Mondjahre

Mondjahre is a term used to describe a year reckoned by the Moon’s cycles rather than the Sun, a concept common in discussions of lunar calendars. In many contexts it is synonymous with a lunar year, though usage and definitions can vary between languages and traditions.

In a purely lunar system, a year consists of 12 lunar months. A synodic month, the time from one new moon to the next, averages about 29.53 days. Therefore a lunar year is about 354.37 days long. Some traditions insert a 13th month in certain years to re-align with the solar year, producing a lunar year of about 383.90 days.

The Islamic calendar is a well-known example of a purely lunar year: each year has 12 months with 29 or 30 days, totaling roughly 354 or 355 days, with no leap months added. Over time, the lunar year drifts relative to the solar year by about 10–12 days per year, causing holidays to move through the seasons.

In contrast, lunisolar calendars also use lunar months but insert intercalary months to keep the calendar roughly aligned with the solar year, preserving seasonality. In German-language contexts, Mondjahre may refer to lunar-year concepts or to the broader category of calendars based on lunar months.

Today the term Mondjahre is not a fixed calendar system but a descriptive label used in historical, linguistic, or comparative discussions of timekeeping. See also lunar calendar, synodic month, intercalation, Islamic calendar.