Calendars and time
Tropical Month
Direct answer
Tropical Month: A Tropical Month is the mean interval for the Moon to return to the same celestial longitude measured from the moving equinox.
A Tropical Month measures lunar return relative to the equinox.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Draconic Month, Calendar Month, Calendar, Julian Date
A Tropical Month measures lunar return relative to the equinox.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Tropical Month
The distinction became necessary when astronomers accounted for precession in positional calculations.
Why Tropical Month matters
Tropical Month is slightly different from the sidereal month because precession changes the equinox reference direction.
Tropical Month in practice: an example
The tropical month is a few seconds shorter than the mean sidereal month.
Where readings of Tropical Month go wrong
It is a mistake to call it a calendar month; the term describes an astronomical return interval.
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
Name the lunar reference frame
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Common questions
What is a Tropical Month?
A Tropical Month is the mean interval for the Moon to return to the same celestial longitude measured from the moving equinox.
What do people get wrong about a Tropical Month?
It is a mistake to call it a calendar month; the term describes an astronomical return interval.
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