默冬章或默冬周期是古希腊天文学家默冬发现的一个天文学周期（一说由巴比伦历习得）。在中国古代，类似的概念为章；在玄始历与大明历前，两者几乎相同。

19个回归年的时间长度和235个朔望月几乎相同。所以阴阳历若采十九年七闰法，每19年加入7个闰月，就可以协调回归年（太阳的周期）与朔望月（月亮的周期）间的时间差距，以较为简易的方式制历。中国古称此周期为一章。

235个朔望月较19个回归年仅约长2小时。若每年12个月，则须设置个闰月。

For astronomy and calendar studies, the Metonic cycle or Enneadecaeteris (from Ancient Greek: ἐννεακαιδεκαετηρίς, "nineteen years") is a period of very close to 19 years that is remarkable for being nearly a common multiple of the solar year and the synodic (lunar) month. The Greek astronomer Meton of Athens (fifth century BC) observed that a period of 19 years is almost exactly equal to 235 synodic months and, rounded to full days, counts 6,940 days. The difference between the two periods (of 19 years and 235 synodic months) is only a few hours, depending on the definition of the year.