Bad Astronomy

Kryptonite for the supermoon

[NOTE (added March 19): It occurs to me that some people might see the Moon rising today and think it looks HUGE because it’s a “supermoon”. However, it’s far more likely they’re falling victim to the famous Moon Illusion. You can read all about it here.]


Illustration by Phil Plait. Ask about my rates!

If you believe the mainstream media, you might think this weekend’s “supermoon” will cause earthquakes, volcanoes, bad weather, halitosis, dust bunnies, and hangnails.

Guess what I think of this idea! Hint: check the name of my blog. Got it? Good.

In reality, this “supermoon” nonsense is, well, nonsense. I have some details below, but for those of you who are impatient (the tl;dr crowd) here are the bullet points:

  • Yes, the Moon is closer today than usual, but only by less than 2%.
  • This does happen around full Moon, which is when we get bigger tides, but that happens every single month. The Moon being closer amplifies that, but only a tiny little bit.
  • The Moon’s possible effect on earthquakes has been studied for a long time. The result? Major earthquakes are not correlated with the Moon’s position or distance.
  • Therefore,
  • Anyone claiming this “supermoon” can cause earthquakes or whatnot is, to be blunt, totally, completely, utterly, wrong.


OK, so, how about some details?

Brief overview

I went over a lot of this in my post showing the Moon had nothing to do with the Japanese earthquake. Briefly, the Moon orbits the Earth in an ellipse, so sometimes it’s closer than other times. On average, it’s most distant point (apogee) is about 405,000 km (251,000 miles) and at closest (perigee) it’s about 363,000 km (225,000 miles).

Those numbers change month to month due to the gravity of the Sun and other effects. As it happens, on March 19 at around 02:00 UT (early evening Friday March 18 for most of the US) 19:10 UT [D’oh! Cut and paste accident there, sorry; note this doesn’t affect my argument] the Moon reaches an unusually close perigee distance of a bit more than 357,000 km. Gravitationally, this doesn’t mean much. That extra 6000 km closer than on an average perigee is only about 1.6%, which is pretty trifling. It means the gravity of the Moon on the Earth is only 3% stronger.

The Moon also affects us through tides, which are similar to gravity. But the tides will only be 5% stronger than usual for a perigee due to the Moon’s proximity!

Now to be clear, this is happening at a time of the full Moon (which happens tomorrow, March 19, at 18:10 UT). That means the Sun, Earth, and Moon are roughly lined up in space, so the Sun and Moon’s tidal pulls add together. Every full Moon we get what are called spring tides, with extra high high tides, and extra low low tides. Places prone to flooding do see more on spring tides, every single month of the year. This extra 5% tug this weekend makes that a bit worse, but only a bit.

Earthquakes, volcanoes, panic?

Does this extra tweak have any other effect on the Earth? Could it cause quakes, volcanoes or anything else?

Nope. Again, go read my supermoon post from last week. Earthquakes and the Moon have been studied extensively. Mind you, the Moon orbits the Earth every month, and there are thousands of earthquakes every year, so any correlation between the two would scream out of the data. The best that’s seen is a weak connection between the Moon and shallow, low-magnitude earthquakes. Big earthquakes, like the the ones in Japan, Christchurch, or Chile in the past few months have clearly not been triggered by the Moon. In fact, the Japan quake happened when the Moon was closer to apogee than perigee! That right there is a bit of a showstopper for this “supermoon” idea.

So why do people keep talking about this?

We humans love to seek correlations, and will see them even when they aren’t there. That’s why astrologers are still in business, despite having no scientific evidence whatsoever that their predictions are any better than random guessing.

In fact, this “supermoon” idea was started by an astrologer named Richard Nolle. On his website, he defines the term as a new or full Moon when the Moon is closer to Earth than usual. He goes and gives a more precise definition, but it’s rather arbitrary*. He says quite bluntly – and quite incorrectly – that lots of seismic events (plus bad weather) can be attributed to the Moon.

For example, about this month’s Moon he says:

That makes this a major geophysical stress window, centered on the actual alignment date but in effect from the 16th through the 22nd.

Note that time period: seven full days. The lunar orbit is about 27 days long, so there’s a 25% random chance of something happening during that time, Moon or no Moon!

He goes on:

Of course you can expect the usual: a surge in extreme tides along the coasts, a rash of moderate-to-severe seismic activity (including Richter 5+ earthquakes, tsunami and volcanic eruptions), and most especially in this case a dramatic spike in powerful storms with heavy precipitation, damaging winds and extreme electrical activity.

Wait a sec: the USGS has records of earthquakes, and there are about 1469 earthquakes every year greater than magnitude 5. That’s 4 per day, so the odds of having at least one quake that size or greater during his “supermoon” period are virtually 100% – just as they would be if you picked any random one-week period. Heck, pick any random day of the year and there’s a near-certainty there will be a mag 5 quake somewhere.

Given that there are tens of thousands of thunderstorms every year, and basically continuous volcanic eruptions, suddenly his predictions seem a little less shiny. But this is typical of this kind of prediction. If you go to his site and read his claims, he picks out all the times there were big earthquakes or volcano eruptions during his “supermoon” periods, but doesn’t tell you how many happened the other 3/4 of the time. This is called cherry-picking and is a big no-no when making actual scientific claims.

But this sort of verbal slipperiness gets eaten up by the media. You can find tons of breathless news media (on the web, on TV, on the radio, everywhere) uncritically accepting these claims. Although most do consult with actual scientists who are clear this is all bunk, the writers tend to put that several paragraphs down where people are less likely to read it. [Note: as pointed out in the comments below, quite a few online news sites reported this non-event responsibly. As I replied to that commenter, I’m glad! Those articles were posted today, and I missed them as I posted my own, and was referring to things I had seen previously about this, as well as the abysmal reporting of other science claims (superstorms, the extra zodiac sign, Apophis, Betelgeuse exploding, a giant planet in the outer solar system and so on) that has been going on lately. For this specific “supermoon” reporting, I was using too broad a brush to paint the media.]


I think I’ve made my point. We go through a lunar perigee every month, and you don’t hear about ginormous effects. We go through new and full Moon each once per month, with the same lack of planetary distress. Putting them together barely nudges the gravitational needle. You might see a little more flooding in some low-lying areas, but that’s it.

Earthquakes? Nope.

Volcanoes? Nope.

Scary weather? Nope.

To be clear, we almost certainly will see earthquakes, volcanoes, and scary weather during this time… just as we do every single day of every single year!

So this “supermoon” is nonsense, pure and simple. Don’t buy it.

But I’ll add that the Moon will actually be a bit closer than usual, and while you might not notice the size or brightness difference by eye, the full Moon is always a lovely and compelling sight in the sky. So I urge everyone to go out and take a look. And while you’re looking think on this: a dozen men have walked on the Moon, dozens of probes have been sent there, and the Lunar Reconnaissance Orbiter is still snapping away, mapping our friendly satellite and taking dazzling images of its surface.

That’s real, that’s tangible, and that’s what we humans can do when we stick with science.

* At the bottom of his supermoon page he explains his definition: take the difference between the Moon’s nearest and farthest distances (about 50,000 km), take 90% of that (about 45,000 km) and then subtract that from its farthest distance (406,000 km - 45,000 km = 361,000 km. Any time there is a new or full Moon when it’s less than that distance is a “supermoon”. But why 90%? Why not 80, or 95? He never says. It’s almost as if he pulled that number out of thin air.