Blue Moon Set to Appear in UK Skies on 31 May 2026

The term “Blue Moon” describes a rare calendrical event in which either two full moons occur in a single calendar month or four full moons occur in one astronomical season, with the third being called a Blue Moon.

In 2026, the UK will observe a Blue Moon on 31 May, as May will feature two full moons within the same month according to the modern definition. The full moon is expected to reach its peak at approximately 9:45 am UK time on that day. This event will also be classified as a micromoon, meaning the Moon will be near its farthest point from Earth and appear about 7% smaller than average.

The phrase “Blue Moon” is not related to the Moon’s color, but rather to its timing within the calendar or season. The modern interpretation, which identifies the second full moon in a calendar month as a Blue Moon, originated from a misinterpretation published in 1946. The earlier, traditional definition comes from the Maine Farmers’ Almanac and ecclesiastical calendar rules, identifying the third full moon in a season with four full moons as a Blue Moon.

Blue Moons, whether defined by the monthly or seasonal method, tend to occur every two to three years. The next seasonal Blue Moon, based on the traditional definition, is scheduled for 20 May 2027.

What the numbers show

• The Blue Moon in the UK will occur on 31 May 2026
• The full moon will reach its peak at around 9:45 am UK time
• In 2026, the Blue Moon will appear about 7% smaller than average due to its micromoon status
• Blue Moons typically happen every two to three years
• The next seasonal Blue Moon is set for 20 May 2027

While the term “Blue Moon” does not refer to an actual change in color, rare atmospheric conditions such as volcanic ash or smoke can cause the Moon to appear blue to observers. However, these occurrences are unrelated to the calendrical Blue Moon event.

The upcoming Blue Moon in May 2026 will be visible as a normal full moon, with its unique status determined by the timing within the calendar month. The event’s classification as a micromoon means it will be slightly smaller in appearance compared to other full moons throughout the year.

Both the modern and traditional definitions of Blue Moon are recognized in astronomical references and calendars. The distinction between the two definitions is based on either the monthly occurrence or the seasonal cycle of full moons.

Blue Moons remain a point of interest for skywatchers and those following lunar calendars, with the next opportunities for observation already documented by astronomical organizations.

* This article is based on publicly available information at the time of writing.