The Supermoon is Coming! Do Not Panic!

In once sense, the astronomical event that happens this Saturday, May 5, is unexceptional: it’s the full moon, which happens every 29 days or so. This month, however, is a little bit unusual. We’re being treated to a so-called “Supermoon,” scheduled to make its appearance on Saturday night (exactly when depends on where you live: the U.S. Naval Observatory can give you the exact time of moonrise for your area).

What makes this particular full Moon “super” is simply that our planet’s lone (natural) satellite, whose orbit is slightly oval rather than perfectly circular, will be closer to Earth than average — and this close approach happens to coincide (it doesn’t always) with a full Moon. So the full moon will look bigger, and 16 percent brighter, than average.

Credit: OnlyChozen/flickr

Will it be spectacular? Absolutely, if you’ve got clear skies and if you catch it just after it rises. But then, a full Moon is always spectacular when you see it close to the horizon. This has nothing to do with the Moon itself, but with a trick of the mind called the Moon illusion: when it’s way up in the sky with nothing but blackness around it, the Moon looks relatively puny. Near the horizon, you’re comparing it to trees and houses and the landscape, so it looks big.

In fact, if you hadn’t read this or one of the other hundreds of “Supermoon” stories online, you might not even notice that the Supermoon is all that much more spectacular than a normal full Moon. Phil Plait is especially cranky about the hype, and says so his Bad Astronomy blog. It doesn’t help that the term “Supermoon” was created, not by an astronomer, but by astrologer Richard Nolle (or so he says). It also doesn’t help that stories are flying around that Supermoons cause earthquakes — the 9.0 quake that precipitated the Fukushima nuclear crisis last year, in particular. Here’s just one story that debunked that nonsensical notion.

But a closer Moon does have at least one effect, beyond dazzling onlookers. When the Moon is closer than average, its gravitational pull on the Earth is stronger than usual — 42 percent stronger, says — and since the Moon has the greatest influence on tides (the Sun has more gravity, but it’s much farther away), high tide is higher than it would normally be. Since sea level itself is about eight inches higher than it was a century ago, thanks in large part to manmade global warming, those tides will have more of an effect than they would have back, say, in my great-grandparents’ day. This could cause problems for some coastal communities already feeling the effects of sea level rise, such as southern Florida.

But calling them Supertides would be a bit over the topIt would also be confusing, since they aren’t the highest tides of the year anyway: those are King Tides, which happen when the Sun and Moon are both lined up on the same side of the Earth, their gravity pulling in the same direction, at the same time as the Moon is making a relatively close approach. By definition, these happen not during a full Moon, but during a new Moon.

Now that you know the Moon will be big and bright on Saturday, go take a look. You’ll be awed by how big and bright it is, I promise. And if you have a boat pulled up on the beach, pull it up a little further so it doesn’t set sail without you.