Old instruments
Computus
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Computus is the body of calendrical methods used to determine the annual date of Easter from weekday, solar-calendar, and ecclesiastical lunar cycles.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Almanac, Astronomical Almanac, Nautical Almanac, Astrolabe
Computus is the calendrical calculation used to determine Easter's date.
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The story behind Computus
Computus became an important field of late-antique and medieval learning as communities sought a common annual observance from repeating calendar cycles.
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Why Computus matters
Computus is a major historical example of astronomy-inspired cycles, religious rules, and calendar arithmetic interacting without being identical to direct sky observation.
Computus in practice: an example
Gregorian computus uses an ecclesiastical equinox and tabulated lunar cycle to identify the relevant Sunday, not a fresh local observation of the full Moon.
Where readings of Computus go wrong
A common error is to reduce the rule to the date of the observed astronomical full Moon or assume all Christian traditions produce the same civil date.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: The Date of Easter - Historical and calendrical explanation of ecclesiastical Easter calculation.
- U.S. Naval Observatory: Introduction to Calendars - Calendar definitions and the astronomical basis of solar, lunar, lunisolar, Julian, and Gregorian systems.
- Source
- The Great Almanac Dictionary
- Category
- Historical Almanacs & Instruments
- Last updated
- 2026-09-26
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Common questions
What is computus?
Computus is the body of calendrical methods used to determine the annual date of Easter from weekday, solar-calendar, and ecclesiastical lunar cycles.
What do people get wrong about computus?
A common error is to reduce the rule to the date of the observed astronomical full Moon or assume all Christian traditions produce the same civil date.
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