This week's full moon is a blue moon, a term that does not refer to the moon's color but to a quirk of our calendar system. The moon takes 29.5 days to orbit Earth, slightly less than an average month. If our calendar were based on 12 lunar months, the year would be about 11 days short, causing it to drift out of sync with the seasons.
Why Blue Moons Occur
By defining our year based on the stars' positions, we must accept that some years have 13 full moons instead of 12. The extra full moon is called a blue moon, defined as the second full moon in a calendar month. Having had a full moon on 1 May, the second one on 31 May is the blue moon.
Alternative Definition
There is a stricter astronomical definition known as the seasonal method, but by that reckoning, this week's full moon is not blue. Under that definition, the next blue moon would occur on 20 May 2027.
