This week, skywatchers will witness a rare blue moon. Despite its name, the moon will not appear blue. The term originates from the peculiarities of our calendar system, which is based on the stars rather than the moon.
Why Does a Blue Moon Occur?
The moon takes approximately 29.5 days to orbit Earth, slightly less than an average month. If our calendar were based purely on lunar months, the year would fall short by about 11 days, gradually drifting out of sync with the seasons. To avoid this, we define a year by the time it takes for the stars to return to the same positions in the sky. This means some years have 13 full moons instead of 12.
The extra full moon is called a blue moon. It is defined as the second full moon occurring within a single calendar month. This May, the first full moon was on 1 May, and the second falls on 31 May, making it a blue moon.
Alternative Definitions
There is a stricter astronomical definition known as the seasonal blue moon, which considers the timing relative to the seasons. By that measure, this week's full moon is not blue. The next seasonal blue moon will occur on 20 May 2027.
Whether you follow the calendar or seasonal definition, this week's full moon offers a delightful opportunity to observe our celestial neighbour and reflect on the intricate ways we measure time.
