation.
- U.S. Naval Observatory: Universal Time - Official explanation of Earth-rotation time and the Universal Time family.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Plan by stellar rotation
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More in Calendars & Timekeeping
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- Delta T
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- Intercalary Day
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- Modified Julian Date
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Common questions
What is a Sidereal Day?
A Sidereal Day is one rotation of Earth measured relative to the equinox-based sidereal reference, lasting about 23 hours 56 minutes.
What do people get wrong about a Sidereal Day?
It is a mistake to call the sidereal day simply a shorter solar day; it uses a different reference direction.
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