March 2011, Week 3


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Mon, 21 Mar 2011 00:14:58 -0400
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Kryptonite For The Supermoon
Phil Plait
Discover Magazine - Bad Astronmy Blog
March 18th, 2011

[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.]

If you believe the mainstream media, you might think
this weekend's "supermoon" will cause earthquakes,
volcanoes, bad weather, halitosis, dust bunnies, and

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.


      * 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%

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

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

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.


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