A lunation, or lunar month, is the time it takes the Moon to pass through all of the Moon phases, measured from one new moon to the next New Moon. The astronomical term for a lunation is a synodic month, from the Greek term synodos, meaning meeting or conjunction. The synodic revolution of the Moon begins each time at new moon, when the Sun and Earth are aligned on opposite sides of the Moon, and stops at the next new moon. It lasts about 29.5 days. The exact length varies slightly, due to the elliptical shape of the Moon’s orbit.
In astronomy, it is most common to use the Brown lunation number system for lunar months. This system was invented by Professor Ernest W. Brown from Yale University (USA) and presented in the book Planetary Theory from 1933, which Brown co-wrote with Assistant Professor Clarence A. Shook.
Brown starts his count with lunation number 1 at the first New Moon of 1923, which was on January 17, 1923. Hence, lunation numbers listed for years before 1923 are negative.