Calendars and time
Intercalary Month
Direct answer
Intercalary Month: An Intercalary Month is an additional month inserted in some lunisolar calendars so lunar months remain near the same seasons.
An Intercalary Month is a leap month used to reconcile lunar months with the solar year.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Intercalary Day, Epact, Calendar, Julian Date
An Intercalary Month is a leap month used to reconcile lunar months with the solar year.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Intercalary Month
Several ancient calendars moved from ad hoc observation to patterned leap-month cycles as astronomical prediction improved.
Why Intercalary Month matters
Twelve synodic months are about eleven days shorter than a seasonal year, so periodic leap months prevent long-term seasonal drift.
How to read Intercalary Month
Intercalary Month comes up when comparing Hebrew, Chinese, Babylonian, and other lunisolar calendar structures.
Intercalary Month in practice: an example
A lunisolar calendar can place a thirteenth month in selected years of a nineteen-year cycle.
Where readings of Intercalary Month go wrong
It is a mistake to assume leap months share one name, position, or rule across lunisolar traditions.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Introduction to Calendars - Official overview of calendar systems, leap rules, eras, and historical calendar reform.
- 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
Trace the lunisolar leap cycle
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More in Calendars & Timekeeping
- Leap Year
- New Style Date
- Sidereal Month
- Tropical Year
- Apparent Solar Day
- Calendar Year
- Double Dating
- Era
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Common questions
What is an Intercalary Month?
An Intercalary Month is an additional month inserted in some lunisolar calendars so lunar months remain near the same seasons.
What do people get wrong about an Intercalary Month?
It is a mistake to assume leap months share one name, position, or rule across lunisolar traditions.
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