Calendars and time
Sidereal Year
Direct answer
Sidereal Year: A Sidereal Year is the time Earth takes to complete an orbit relative to a fixed stellar reference direction.
A Sidereal Year measures Earth orbit against the stars.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Tropical Year, Anomalistic Year, Calendar, Julian Date
A Sidereal Year measures Earth orbit against the stars.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Sidereal Year
Recognition of precession explained why the sidereal and tropical years are not the same length.
Why Sidereal Year matters
Sidereal Year separates orbital return relative to distant stars from seasonal return relative to the precessing equinox.
Sidereal Year in practice: an example
At roughly 365.2564 days, the sidereal year is about twenty minutes longer than the mean tropical year.
Where readings of Sidereal Year go wrong
The usual misreading is to use the sidereal year as the direct basis for Gregorian seasonal datekeeping.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Astronomical Almanac Glossary - Reference definitions for astronomical periods, coordinates, dates, and time scales.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Compare stellar and seasonal return
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More in Calendars & Timekeeping
- Unix Time
- Astronomical Day
- Callippic Cycle
- Draconic Month
- Golden Number
- International Atomic Time
- Local Mean Solar Time
- Ordinal Date
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Common questions
What is a Sidereal Year?
A Sidereal Year is the time Earth takes to complete an orbit relative to a fixed stellar reference direction.
What do people get wrong about a Sidereal Year?
The usual misreading is to use the sidereal year as the direct basis for Gregorian seasonal datekeeping.
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