Stargazers in the UK are in for a treat this week as a rare blue moon is set to appear on 31 May. Despite its name, the moon will not actually appear blue; the term refers to the occurrence of a second full moon within a single calendar month.
The phenomenon arises from the discrepancy between the lunar cycle and our calendar system. The moon takes approximately 29.5 days to orbit Earth, but our calendar year is based on the solar cycle, which is about 365 days long. This mismatch means that roughly every 2.5 years, a calendar month contains two full moons.
This month, the first full moon occurred on 1 May, making the one on 31 May the blue moon. However, there is an alternative definition known as the seasonal blue moon, which is based on astronomical seasons. By that reckoning, this week's full moon is not a blue moon; the next seasonal blue moon will occur on 20 May 2027.
Regardless of the definition, the full moon on 31 May will be a bright and impressive sight in the night sky, weather permitting. Skywatchers are advised to look east after sunset for the best views.
