Calendars and time
Sidereal Month
Direct answer
Sidereal Month: A Sidereal Month is the time the Moon takes to return to the same direction relative to the stars.
A Sidereal Month measures the Moon orbit against a stellar reference.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Synodic Month, Anomalistic Month, Calendar, Julian Date
A Sidereal Month measures the Moon orbit against a stellar reference.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Sidereal Month
Star-based observation provided a stable way to recognize the Moon orbital return independently of phase.
Why Sidereal Month matters
Sidereal Month distinguishes the Moon orbital revolution from its longer phase cycle, during which Earth also moves around the Sun.
Sidereal Month in practice: an example
The mean sidereal month is about 27.32 days, roughly two days shorter than a synodic month.
Where readings of Sidereal Month go wrong
The usual misreading is to use the sidereal month to predict the next identical lunar phase.
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 orbital and phase months
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More in Calendars & Timekeeping
- Tropical Year
- Anomalistic Year
- Calendar Month
- Dominical Letter
- Era
- Intercalary Month
- Leap Year
- New Style Date
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Common questions
What is a Sidereal Month?
A Sidereal Month is the time the Moon takes to return to the same direction relative to the stars.
What do people get wrong about a Sidereal Month?
The usual misreading is to use the sidereal month to predict the next identical lunar phase.
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