May 2026 brings with it an astronomical curiosity: during the same month there will be two full moons. The first is scheduled for May 1st, the second for the 31st. The latter is popularly called “blue moon” — a term that has nothing to do with color, but which recalls the English expression once in a blue moonequivalent to our “every death of a pope”, to underline the rarity of the event.
Over the years, two definitions of Blue Moon have spread: the “original” is actually the one attributed to the third full moon in a season that contains four, but now also the second full moon in the same month is called the same way, due to the confusion created by the amateur astronomer James Hugh Pruett (1886–1955) and broadcast first by the radio program StarDate in 1980, therefore from the game Trivial Pursuit,
Why does this happen?
The cause is purely mathematical, the lunar cycle lasts on average about 29 and a half days, while most calendar months have 30 or 31. When a full moon falls on the first day of the month, there is enough time for the cycle to complete again before the end. February, with its only 28 or 29 days, is the only month in which this cannot happen.
The phenomenon repeats itself every two or three years; the latest occurred in the summer of 2023.
A smaller than usual full moon
The full moon on May 31st will also be one micromoon: at that moment our satellite will be near the apogee, the point of its orbit furthest from the Earth, therefore being slightly smaller and dimmer than average. The difference compared to a supermoon can be up to 14% in apparent diameter, but to the naked eye it is almost imperceptible.
And when a month remains without a full moon?
Again due to the 29.5 days of the lunar cycle, the opposite can happen: February sometimes remains completely devoid of a full moon, a phenomenon known as “black moon”. However, it is decidedly rarer — the last time was in 2018 and the next one is expected in 2037.
