Unusual lunar event combines “Blue Moon” and “Micromoon” phenomena

A rare lunar sight known as a “Blue Micromoon” is expected to be visible in the night sky on 31 May 2026, offering astronomy enthusiasts a less common variation of the full moon.

The phenomenon occurs when two distinct lunar conditions coincide: a “Blue Moon” and a “Micromoon”.

What is a Blue Moon?

Despite its name, a Blue Moon does not refer to a change in the Moon’s colour.

It is a term used when a second full moon appears within a single calendar month. This happens because the lunar cycle lasts approximately 29.5 days, meaning most months typically feature only one full moon.

In this case, the previous full moon occurred on 1 May, making the 31 May full moon the second of the month.

There is also a seasonal definition of a Blue Moon, which refers to an extra full moon in an astronomical season that contains four full moons instead of the usual three.

What is a Micromoon?

A Micromoon occurs when a full moon coincides with the point in its orbit where it is farthest from Earth.

Because the Moon follows an elliptical orbit, its distance from Earth changes throughout the month. When it is at its farthest point, known as apogee, it appears slightly smaller and dimmer than usual.

Astronomers say a Micromoon can appear about 6% smaller than a typical full moon and significantly smaller than a “Supermoon”, which occurs when the Moon is closest to Earth.

Why the 31 May 2026 event is unusual

While Blue Moons occur roughly every two to three years and Micromoons can be seen several times a year, the combination of both events is far less common.

Experts say a Blue Micromoon is a relatively rare occurrence that may only happen once in several decades.

On 31 May, the Moon will be approximately 406,000 kilometres from Earth. It is expected to appear around 7% smaller and about 30% less bright than an average full moon.

No Change in Colour Despite “Blue” Name

Astronomers emphasize that the Moon will not appear blue in colour. The term “Blue Moon” is purely calendrical, not visual.

Observers will still see a normal pale white full moon, though slightly smaller and dimmer than usual.

A Rare Celestial Combination

The event brings together two separate lunar concepts that are usually discussed independently, making it a noteworthy moment for skywatchers and astronomy enthusiasts.

While not visually dramatic, the Blue Micromoon is considered an interesting reminder of the Moon’s changing orbit and the precision of astronomical cycles.