Have you heard? There's gonna be a Supermoon tonight! It's a very cool sight to see, especially on a clear night. But it's way more common than you might think. In fact, the next three full moons this summer will be Supermoons. And astronomers are kind of puzzled as to why we seem to only notice them at random.
"Supermoon" is a nonscientific term coined in 1979 to describe the point on the moon's oval-shaped orbital path that brings it closest to our planet. The technical term is perigee. Whatever you call it, at that point the moon is just a smidge over 31,000 miles closer to earth than it is at its farthest point, the apogee.
What kind of difference does 31,000 miles make? On a perfectly clear night, when the timing of the full moon coincides exactly with the perigee, planet earth's pet rock will show up about 14 percent bigger and, say, 30 percent brighter. Here's a comparison between an average full moon and a Supermoon to show you what that 14 percent bonus looks like.
So, yeah, it's bigger, but it's not massively, eye-poppingly doubled in size or anything. And it's not really even all that rare. Geoff Chester of the U.S. Naval Observatory explains that a full moon occurs at or near perigee every 13 months and 18 days, on average. There were three "Supermoon" events last year, but only one was widely reported. Yes, even here on Gizmodo.
What made the one everyone talked about on June 22, 2013 so much more special than the other two? Probably just the ol' internet hype machine. Someone started talking about it, and other people talked about it, and soon you had Supermoon fever—despite the fact that this was neither the first, nor the last, nor even the biggest Supermoon.
In fact, people's fascination with the Supermoon is probably due to a poorly-understood optical illusion more than anything. As NASA explains:
The illusion occurs when the Moon is near the horizon. For reasons not fully understood by astronomers or psychologists, low-hanging Moons look unnaturally large when they beam through trees, buildings and other foreground objects. When the Moon illusion amplifies a perigee Moon, the swollen orb rising in the east at sunset can seem super indeed.
So take a moon that is, truthfully, a little bigger than usual, add the sudden attention of people who may not realize how common this event is, and you've got a recipe for some false memories. "I guarantee that some folks will think it's the biggest Moon they've ever seen if they catch it rising over a distant horizon, because the media will have told them to pay attention to this particular one," Chester says.
As for Chester himself? He's kind of sick of this crap.
There's a part of me that wishes that this 'Supermoon' moniker would just dry up and blow away, like the 'Blood-Moon' that accompanied the most recent lunar eclipse, because it tends to promulgate a lot of mis-information. However, if it gets people out and looking at the night sky and maybe hooks them into astronomy, then it's a good thing.
So yes, absolutely go outside and admire the full moon tonight. Take pictures. Smooch someone you love under romantic moonlight. Howl at it, I don't care. If it's a clear night where you are, the view will probably be gorgeous. But it's not nearly as unusual as you might expect.
In fact, the next full moon, on August 10th, will be even bigger, becoming full during the exact same hour it hits perigee. Maybe we should call that the Super Duper Moon.