What is a Blue Micromoon?
This weekend, the night sky will feature a rare spectacle: a blue micromoon. While the name suggests a tiny, blue-tinted moon, the reality is slightly different. Here is what the term means and how to catch a glimpse of the event.
What is a Blue Moon?
There are two definitions of a blue moon. The more common one, according to Dr Greg Brown, an astronomer at the Royal Observatory Greenwich, is a monthly blue moon, which occurs when there are two full moons in a single calendar month. This is unusual because the moon takes about 29.5 days to complete its phases, so a full moon typically happens once a month. The second full moon in a month is called a blue moon, and this type is occurring this weekend.
The second definition is a seasonal blue moon, which refers to an extra full moon during an astronomical season (between a solstice and an equinox). Normally, a season has three full moons, but occasionally there are four, and the third one is called a blue moon.
What is a Micromoon?
A micromoon refers to the moon's apparent size when it is at its furthest point from Earth in its elliptical orbit. If a full moon occurs near the closest point, it is a supermoon; near the furthest point, it is a micromoon. Despite the name, a micromoon appears only about 14% smaller than a supermoon and 6% smaller than a typical full moon, so it won't look like a tiny speck.
How Rare is a Blue Micromoon?
Blue moons happen every couple of years, and micromoons occur two to three times a year. A blue micromoon, however, is rarer, occurring roughly once every few decades. According to Brown, the next blue micromoon for the UK will be in 2066, while some parts of the world, like the US, will see one in 2053. Time zone differences and the international date line can affect whether a full moon is considered a blue moon in different regions.
Is it Actually Blue?
No, the moon will not appear blue. However, an unrelated phenomenon—such as dust from forest fires or volcanic eruptions—can cause a bluish tinge due to scattering of light in the atmosphere.
Best Time to View the Blue Micromoon
The exact full moon occurs on 31 May at 9:45 am BST (4:45 am ET in the US, 6:45 pm AEST in Australia). For the UK, this is after moonset, but the moon will appear full on the night before and after. For the best view, head out on Saturday or Sunday night. In the northern hemisphere, the moon will be relatively low in the sky; in the southern hemisphere, it will be very high.
Is it Related to the Phrase 'Once in a Blue Moon'?
It is unclear. Blue moons are somewhat rare but occur every couple of years. The phrase may instead be linked to the rarer blue tint caused by atmospheric smoke or ash.
