A rare blue micromoon is due on 31 May, but the exact full moon arrives at 9.45am BST, when it is already below the horizon in the UK. That means people looking up over Britain will miss the peak moment, even though the moon will look full the night before and the night after.
That is why searches for is it a full moon tonight are spiking now: the answer depends on where you are and when you look. Dr Greg Brown said the best view in the UK should be on Saturday or Sunday night, when the moon will be indistinguishable from full for the whole of the night beforehand and basically the night after as well.
The event is being billed as a blue micromoon, a label that sounds more dramatic than the sky will look. Brown said the moon is not actually blue unless dust in the atmosphere from major forest fires or volcanic eruptions gives it a bluish tinge. And a micromoon is only about 6% smaller than a typical full moon, so the difference is subtle even when you know what you are seeing.
Brown explained that a blue moon usually means either a second full moon in a single calendar month or, in another definition, the third full moon in an astronomical season. A micromoon happens when a full moon occurs close to the moon’s furthest point from Earth, while a supermoon is the opposite, when a full moon lines up near the closest point. Because the moon’s orbit is elliptical, its apparent size changes from month to month.
That is what makes this one unusual. Blue moons come every couple of years and micromoons two or three times a year, but the combination is rare enough to turn up only about once every couple of decades. Using the same definition, the next one for the UK appears to be in 2066, while some parts of the world, including the US, should get one in 2053.
For people outside Britain, the timing is different again: the full moon falls at 4.45am Eastern Time in the US and 6.45pm AEST in Australia. In the northern hemisphere it will sit relatively low in the sky through the night, while in the southern hemisphere it will be very high. For anyone hoping to see it from the UK, the answer to the question is yes, there is a full moon tonight — but the best of this one comes on Saturday or Sunday night.
