A blue moon is set to illuminate UK skies on 31 May, marking the first such event since August 2024. Despite its name, a blue moon is not actually blue in colour. Instead, the term refers to either the second full moon occurring within a single calendar month, or the third of four full moons observed in one astronomical season.
What is a Blue Moon?
According to the Royal Observatory Greenwich, blue moons are relatively rare, occurring only every two to three years. However, two blue moons took place in 2018 alone. The phenomenon is a quirk of the lunar calendar, where the 29.5-day cycle of the moon sometimes aligns with our Gregorian calendar to produce an extra full moon.
When to See the Blue Moon
This month's blue moon is expected to reach its peak at 9:45am BST on 31 May, as confirmed by the Royal Observatory. While the moon will be at its fullest during the day, it will still appear bright and full in the night sky on both 30 and 31 May. For the best viewing experience, find a location with minimal light pollution and a clear horizon.
The next blue moon after this event is scheduled for 20 May 2027, giving skywatchers a few years to wait for the next occurrence.
