A rare blue micromoon will reach its exact full phase on 31 May at 9.45am BST, but for viewers in the UK the moon will already have set by then. Dr Greg Brown said the better way to catch it is to step outside on Saturday or Sunday night, when it will look full to the eye.
The timing is why people are searching for Blue Moon Tonight now: this weekend is the only real chance to see the event at its best, even if the moon’s exact phase comes in the middle of the morning. Brown said it will be indistinguishable from full throughout the night before and basically the night after as well, so skywatchers do not need to wait for a single minute on the clock.
A blue micromoon is a double rarity. A blue moon is usually either the second full moon in one calendar month or, in another definition, the third full moon in an astronomical season. A micromoon is a full moon that happens near the moon’s furthest point from Earth. Brown said the moon’s orbit is elliptical, and because of that a full moon near its closest point is a supermoon, while one near its furthest point is a micromoon.
That does not make this one as blue or as micro as the name suggests. A micromoon is only about 14% smaller than a supermoon and about 6% smaller than a typical full moon, while a blue appearance can happen for entirely unrelated reasons such as forest fires, volcanic eruptions or dust in the atmosphere. Brown said a blue micromoon comes along about once every couple of decades, which is why this one stands out even though the moon itself will look very ordinary.
For people in the northern hemisphere, the moon will sit relatively low in the sky through the night, while it will be very high in the southern hemisphere. The exact full moon time is 4.45am Eastern Time in the US and 6.45pm AEST in Australia, but Brown said the UK will miss the exact moment because the moon is long gone by then. The next UK occurrence appears to be in 2066, while some parts of the world, including the US, are next due in 2053.
So the answer for readers is simple: go out this weekend, not at dawn on 31 May, and do not expect a dramatic color change or a visibly shrunken moon. The event matters because it is rare, it is happening now, and for most people the best view will be the one that looks almost completely ordinary.
