Calendar history
Why Lunar Debt Matters
Lunar debt is the plain-language name for a real calendar problem: twelve Moon cycles do not make a solar year.
The story explains the roughly 10.88-day gap between twelve synodic lunar months and the tropical year, then points to the calendars that drift or add leap months in response.
Story
The shortfall
A synodic month is about 29.53 days. Twelve lunar months add up to about 354.37 days, leaving roughly 10.88 days between the lunar year and the seasonal solar year.
Story
Two calendar choices
A lunar calendar can let months travel through the seasons. A lunisolar calendar can insert leap months so named months remain connected to seasonal anchors. Both are coherent answers to the same arithmetic pressure.
Story
CalculationTime angle
The Lunar Debt Calendar turns the gap into a visible ledger: change the span, watch the owed days accumulate, and compare that drift with Hebrew and Chinese leap-month cycles.
Related CalculationTime Tools
Open the calculators behind the story
Lunar Debt Calendar
10.88-day shortfall and leap-month ledger
Multi-Calendar Visualizer
Compare calendar labels for one civil date
Full Moon Calendar
Engine-backed full moon timing