This week’s full moon is a blue moon. The label does not refer to its color but results from how calendars are set by the stars rather than lunar phases. The moon completes an orbit of Earth in 29.5 days. Using twelve lunar months for a year would leave it about eleven days short, throwing it out of sync with the seasons. Measuring the year by the return of stars to the same sky positions means some years contain thirteen full moons. The extra one is termed a blue moon and is defined as the second full moon in one calendar month. With the first occurring on 1 May, the second on 31 May qualifies. A stricter seasonal definition of a blue moon also exists, but under that system this week’s moon does not count. The next such event would occur on 20 May 2027.
