蔀或篰是中国古代历法的时间单位。指回归年与朔望月与平太阳日的最短循环（时间长度极接近的最小公倍数）。在西方，类似的概念为 Callippic cycle。

由于回归年与朔望月之最小公倍数并未恰好一日长，若再取与太阳日之公倍数，则能更妥善的制历，因此出现「蔀」。

For astronomy and calendar studies, the Callippic cycle (or Calippic) is a particular approximate common multiple of the year (specifically the tropical year) and the synodic month, that was proposed by Callippus during 330 BC. It is a period of 76 years, as an improvement of the 19-year Metonic cycle.