Lunation

(glossary entry)

Description

The cycle of phases, as seen from Earth, from one (geocentric) New Moon to the next.

The length of the cycle is said to vary from about 29.27 to 29.83 (Earth) days, with an average of 29.530589 days.
Lunations are sometimes numbered from an arbitrary starting point. The most popular system seems to that of E. W. Brown who counted the lunation commencing with the New Moon of January 17, 1923 as "1".