Intercalary Month

Overview

An intercalary month (also called an embolismic or leap month) is an additional month inserted into a lunisolar calendar to correct the drift between the calendar year and the solar year. A 12-month lunar year is approximately 354 days, about 11 days shorter than the solar year of roughly 365.25 days. Without periodic corrections, the months would gradually shift out of alignment with the seasons.

The Metonic Cycle

In 432 BCE, the Greek astronomer Meton discovered that 19 solar years correspond almost exactly to 235 lunar months. Based on this 19-year cycle, 7 intercalary months are inserted over the span of 19 years. The Chinese agricultural calendar, the traditional Japanese calendar, and the Hebrew calendar all use this pattern. The specific rules for which month receives the intercalation vary by system - in the Chinese calendar, the intercalary month is the one that contains no major solar term (zhongqi).

Modern Relevance

Although the Gregorian calendar is now the international standard, intercalary months still affect cultural events across East Asia. The shifting date of Lunar New Year each year is a direct consequence of the intercalation pattern. For software handling traditional calendar dates, the existence of intercalary months means the "month" value can reach 13, a case that must be accounted for in date libraries and validation logic.