Science & Technology

What's a Supermoon, Anyway?

If you've heard the word "supermoon," you've probably heard the myths: This extra-special moon looms huge in the sky, exerting a greater gravitational pull that can cause everything from earthquakes to flooding to health effects. Fear not: A supermoon isn't as big of a deal as its made out to be, and there's not really much that it can do to you. But what is it, anyway?

Look! Up in the Sky!

The moon goes around the Earth pretty much as regularly as you could hope for — and that's a good thing because our planet without its kid sister would be a lot less habitable. But its orbit is a long way from a perfect circle. It takes the form of an ellipse, and that means that there are times when it's farther from the planet and times when it's a whole lot closer. The situation is further complicated by its relatively small size, which makes it more susceptible to the gravitational effects of other cosmic bodies. In other words, you can definitely count on seeing a few more supermoons in your life — but you won't be able to guess where they are by spotting a pattern on the calendar.

Actually, 2018 started out with a rare supermoon twofer, with one on the first day of January and another on the 31st. Today, you can see another one — or you would if it weren't also a new moon. But even if it's a little harder to spot in the sky, it's still pretty significant. It's going to be the closest of the summer's three new supermoons in a row, the first one being a month ago on June 13 and the last on August 11. That's right: three supermoons in a row and all of them are new moons (meaning they're caught between the Earth and the sun so that the side facing away from us is completely illuminated and the side that faces us completely dark). That's got to be a huge coincidence, right?

Well ... not really. In general, it's considered a supermoon if the full moon coincides with perigee, and only recently have people begun to consider new moons supermoons as well. In other words, all supermoons are either full or new, so three new moons in a row isn't that unusual.

Near, Far, Wherever Lunar

If the significance of a supermoon seems a little fuzzy to you, then this might make it more clear: Its origins lie not with astronomers but with astrologers. Astronomers do, of course, concern themselves with the phenomenon, but traditionally they refer to these moons as either the perigean new moon or the perigean full moon. But the effects of this particular cosmic alignment have less to do with your star charts and more with the movement of the tides.

Remember, every time there's a full or a new moon, it means that the Earth, moon, and sun have gotten themselves in a line. The moon always has an effect on the movement of the tides, and when it lines up with the sun, that effect is even more pronounced. The moon's gravitational pull is a lot stronger because it's a lot closer, but when it's combined with the comparatively weaker pull of the sun, it will cause the daily tides to rise even more. Now, when the full or new moon also coincides with the perigee — which is to say, when there's a supermoon — the moon's gravitational pull on the tides is even stronger because it's even closer than normal. Called "perigean spring tides," these are the most pronounced tides of the year. At their most extreme, they can seep six inches further up the shore than during other times. It's generally not enough to put any ships at risk, but it's still a pretty astonishing reminder of the cosmic forces that invisibly shape our world.

For a hilarious take on the strange details involved in our next big interplanetary expedition, check out "Packing for Mars: The Curious Science of Life in the Void" by Mary Roach. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Does the Moon Rotate?

Written by Reuben Westmaas July 9, 2018

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