In astronomy, new moon is the first phase of the Moon, when it orbits as seen from the Earth, the moment when the Moon and the Sun have the same ecliptical longitude.  The Moon is not visible[note 1] at this time except when it is seen in silhouette during a solar eclipse when it is illuminated by earthshine. See the article on phases of the Moon for further details.
A lunation or synodic month is the mean (average) time for 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 3 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.