Synodic months

Metonic cycle, in chronology, a period of 19 years in which there are 235 lunations, or synodic months, after which the Moon’s phases recur on the same days of the solar year, or year of the seasons.

The cycle was discovered by Meton (fl. 432 bc), an Athenian astronomer.

Computation from modern data shows that 235 lunations are 6,939 days, 16.5 hours; and 19 solar years, 6,939 days, 14.5 hours

Source

http://www.britannica.com/science/Metonic-cycle#ref102868