Big moon rising!

As I was cycling back from the bus stop this evening and glancing up at the full moon (all smudgy-looking, as it was shining through clouds), I was struck by how unusually large it looked in the sky. I’m very familiar with the “moon illusion” – caused by our brains playing tricks on us when the moon is near the horizon and making it look larger than when it’s higher in the sky (see this page for more details) – and wondered if this was just another example of that, but this seemed a bit different.

As it turned out, I actually wasn’t imagining things – tonight’s full moon is bigger in the sky than it usually is, because the moon is approaching its perigee. Like almost all orbiting bodies, the moon’s orbit is actually slightly elliptical (i.e. it’s not a perfect circle), and over the course of its roughly 28-day orbit around the Earth its orbital distance varies from a closest approach to the centre of the Earth (“perigee”) of about 356,000 km to a furthest approach (“apogee”) of about 406,000 km. (Incidentally, those italicised suffixes are quite versatile, and can be applied to whatever suffix is appropriate to the central body as shown here).

This month, the Moon will actually be at perigee on February 19th (in two days, at the time of writing), but that’s a day after it’s officially a “Full Moon”. According to this Lunar Perigee and Apogee calculator, the Moon is currently about 359,000 km from the centre of the Earth and will getting closer until Feb 19th, whereupon it will move away again on its orbit and will be at apogee on March 6th when it’ll be 406,582 km from Earth’s centre.

However, if you’re looking at the tables on that calculator page you’ll notice that the perigee and apogee distances aren’t always the same – sometimes the Moon is even closer at perigee, and sometime it’s further away at apogee – because the sun’s gravity also affects the eccentricity of the Moon’s orbit. It turns out that next month – on March 19th – the moon will be even closer to Earth at perigee (356,577 km) and it’s going to be a full moon on that very day – and March’s full moon is also going to be the closest it’s been to Earth since 1993! (it’s actually going to be slightly closer in November 2016, and even closer in November 2034!)

The full moon should still loom larger in the sky over the next couple of nights at least, so see if you can notice whether it looks larger than normal to you. But definitely have a look at it again on March 19th, when the full moon will be the closest it’s been for eighteen years!

(more info can be found on this page, which is also where the moon graphic is taken from)

March 19th EDIT: And as March 19th rolls around, there’s a lot of rubbish going around the internet about how the so-called “supermoon” causes earthquakes etc etc… it won’t. There is no correlation between earthquakes occuring and the moon’s orbital distance – and it’s only a few hundred km closer than it was last month anyway. Just go out tonight and enjoy the best view of the full moon you’re going to get for a while!

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