From Wiki
Lunar month:
"Therefore, the synodic month takes 2.2 days longer than the sidereal month. Thus, about 13.37 sidereal months, but about 12.37 synodic months, occur in a
Gregorian year."
So in 36 calendar months, there are 40.11 sidereal months.
Alternatively, 1 mean year = 365.2425 days.
1 sidereal month = 29.531 days. Ratio 12,368 sidereal months/calendar year.
Every 3 calendar years there are 37.10 sidereal months = 37.10 full moons.#
i.e., approx. 12 + 12 + 13 or two years of 12 full moons and one year of 13 full moons.
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