Why a supermoon is just, so, well, super

Last night the Moon shone big and bright. A supermoon.

A supermoon happens when the perigee point of the lunar orbit happens to fall at the point furthest away from the sun, so the Moon is fully illuminated as seen from Earth. A sketch I made with the help of Celestia and a drawing tool.
How a supermoon works.  The perigee isn’t always at this point – the alignment happens about every fourteen months. A sketch I made with Celestia and a drawing tool.

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.

Colour photo of the Moon taken by the Galileo probe in 1990 - a view we never see from Earth. The - uh - 'dark side' is to the left, fully illuminated. NASA, JPL, public domain.
Colour photo of the Moon taken by the Galileo probe in 1990 – a view we never see from Earth. The – uh – ‘dark side’ is to the left, fully illuminated. NASA, JPL, public domain.

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.


15 thoughts on “Why a supermoon is just, so, well, super

    1. Your post’s pretty cool too. We don’t often think about how important and pivotal the Moon really is to the precise nature of the Earth, and hence our lives upon it. There’s an ongoing search for ‘Earth like’ planets just now – but I wonder, I really wonder, whether we’d ever find one exactly like ours – with an iron core formed by impactor, early on, big enough to create a magnetic field to shield life; with a moon that stabilises the axial tilt and reduced the madly spinning globe to a reasonable day length, and so on. Underscores the real value of our planet, of course – this just at a time when we seem bent on destroying its ability to support us.

  1. I was lucky enough to have a telescope pointed at the moon as it rose last night. I didn’t even know it was a supermoon, but it was a beautiful sight nonetheless. Over here it rose with a beautiful golden hue.

    1. Seriously cool. I saw it last night at about 40 degrees above the horizon, over Wellington NZ – the first clear night we’ve had since Wednesday last, in fact. Then I saw it again, yellow and setting, when I got up before dawn. I couldn’t tell the real difference in brightness and size just by looking at it, of course, but it’s a great sight even at ordinary times. The only celestial body beside the Sun that presents a disc to the naked eye. How cool is THAT?

  2. Your last couple of paragraphs were very uplifting, Matt. Knowing that all of human history with be obliterated and our existing ultimately useless, makes my morning sooo enjoyable. 🙂 In the mean time, I’ll be watching the moon and the stars as I always have.

    1. That’s OK. I figured I’d try and leave my mark on the place before it happened, but they came along and painted it out… 🙂

      Optimistically, we’ll have transcended Earth and gone somewhere else. Arthur C. Clarke (who else!) laid out how it might happen in 2001.

  3. Beautiful supermoon in California (USA) last night too. Though I didn’t know why it was so beautiful until reading this post. As usual, you are a wealth of amazing information! Thanks for illuminating us on this subject. 😉

    1. Thank you! It’s just such a cool phenomenon – and to me, even cooler because the Moon’s the only celestial body we can see as a disk with the naked eye, apart from the Sun.

    1. Thanks. Yes, it’d be great to see it from above the troubled airs…We got it over Wellington (at sea level) on the first clear night after 5 nights of overcast and storms – quite a sight and I can imagine how much brighter it would be a few thousand feet higher. Very cool.

    1. It’s all to do with the way momentum is conserved – exactly the same phenomenon as when a ballet dancer slows a pirouette by throwing their arms out. Which to me is one of the coolest parts of the whole thing – the fact that the physics principles guiding the Earth-Moon system, also apply in very ordinary ways like that, down here on Earth.

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