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That’s No Supermoon

June 24, 2013 — 4 Comments

The periodic mention of a ‘supermoon‘ in the news cycle is starting to annoy me. A supermoon is simply not that much bigger than any other Moon!  It’s apparently just perceptible but by no means would you call it ‘super’. Annoyingly though, observation of the so-called supermoon is wrapped up in another effect: the Moon Illusion. This means that people enthusiastically report seeing a really big Moon, but don’t realize that they would likely have thought it big on any other Full Moon night too.

So let me put my rant in some context. The term supermoon was coined by astrologer Richard Nolle about 30 years ago. It refers to a Full Moon or New Moon that occurs when the Moon is in the closest part of its orbit around the Earth. The Moon’s orbit is not perfectly circular and there is a closest point in every cycle (perigee) and a most-distant point too (apogee). At perigee the Moon is closer to the Earth by about 50,000 km (30,000 miles), which is enough to make the Moon appear slightly larger in the night sky. In fact it is about 1.1 times larger in it’s angular diameter on the sky. Expert Moon watchers can see a subtle difference but it’s pretty slight and hardly warrants the title of a ‘super’ moon.

The Moon's motion over one cycle.

The Moon going through one complete orbit as seen from the Earth.

This animated GIF shows a Moon going through one entire orbit (apogee-perigee-apogee) and you can see the changing size (you can also see it undergoing libration, which is the wobbling motion). You can a direct size comparison below. In both these cases you’re seeing it close-up – imaging these things hanging in the sky at a distance. The size change is happening in every cycle, but is most prominent when the Full Moon coincides with perigee, as was the case this week.

Size Comparison for the Moon at Apogee and Perigee

Size Comparison for the Moon at Apogee and Perigee [Source: http://www.fourmilab.ch/earthview/moon_ap_per.html%5D

So there is a difference in the appearance of the Moon but it is very small and you’re unlikely to be seeing this when you go outside to look at a supermoon. What you’re actually experiencing is most likely the Moon Illusion: the optical illusion that the Moon looks larger when it is near the horizon than when it is high in the sky. The Moon Illusion is not well understood but most astronomers are very familiar with it. It may be partially caused by the Ebbinghaus illusion, which is the one that makes the two central circles in the following image appear to be different sizes when they are, of course, the same. When close to the horizon the Moon is compared to objects like rooftops, hills and clouds. When high in the sky is mostly seen in wide-open space. Another explanation may lie in the processes that govern our binocular vision; it might be that the Moon Illusion does not occur of you stand on your head, for example. This has not (yet) been tested widely.

Ebbinghaus Illusion

So what happened over the weekend was that people heard about a supermoon and so went outside to see it. Given that it any observable supermoon is a Full Moon, this means people went out to see it when it was low down in the sky, because Full Moon’s rise late in the evening. Thus they probably experienced the Moon Illusion and reported that indeed the Moon looked very large.

On a final point: the supermoon is also given silly superpowers by some new outlets too. The natural oscillation of the Moon’s distance does indeed affect tides a little, but it does not cause earthquakes, madness or werewolves.