Supermoon: what it is, and what it definitely isn’t (a repost)

A repost, apropos of this weekend’s supermoon and the fact that people are going bugnut over it… yet again… and Taslima seemed lonely in being the only other FtBer covering this one. My original post is here, published March 17, 2011.

Look up in the sky! It’s a bird! It’s a plane! No, it’s a SUPERMOOOOOOON!!

I have written at some length about the moon, with its wobble called libration, and how its elliptical orbit means that it varies in its distance to us between roughly 360,000km and 406,000km. That’s a difference of ~46,000km, or about ten percent of its distance at apogee. Apogee is what you call the moon’s furthest point in its orbit, and perigee the closest. As the moon orbits us about once a month (thus the lunar cycle), that means that during a predicted perigee, the moon is about two weeks away from apogee.

The moon orbits us at an inclination of about 5 degrees to the solar plane — it is for that reason that we do not see a total lunar eclipse somewhere on the planet once a month, but rather only on those months where the moon is roughly aligned with the plane of the Earth’s orbit around the sun. That means we get to see the sun illuminating the moon most of the time; its phase indicates where in the rotation it is, and what face is pointed toward the sun. A supermoon is when the full moon phase happens to coincide with the perigee — meaning the moon is not only at its visibly largest point, it’s also got the full face illuminated.

Astrologers have consistently through the ages attempted to link supermoons with natural disasters. Because the Sun, Earth and Moon are all in a sorta-kinda-straight line, and the moon is SOOOO MUCH CLOSER TO US THAN NORMAL, naturally this must mean there’s a lot of possibility that the moon is going to cause calamity on our planet. This makes sense, if you believe the heavenly bodies exert any more influence on the planet than the evidence shows them to exert. The problem with this line of thinking is, if you claim an effect and give a window, and the window’s large enough that statistically, it’s very likely something within the range you’re claiming will happen, then selection bias will do the rest of your work for you. Because you’ve primed people to watch for natural disasters, when they happen, SURPRISE, suddenly that means you were right!

To wit:

On March 19, Earth’s satellite will be at its closest point to our planet in 18 years — a mere 356,577 kilometers away. The event — also called a lunar perigee — was dubbed a “supermoon” by astrologer Richard Nolle back in the 1970s. The term is used to describe a new or full moon at 90% or more of its closest orbit to Earth. Next week, it will be at 100%.

Nolle is responsible for coining the upcoming event, and he’s also responsible for the latest buzz sweeping the Internet about how the supermoon will affect the planet. On his website Astropro, Nolle warns Earth’s inhabitants to prepare themselves during the “supermoon risk window,” which ranges from March 16 – 22. During this time, Nolle claims there will be an increase in supreme tidal surges, magnitude 5 or higher earthquakes, and even volcanic activity.

I’m amazed that Fox News got most of the science right in this, excepting contradicting his quote — there’s a closest perigee every year, where perigee and apogee actually fluctuate year after year, and this “closest point in eighteen years” only means “one or two percent closer than normal”, and anyway, it’s still further away than its closest recorded perigee of 356,375km on Jan. 4, 1912. The only earthquake noted on that day (given humankind’s lower population density) was a 5.5. I’ll tell you later why that’s unimpressive. Still, they deserve credit for siding with the astronomer that says there’s no risk, and that the only major effect of this alignment is the increased risk of moonquakes — which only affects you if you live on the moon. My problem with this article is that the astrologer is given any kind of credence at all. There’s no reason for that. The fact is, there’s absolutely no reason to consult an astrologer about any nonsense that they themselves proffer without evidence and continue to cling to, despite evidence to the contrary. They should not be consulted pretty much ever, when you have a credulous people willing to attribute natural disasters to anything but nature.

The reason I say this, is because people are blaming the impending supermoon for the Japanese earthquake last week.


The supermoon event occurs on March 19th. The astrologer in question painted a “danger window” around that date of March 16th through March 22nd, predicting “supreme tidal surges” (like predicting the sun will rise — the moon affects tides, and having the moon and sun at opposite sides of us actually does create really high and low tides!), “volcanic activity” (what counts? Tiny amounts of ejecta happen from most volcanoes every few months!), and “earthquakes above 5.0 magnitude.”

It’s this last one that people are blaming the Japanese quake on. Never mind that a) the quake happened on March 11th, when the moon was roughly at a 75 degree angle to us compared to the sun — e.g., at not quite half phase — and b) it was about 400,000km away, since apogee happened on March 6th.

Also, according to these statistics, there are 1319 earthquakes every year that fall between 5.0 and 5.9. That means there will be, on average, (1319 / 365) * 8 = 28.9 earthquakes that fall within that range, somewhere on Earth during the eight days given for the supermoon “danger window”.

That’s not even counting the earthquakes that surpass that level. Again, according to the site, there’s on average 134 6.0-6.9 magnitude earthquakes per year, and 15 7.0-7.9’s, and one 8.0+ per year. That means statistically, (134/365) * 8 = 2.9 earthquakes that measure between 6.0 and 6.9 during that window. It also means 0.32 7.0-7.9’s, or a 32% chance of a whopper. And though the 8.9 earthquake that just happened in Japan probably relieved a lot of the tectonic stress of the rest of the plates, there’s still a 2% chance that an 8.0+ would happen during that eight day window.

And since nobody will remember this astrologer’s predictions if they’re wrong, selection bias means he’s making an excellent bet by predicting 5.0+ quakes.

The Earth is not in any kind of danger, having a perigee fall on the same night as a full moon. Think about it for half a second, and you’ll realize that the rotational differences that mean such “supermoons” are possible on the “every several years” order of magnitude, means the month prior and following a supermoon must needs have a very close proximity between perigee and full moon. And it means that on off years, the perigee and full moon might be off by a day or two. Why wouldn’t those types of syncronicities cause increased earthquake activity, if this astrologer is giving us a “danger window” of eight days?

Nonsense, top to bottom. Or apogee to perigee.

Supermoon: what it is, and what it definitely isn’t (a repost)