A lunation or synodic month is the average time from one new moon to the next. In the J2000. 0 epoch, the average length of a lunation is 29. 530588 days (or 29 days, 12 hours, 44 minutes, and 2. 8 seconds). However, the length of any one synodic month can vary from 29. 26 to 29. 80 days due to the perturbing effects of the Sun's gravity on the Moon's eccentric orbit. In a lunar calendar, each month corresponds to a lunation. Each lunar cycle can be assigned a unique lunation number to identify it.