The Supermoon stuff? AGAIN?

Bad Astronomy
The entire universe in blog form
May 2 2012 10:37 AM

The Supermoon stuff? AGAIN?

Sigh.

You may've seen some folks writing about this weekend's so-called Supermoon. I suppose I'm not surprised, but it's still irritating. Why? Because it's just hype (and to get this out of the way immediately, will have no real effect on the Earth, either). Here's the scoop.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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This weekend, on the night of May 5/6, the Moon will be full. This happens every 29 days or so when the Moon is opposite the Sun in the sky, and we see its face fully illuminated.

As it happens, the Moon's orbit is elliptical, and so sometimes the Moon is a bit closer to the Earth than other times. Every now and again the Moon is full when it's also closest to Earth -- the point in its orbit called perigee. May 5th is one of those times.

What does this mean? Well, it means the Moon is closer, so it will appear a bit bigger and brighter than usual. But here's the thing: you'd never know. Seriously, to the eye it'll look exactly the same as it always does when it's full. The Moon is actually pretty small in the sky -- if you don't believe me, go outside tonight, find the Moon, and hold your thumb up at arm's length next to it; it'll easily cover the Moon entirely (my thumb is 2 - 3 times wider than the Moon). A small change in its size is something that's really hard to see.

To be specific, according to Fourmilab, the Moon will be 356,953 kilometers from Earth when it's full. However, last month, on April 7, when it was full it was about 358,313 km away. That's a difference of 1400 km, less than 1%. So really, the size of the full Moon this weekend won't be any different than it was last month, and no one was writing about it then. And to show I'm not being biased, take a look at when the Moon was full near apogee -- the most distant point in its orbit. That'll happen in late November of 2012, when it'll be at a distance of 406,364 km. That's still only a difference of less than 14%.

That's a pretty small change, not enough to notice by eye. To see the difference in this weekend's full Moon, you'd need to take a picture and compare its size to how it looks at some other full Moon. The picture above shows that pretty well (click to enlunenate). You can see the difference between perigee and apogee Moons there easily, but that's because the Moons are side-by-side. This weekend, with the rising Moon all by itself in the sky, you have nothing to compare it to. It'll look the same as it always does (and don't confuse the rising Moon looking huge due to the Moon Illusion with this Supermoon silliness).

That hasn't stopped some news venues touting this as a "Supermoon". I'm seeing it on websites, on Twitter, and getting email about it, and like I said, it's irritating (and I'll add the idea for this whole as well as the term "Supermoon" were started by an astrologer, so draw your own conclusions there). I'm all for encouraging people to go out and look at the Moon, but it shouldn't be under false pretenses. I mean, c'mon: it's the Moon! It's bright and silvery and lovely and you can see features with your naked eye and with a telescope you'll see tons more!

Even though it's not super, the Moon isn't exactly mild mannered either. So if you can, go out and look. Not just this weekend, but any time! Use a telescope, or binoculars, or just go and look anyway. Because it's pretty, and it's ours, and it's always worth a look.

Image credits: Apogee/Perigee: Anthony Ayiomamitis; Rising Moon: me!



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