metonic cycle

The Metonic cycle is a period of 19 years (235 lunar months), after which the new and full Moons return to the same days of the year. New Moon to new Moon is one cycle.

It appears as a thin curved shape, but I think of it as the time when we cannot see the Moon. The Metonic cycle was the basis of the ancient Greek calendar and is still used for calculating movable feasts such as Easter.

1711 to 2300 = 589 years
589/19 (the Metonic cycle) = 31
365 days/31 = 11.7741935 (months in a year, not 12)