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Yes, there will be three Supermoons this summer. No, this is not cause for alarm

by on 2014/07/17

I write this post as something of a pre-emptive strike.  The slightly ominous sounding “Supermoon” is a common occurrence in our night sky, and NASA has confirmed that we will have three consecutive Supermoons this summer, in July, August and September.  There is often a lot of bunkum on the web about the dangerous effects of Supermoons.  However, they’re absolutely nothing to worry about, and I’ll tell you why.

We’ve known since the days of Johannes Kepler that the motions of the planets and moons do not follow circles, but instead they follow ellipses.  For example, the Earth’s orbit around the Sun is an ellipse, but this ellipse is close to being a circle.  In other words, the eccentricity of the Earth’s orbit (a number that describes how elliptical an orbit is, with zero eccentricity orbits being circles) is around 0.06.

The Moon’s orbit around the Earth is also elliptical, also with an eccentricity of around 0.06.  However, this means that as the Moon moves along its orbit, it moves closer to, and further away from the Earth.  The point where the Moon is closest is called perigee, and the point where the Moon is furthest away is called apogee.

As we all know, the Moon has phases.  The Sun emits light that bounces off the Moon and arrives at our eyes on Earth, and depending on the positions of the Sun, Earth and Moon, parts of the Moon are in shadow.  A Supermoon occurs when a full Moon is in our sky, and the Moon is near its perigee (the usual definition is that both must occur within 24 hours of each other).

So what does the Supermoon do to the Earth? Not a lot.  The Moon’s eccentricity is really small, so the change in distance over its orbit is also really small (a few percent).  To the naked eye, it’s actually very hard to tell if the Moon is any bigger during a Supermoon (as this graphic on Bad Astronomy shows).

Now, it is true that during a full moon, the tides the Moon produces on the Earth are at their peak: this is because during a full moon, the Sun, Earth and Moon are in alignment, so the tidal force the Sun generates adds to the force the Moon generates.  This alignment is called a syzygy, and as such is an excellent get-out-of-jail-free card when playing Scrabble.  But, because the Moon isn’t getting much closer to the Earth, the tidal force it generates doesn’t get much bigger.  Despite a great deal of scientific effort, there are no discernible correlations between Supermoons and volcanoes, earthquakes or other activity that extreme tidal forces could cause (and alarm bells should ring if you hear anyone suggesting otherwise without solid evidence).  In fact, the term ‘Supermoon’ was coined by an astrologer, who attempted to link the Moon’s orbit with bad weather and natural disasters.  As you might expect, Research the Headlines does not endorse the view that astrology has anything scientifically meaningful to contribute to human knowledge.

The reason I’m writing this is because Supermoon stories come up all. the. time. in both press articles and blogs, in some cases linking the Moon to appalling tragedies like the tsunami that hit Fukushima in 2011 (see Phil Plait‘s excellent takedown of this hokum).  Equally, there are now a great number of articles from the press and the general public debunking these stories.

In any case, we are being treated to three full moons that are bigger than the average this summer – it’ll be the best time to get the telescope out and admire the lunar surface, which I thoroughly recommend.


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