This week’s full moon is a blue moon. The term does not describe the colour of the moon, but instead arises from the way we define our calendar in reference to the stars rather than the moon.
The moon takes almost a month to circle our planet. The exact time is 29.5 days but if we were to define our calendar as 12 lunar months, the year would fall short by around 11 days. Thus, the calendar would fall out of step with the seasons.
By defining our year by how long it takes for the stars to return to their same positions in the sky, we must accept that some years will have 13 full moons instead of 12. The extra full moon is the blue moon, defined as the second full moon in a calendar month.
Having had one full moon already on 1 May, the second one on 31 May this week is the blue one. There is, however, another way to define a blue moon.
There is a stricter astronomical way to define a blue moon, known as the seasonal method, but by this reckoning the full moon this week is not blue. Instead, we would have to wait for the full moon on 20 May 2027.
