Sometimes in one calendar month, two full moons occur at once, although this is a fairly rare event. But very rarely, two full moons occur at once in two calendar months of the same year. In what months of the year can this happen?
For two full moons to occur in one calendar month, the duration of this month must be greater than the period between two successive full moons (the latter is called synodic month and is 29.5 days). Then a situation is possible that one full moon occurs at the very beginning of the month, and the second at the very end of it. However, if the next month is so short that during it the full moon does not have time to occur, then in the next month after it the situation will repeat itself: during it two full moons will again be observed, one at the very beginning, the other at the end.
If the length of the synodic month is known, then it is quite obvious that the intermediate month, when there may be no full moons at all, can only be February. If it is unknown, then a similar conclusion can be made by realizing that the desired calendar month should, firstly, be shorter than the synodic one, and secondly, months with such a duration should be quite rare (otherwise the situation described in the condition will not be “quite rare “), Ie months with a duration of 30 days are probably no longer good.
Well then, two months of the year, in each of which two full moons are possible – January and March.