Calendar history
Lunar Debt: Why Moon Calendars Need Payback Days
Twelve lunar months fall short of the solar year, so calendars must either drift through the seasons or add leap months.
The lunar debt story turns a hard calendar idea into a visible calculation: the difference between 12 synodic months and a tropical year.
Story
The calculation problem
A synodic lunar month is about 29.53 days. Twelve of them make roughly 354.37 days, about 10.88 days shorter than the tropical year. That shortfall is the debt.
Story
Two calendar answers
A lunar calendar can let months drift through the seasons, as the Islamic calendar does. A lunisolar calendar can add leap months to pull lunar months back toward the solar year, as Hebrew and Chinese calendar systems do in different ways.
Story
CalculationTime angle
The Lunar Debt Calendar should be a doorway into multi-calendar learning: it explains why calendar systems disagree before the visualizer shows what those disagreements look like on actual dates.
Related CalculationTime Tools
Open the calculators behind the story
Lunar Debt Calendar
10.88-day shortfall and leap-month ledger
Multi-Calendar Visualizer
Calendar labels side by side
Full Moon Calendar
Engine-backed full moon dates