:format(webp)/cdn.vox-cdn.com/uploads/chorus_image/image/36687628/183460906.0.jpg)
If you look to the sky Saturday or Sunday night, you might be struck by something: the uncommonly large size of the full moon.
That's because this month brings us a "Supermoon" — an unofficial astronomical term that refers to a full moon that looks slightly bigger than usual.
A comparison between an average moon and a supermoon. (Marcoaliaslama)
The reason for this: the moon's orbit around Earth is elliptical, so each month, it spends some time a bit further away from us (with a maximum of about 252,000 miles) and a bit closer to us (with a minimum of about 222,000 miles).
A diagram of the moon's elliptical orbit, which has been exaggerated here for effect. (NASA)
Due to the moon's position relative to the sun, it also varies on a (roughly) monthly basis in its phase — in other words, how much of the moon is illuminated by the sun, and thus how much of it we can see. This is the lunar cycle (you know, full moon, quarter moon, etc).
Every so often — roughly every 14 months — the full moon happens to occur at the same time that the moon is especially close to us. When this happens, it looks a little bigger and brighter than usual, leading to the term Supermoon. (Near-Supermoons can also happen a bit more often — one occurred in July — but this one is the real deal.)
How you can see the Supermoon
(Susa Ark/Getty Images)
The official moment when the full moon begins is Sunday at 2:09 p.m. Eastern time, but it'll look rather full both Saturday and Sunday night.
If you go out expecting to see a giant orb hovering in the sky, though, you might be disappointed. Estimates are that the moon will appear somewhere between 10 percent and 14 percent bigger than normal, and 30 percent brighter — certainly something to take note of, but not an astronomical phenomenon of huge proportions.
One tip: experts say that the this moon's size will be most striking when it's near the horizon, so you're able to compare it to objects of familiar size and more easily note the discrepancy. Additionally, the moon generally looks biggest to us when it's near the horizon, due to an optical illusion that's still being debated.
To take advantage of this and catch the moon near the horizon, look up the precise moment of moonrise in your location here, and head outside then.
