2012 is a leap year, meaning that an additional day is added to account for the fact that the actual time it takes for the Earth to make one revolution around the sun is slightly more than 365 days. A leap year occurs every four years with one exception: a year that is divisible by 100 is a leap year only if it is also divisible by 400.

I recently learned that the Chinese calendar has a different definition for a leap year. The Chinese calendar is primarily a lunar calendar. That means that months are determined by the phases of the moon. Specifically, the start of each month always occurs on a new moon. A typical year has twelve months and 353, 354, or 355 days. The correction to account for the difference between a Chinese calendar year and an astronomical year is a little more drastic: every so often, a new *month* is inserted because there will be thirteen new moons within a year.

Complicating matters even further is a rule that states the winter solstice must occur in the 11th month. Therefore, the point in the calendar where the new month is added will vary from leap year to leap year. How often do leap years occur in the Chinese calendar? The intervals are not regular, but it’s once every three years!