New moon western horizon shortly after sunset and before moonset. 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. The
Lunar month respect to the Sun as seen from the Earth. While the Moon is orbiting the Earth, the Earth is progressing in its orbit around the Sun. After completing a sidereal month, the Moon must move a little further to reach the new position having the same angular distance from the Sun, appearing to move with respect to the stars since the previous month. Therefore, the synodic month takes 2.2 days longer than the sidereal month. Thus, about 13.37 sidereal months, but about 12.37 synodic months, occur in a Gregorian year. Since Earth's orbit around the Sun is elliptical and not