Last night the Moon shone big and bright. A supermoon.
A ‘supermoon’ happens when the lunar perigee – the point at which it is closest to Earth – occurs on the side of Earth’s orbit furthest from the sun, which means the lunar disk is fully illuminated from Earth’s perspective. Hence we get a moon that not only has a visual diameter 12 percent greater than that of the Moon at apogee, but which is also full.
The difference isn’t huge. The Moon’s distance varies from about 405,000 to about 360,000 km from Earth. It doesn’t add much to the tides – a few inches at most – and is a perfectly normal occurrence.
For me it highlights how unique our Moon is. No other planet in the solar system, except Pluto, has a moon that is such a large proportion of its own size. (Don’t get me started on Pluto’s demotion to ‘dwarf’ planet).
Current theory – devised mainly from Apollo data – suggests that the Moon formed about 4.5 billion years ago, and only 30-50 million years after the Solar System began coalescing, when an impactor the size of Mars - ‘Theia’ – ploughed into the proto-Earth, probably on a slaunchwise angle.
The impact rendered both bodies fragmented and molten. The iron cores of both sank to the centre of the Earth – which is why Earth has such a large one compared to Mars or Venus – while the lighter material coalesced to form the Moon.
A variant explains how the Moon is two-faced – the far side (which gets just as much light as the near side, so we can’t call it ‘dark’) is very different from the side we see. Why? One theory suggests Earth emerged from the impact with two moons which, themselves, subsequently collided. Splat. Later, tidal effects meant that the heavier side – the one we see, with all the maria – ended up facing the Earth.
Which brings me to the last cool link between this supermoon and the history of the Earth-Moon system. Way back when, as the moon coalesced, it was only about 14,000 miles from Earth and both bodies were rotating far more rapidly than today.
Does the Moon rotate, you say? Actually, it does spin – it rotates once in exactly the same time as it takes to orbit the Earth. This outcome is known as ‘tidal locking’ and occurs when a small body – in the Moon’s case, 1/81 times the mass of Earth – orbits a larger one.
Each induces tides in the other, robbing each other of rotational energy. Naturally the larger body ‘wins’, though it’s also slowed in the process. The energy doesn’t vanish, of course. It’s turned partly into heat, and then radiated; but it also partly gets translated into orbital momentum.
What this means in practise is that the Moon moves away from the Earth. In about three billion years, it will be far enough away that it doesn’t stabilise the Earth’s axial tilt.
Can Earth ever be tidally locked to the Moon? Theoretically – yes, but apparently it’ll take 50 billion years. The orbital period of the Moon – and the length of our ‘day’ – would be about 47 of our existing days. But it won’t ever happen. Earth will be swallowed up by the Sun, as it turns into a red giant, well before then.
But that’s way off in the future, and I can guarantee humans won’t be around to see it. Unlike today’s supermoon.
Copyright © Matthew Wright 2013
Coming up this week: My take on Eta Carinae, more writing tips, and more.