Skywatchers across the UK are set for a rare lunar event later this week as the second full Moon of May reaches its peak. Known as a Blue Moon, the phenomenon only occurs every two or three years and is expected to attract keen stargazers hoping for clear skies.
This year’s event will be even more unusual because the Blue Moon will also be a micromoon, meaning it will appear slightly smaller and dimmer than a typical full Moon.
What Is a Blue Moon?
The phrase “once in a blue moon” is commonly used to describe something that happens very rarely. While many people recognise the saying, fewer realise it has genuine astronomical origins.
Despite the name, a Blue Moon does not actually appear blue. Instead, the term refers to a specific type of full Moon event.
Astronomers use two recognised definitions for a Blue Moon. The first describes the third full Moon in a season containing four full Moons. However, the more widely used definition refers to the second full Moon occurring within a single calendar month.
The reason this happens lies in the difference between the Moon’s orbit and the modern calendar. The Moon takes roughly 29.5 days to complete its cycle around Earth, meaning 12 lunar cycles add up to around 354 days. Since the calendar year lasts 365 days — or 366 in a leap year — an extra full Moon occasionally appears every two or three years.
That additional full Moon is what creates a Blue Moon.
When Will the Blue Moon Be Visible in the UK?
According to the Royal Observatory Greenwich, May’s Blue Moon will reach peak illumination at approximately 9:45am on Sunday 31 May.
Although the Moon will technically peak during daylight hours in the UK, it should still appear almost completely full on the evenings before and after the event. This gives viewers several opportunities to spot it, weather permitting.
As with most astronomical events, visibility will depend heavily on cloud cover. Rural areas and locations away from bright city lights are likely to offer the clearest views.
Why This Blue Moon Is Also a Micromoon
This year’s Blue Moon comes with an additional twist. It will also qualify as a micromoon, a term used when a full Moon occurs near the furthest point of the Moon’s orbit from Earth.
Because the Moon’s orbit is slightly elliptical rather than perfectly circular, its distance from Earth changes throughout the month. When the full Moon takes place at its most distant point, it can appear marginally smaller and less bright than usual.
The effect is subtle and may not be immediately noticeable to casual observers, but photographers and experienced skywatchers are often keen to compare the difference.
A Rare Sight in the Night Sky
The last Blue Moon was visible in 2024, making this latest appearance one of the more unusual celestial events visible from the UK this year.
While the Moon itself will not change colour, the combination of a Blue Moon and a micromoon makes this weekend’s display particularly noteworthy for astronomy enthusiasts and casual observers alike.
For those hoping to catch a glimpse, the best advice is simple: keep an eye on the forecast, find a clear view of the sky and look up over the weekend.
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