Blue moon: how to see rare lunar event

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A rare blue moon is set to pass overhead — and be visible tonight to most people in the UK and elsewhere.

The moon will be most visible towards the south of the UK, which is forecast to have clear skies. There will be less cloud cover towards the early evening.

Since the satellite itself doesn't actually change, the blue moon will be visible from anywhere that could normally see the non-blue one.

(The International Space Station should also be visible a few times through the night, if you want to kill two sights with one look.)

There won’t be much to see beyond the normal moon — it’s very unlikely that it’ll actually be blue, and that’s not where it gets its name from. The event is actually a quirk of the calendar that happens just over every two and a half years — the moon runs on a 29.5 day cycle, and therefore every so often there’s an extra day — so the moon itself doesn’t change or look any different from Earth.

But it will be extra big and bright, since it’s also approaching very close to the Earth. That will be what accounts for any more spectacular look this evening, rather than it being anything to do with being a “blue moon”.

(The moon itself does have some blue bits, as the Royal Observatory has pointed out, but they aren’t usually visible from Earth.)

Sometimes the moon does look blue from Earth. But that doesn’t have anything to do with the event happening tonight, and is instead caused by volcanic eruptions that spread ash into the sky and catch the light.