I’ll make a prediction: on or around 19 March, when the so-called “Supermoon” occurs, at its closest approach to Earth in two decades, people will indeed report that the Moon looks much bigger than normal. But it won’t really be much bigger in the sky at all. It’s all in our heads!
You’ve probably all seen it before, a huge Full Moon sitting on the horizon. Time and again I have had people ask me why the Moon is so much bigger some times than others, and the answer is: it isn’t, really.
The Moon orbits the Earth in an elliptical orbit, meaning that it is not always the same distance from the Earth. The closest the Moon ever gets to Earth (called perigee) is 364,000km, and the furthest is ever gets (apogee) is around 406,000km (these figures vary, and in fact this Full Moon on 19 March 2011 will see a slightly closer approach of 357,000km).
So the percentage difference in distance between the average perigee and the average apogee is ~10%. That is, if the Full Moon occurs at perigee it can be up to 10% closer (and therefore larger) than if it occurred at apogee.
This is quite a significant difference, and so it is worth pointing out that the Moon does appear to be different sizes at different times throughout the year.
But that’s NOT what causes the Moon to look huge on the horizon. Such a measly 10% difference in size cannot account for the fact that people describe the Moon as “huge” when they see it low on the horizon.
What’s really causing the Moon to look huge on such occasions is the circuitry in your brain. It’s an optical illusion, so well known that it has its own name: the Moon Illusion.
If you measure the angular size of the Full Moon in the sky it varies between 36 arc minutes (0.6 degrees) at perigee, and 30 arc minutes (0.5 degrees) at apogee, but this difference will occur within a number of lunar orbits (months), not over the course of the night as the Moon rises. In fact if you measure the angular size of the Full Moon just after it rises, when it’s near the horizon, and then again hours later once it’s high in the sky, these two numbers are identical: it doesn’t change size at all.
So why does your brain think it has? There’s no clear consensus on this, but the two most reasonable explanations are as follows:
1. When the Moon is low on the horizon there are lots of objects (hills, houses, trees etc) against which you can compare its size. When it’s high in the sky it’s there in isolation. This might create something akin to the Ebbinghaus Illusion, where identically sized objects appear to be different sizes when placed in different surroundings.
2. When seen against nearer foreground objects which we know to be far away from us, our brain thinks something like this: “wow, that Moon is even further than those trees, and they’re really far away. And despite how far away it is, it still looks pretty big. That must mean the Moon is huge!”.
These two factors combine to fool our brains into “seeing” a larger Moon when it’s near the horizon compared with when its overhead, even when our eyes – and our instruments – see it as exactly the same size.
There seems to be a growing excitement about the “Supermoon” that is due to occur on 19 March 2011, when the Moon will be at its closest to Earth in this orbit, and closer than it has been at any time since 1992.
The Moon orbits the Earth in an elliptical orbit, i.e. it is not perfectly circular, and so in each orbit there is a closest approach, called “perigee” and a furthest approach, called “apogee”.
At this month’s perigee the Moon will be 356,577km away from Earth, and will indeed be at its closest in almost 20 years [This is WRONG: see Update 2 below!). But how close is it compared with other perigees?
Let’s start by comparing it to the Moon’s average distance from the Earth, which is ~385,000km. This perigee will be ~8% closer to the Earth than average. OK, that’s a bit closer, but not significantly so.
What about comparing it to the Moon’s average perigee distance, which is ~364,000km. So this “Supermoon” will be ~2% closer to the Earth than it is most months at perigee. Wow!
So what will this mean to you? Nothing at all. The Moon will be a few percent bigger in the sky, but your eye won’t really be able to tell the difference. It will also be a few percent brighter, but your eye will compensate for this too, so altogether this “Supermoon” will look exactly the same as it always does when it’s full.
As to all of those soothsayers claiming that there will be earthquakes and tidal waves. There very well might be, but they’ll be nothing at all to do with the Moon.
UPDATE: I predict that lots of people will report having seen a huge Moon on or around 19 March
UPDATE 2: Thanks to “justcurious” for the comment that inspired this calculation, and to Steve Bell at the UK Hydrographic Office for providing the calculations below:
This is an auspicious time of the year.
The Sun, on its yearly circuit of the sky*, moves gradually along the ecliptic, a line which is the projection of our solar system’s disk onto our night sky. This line, the ecliptic, is also known as the zodiac, a term originating from the late 14th Century, and deriving from the Greek literally translating as “circle of little animals”. (Incidentally our word for zoo derives from the same origin).
To the ancient Greeks this was indeed a circle of little animals, featuring: a ram, Ares; a bull, Taurus; Pisces the Fish; and many other well known (for all the wrong reasons) constellations. It also features three humans: Aquarius the Water Carrier, said to represent Ganymede, beloved of Zeus; and Gemini the Twins, Castor and Pollux.
The only zodiac constellation which is inanimate is Libra the Scales, taken from Babylonian astrology. The Greeks however didn’t recognise Libra; instead they thought that the stars here marked out Scorpius’ claws, which they considered to be a separate sign.
So the twelve constellations that lie along the ecliptic are most well-known due to astrology, a pseudo-science that suggests there is some significance to which constellation the Sun was in when you were born. This is, of course, bullshit.
There are so many reasons why astrology should be laughed off as pre-scientific magical thinking (no evidence, no mechanism by which it might work, inconsistent etc) but next time you meet an astrologer ask them what star sign you would be if you were born between 30 November and 18 December. If they tell you Sagittarius (if they’re a Hindu astrologer they might also say Scorpius) then tell them they are plain wrong.
On these 18.4 days of the year the Sun is wonderfully absent from the usual twelve zodiac signs, It is still there, however, gracefully moving along the ecliptic, but between 30 November and 18 December it is in the constellation of Ophiuchus the Serpent-bearer.
Ophiuchus does not appear in any astrologer’s zodiac. Back in the early days of astrology, when it was first dreamed up several thousand years ago, there were indeed only twelve constellations lying along the zodiac. Over the past few millennia however the Earth’s axis has wobbled slightly (an effect called precession) with the result that the line of the ecliptic has moved with respect to the constellations, and so an interloper, Ophiuchus, has crept in.
So celebrate all those of you born between 30 November and 18 December (one person in twenty share this star sign); you’re not a Sagittarian at all; you’re an Ophiuchan.
It’s still all bullshit though.
* the Sun, of course, does not orbit the Earth, it’s the other way around. It just looks like it does from down here…