Dr Greg Brown explains Blue Moon Tonight’s 9.45am BST peak
A blue moon tonight will pair with a micromoon this weekend, and the full moon reaches its exact phase at 9.45am BST on 31 May. Dr Greg Brown of the Royal Observatory Greenwich said people in the UK should look on Saturday or Sunday night, when it will appear full through most of the night.
Brown said the event is a blue micromoon, a combination that occurs about once every couple of decades. In the UK, the next one using the same definition appears to be in 2066, while some parts of the world, including the US, will have one in 2053.
Dr Greg Brown on blue moons
Brown described the better-known version as a monthly blue moon: “Probably the more common one that’s used at the moment, the one that’s better known, and certainly the simplest, is a so-called monthly blue moon”. He said, “This is where you get a second full moon in one single calendar month.”
He added another definition used by astronomers: “Within those roughly three months you expect to get three full moons, but occasionally you’ll get four”, and “for some reason, it’s the third one which is referred to as the blue moon.”
The moon takes 29 and a half days to cycle through its phases, which helps explain why a second full moon can land within one calendar month or a season with four full moons. Time zone differences can also shift how the event is labeled in different places.
Royal Observatory Greenwich and the micromoon
Brown said, “The moon is not always a constant distance away from the Earth. Its orbit around the Earth is elliptical.” He added, “If a full moon happens to occur close to its closest point, then we call that a supermoon”, and “if it’s close to its furthest point, then we call that a micromoon.”
He said a micromoon looks about 14% smaller than a supermoon and about 6% smaller than a typical full moon. An unrelated blue moon effect can also come from dust in the atmosphere after significant forest fires or volcanic eruptions, which can give the moon a bluish tinge, but that is not the event due this weekend.
31 May in the UK
The full moon reaches its exact full phase at 9.45am BST on 31 May, or 4.45am Eastern Time in the US and 6.45pm AEST in Australia. Brown said the UK will already have missed the exact full-moon moment by then because the moon will have set, but it will still look full the night before and the night after.
Brown said the moon will be relatively low in the sky throughout the night in the northern hemisphere, while it will be very high in the sky in the southern hemisphere. For people in the UK, the practical window is Saturday or Sunday night, not the exact minute on 31 May.
