You may have noticed that, of late, the esteemed ladies and gentlemen of the Fourth Estate – to whit, our blessed Press – have been beside themselves with astronomical concerns centred around the Moon. There is talk this month of a ‘Supermoon’, an event whereby our loyal celestial companion will swing close to our own world, appearing large and bright and lovely (-er than usual) – but also wreaking terrible damage on our poor planet due to the increased force of gravity. Some already attribute the recent distressing events in Japan to these heavenly circumstances.
But in fact, there is no cause for concern. (Move along, there’s nothing to see here.) In fact, the ‘Supermoon’ is a relatively common event – at least astronomically speaking. If you’d like to take a seat and get comfortable, Science will shortly be afoot. And I hope you’ll indulge me, because I think it’s all rather brilliant.
The Moon, as you know, is locked in an orbit around Earth, and has been for over four billion years. In fact, the Moon in its own right is thought to be about fifty million years younger than the rest of the Solar System – including Earth – and speculation abounds as to how it formed in the first place. Astronomers are moderately confident that the Moon did not simply form out of the coalescing gas and dust around the Sun as Earth and many of the other planets did. The prevailing idea at the moment is that the Moon was originally part of Earth, and was formed as the result of an impact: something very large and very fast whammed into the fledgling Earth while it was still all hot and gooey, and splattered out a lump of mass that later solidified into an orbiting moon.
Still, give or take that fifty million years, it’s all still rather well-established by now. So what’s actually happening tomorrow?
Well, first, there’s a Full Moon. These you’re familiar with: we get one every month; sometimes two. What else?
There’s that close approach. The Moon’s orbit around Earth isn’t perfectly circular. Like all natural orbits, it’s actually an ellipse – and that means that there are points where the orbiting body is closest to and furthest from the body being orbited. These points are called the ‘apsides’ (singular ‘apsis’): when the Moon is closest to Earth, it is said to be at ‘perigee’; when furthest away, it’s at ‘apogee’. Tomorrow is perigee and, on top of that – because the apsides go through their own cycles, too – the point of perigee this month is closer than at any time in the last eighteen years. Normally, the Moon’s knocking around somewhere around 380,000 km away – tomorrow, it’ll be just over 350,000. That’s pretty close, by astronomical standards, and that’s what makes tomorrow’s perigee astronomically a little bit special. But only – and I’m sorry to be a wet blanket here – a bit.
But won’t the close perigee have terrible effects here on Earth? More earthquakes and disasters and such?
No. No more than usual (which isn’t to say none – just that any that might happen won’t have been caused by the Moon). Both Earth and the Moon are colossally big and heavy, and because they have lots of mass, and everything with mass generates a gravitational field, they exert quite a pull on each other. It’s enough of a pull to flex the Earth’s seas in a regular pattern and create tides; and the same force is enough to pull the Moon’s rotation into sync with Earth’s, so that we only ever get to see one face of the Moon. The face with, well, the face on it. When you think about it, that’s a lot of force, and it would be easy to imagine that it wouldn’t take much more to start causing some serious problems.
But although the gravitational force between Earth and Moon will be very, very slightly stronger as the two bodies pass close by, it’s not going to be nearly enough of a difference to create any noticeable effect beyond its regular ones. Tides will continue, the Full Moon will look pretty, and that’s about it.
The Moon may, possibly, look slightly bigger – but the main effect there (besides the expectation of seeing a bigger Moon) will be what’s called the Moon Illusion. If you get to see the Moon low in the sky, near to the horizon, you’ll notice that it looks significantly bigger than when you see it high up. There are several competing explanations as to why we see this effect. All agree that it’s not that the Moon is actually bigger, nor that it looks ‘objectively’ bigger somehow – it’s just a funny effect of how the individual’s brain works out the angles and perspective when the Moon’s close to the horizon.
The truth is it’s just no-one’s quite sure how the Moon Illusion works. But it always does, and there’s a possibility that tomorrow, if you can catch the Moon low down, the Moon Illusion may amplify the minimal effect of the close perigee and make it look quite a bit bigger than normal anyway.
It’s possible that this will make the Moon look even more fascinatingly beautiful than usual, so I’d encourage you to keep an eye on it if you can. But, to be honest, I’d encourage everyone to watch the skies every chance they get. What can I say? I’m a hopeless astronomy nut. 🙂