Rare blue moon and micromoon visible from France this weekend

A rare combination of lunar events will be visible over France tonight, May 31, as a so-called 'blue moon' coincides with a 'micromoon', making the full moon appear slightly smaller than usual.

Despite its name, the moon will not appear blue.

The term 'blue moon' refers to the second full moon in a single calendar month. The first full moon of May occurred on May 1, traditionally known as the 'Flower Moon'. The second, on May 31, therefore qualifies as a blue moon.

The phenomenon arises because the lunar cycle lasts around 29.5 days, slightly shorter than most calendar months. As a result, a month can occasionally contain two full moons. This happens roughly every two to three years.

Why the moon looks smaller

This month's full moon is also a micromoon, meaning it occurs when the Moon is close to apogee - the most distant point in its orbit around Earth.

At that point, the Moon is more than 405,000km from Earth, compared with an average distance of around 384,000km.

The result is a full moon that appears slightly smaller and less bright than average.

The effect is real but subtle. Astronomers note that differences in apparent size between the largest 'supermoons' and the smallest micromoons are difficult to detect with the naked eye, despite being some 14% in diameter between the two extremes.

The coincidence of a blue moon and a micromoon is uncommon - due to happen next in 2053 - making this weekend's full moon a noteworthy event for amateur astronomers.