A little bit of moonshine in the night

posted in: Science fact | 6

A lunar eclipse happened in my part of the world on 28th November, hard on the cosmic heels of a solar eclipse earlier in the month. As it turned out, the penumbral eclipse was a huge disappointment. No shadow across the moon’s disc, not even a reddening of the light. So the cirrus cloud partially obscuring the view didn’t matter much. We had moonshine as we always do and the photos were a fizz.

However, it got me to wondering about moons; ours, and other moons in general. To start with, let’s mention the eclipse – the truly spectacular solar eclipse that happened earlier this month. It was a partial eclipse in my part of the world, but even so it is a special event. But why is it so? The moon is tiny compared to the sun.

An extraordinary cosmic coincidence

The sun is about 400 times the moon’s diameter and about 400 times as far way from the Earth and that ratio means that when the moon comes between the sun and the Earth, that shadow is just about a perfect fit.  That relationship is a coincidence. Evidence indicates that the Moon was once closer to the Earth and is gradually moving away, so enjoy your cosmic moment, knowing that in the distant future, there will be no total solar eclipse.

That factoid is not the only extraordinary thing about our moon. Not at all.

It’s not the largest moon in the solar system. In fact, going by this list it comes a creditable fifth after Ganymede (Jupiter iii), Titan (Saturn vi), Callisto (Jupiter iv) and IO (Jupiter i). Indeed, Ganymede and Titan are both larger than Mercury and let’s not talk about poor Pluto. Really, when you think about it, it makes perfect sense that the largest planets have collected the largest moons.

Except ours.

Why is this so?

I have in my possession a tattered little paperback, a collection of essays on astronomy by Isaac Asimov (Asimov on Astronomy, Coronet, 1974). One of the things I loved about Asimov, who had a PhD in chemistry and an interest in everything scientific, was that he could explain complex physics in a way that an interested amateur with absolutely zippo mathematical ability could understand. He wrote papers regularly for magazines and the like and subsequently, they were published in book form. This little volume is a treasure trove of scientific fact and some intriguing speculations. True, some of it is now dated, since it was published before the epic discoveries of Voyagers I and II. Pluto had not yet been demoted. And yet before it could be proved he predicted that many planets other than Saturn would have rings.

To get back to the point, one of these essays is entitled “Just Mooning Around” in which Asimov talks about the gravitational effects of the sun, the planets and the moons in the solar system have on each other. Without going into all the details of the paper, he talks about the ‘tug of war’  ratio, which argues that in most cases, the gravitational attraction of a planet on its moons is vastly greater than the pull of the sun on those same moons. There is also a kind of ‘goldilocks’ zone around a planet in which a true moon would exist (as opposed to loosely captured satellites like Neptune’s Nereid). A moon must be between a minimum Roche limit and a maximum ‘tug of war’ distance. For the mind-bending number-crunching, go read it yourself – I told you I can’t do maths. However, I can appreciate logic. And you will see it is so.

According to his theory, of the four innermost rocky planets, Mercury could not have a moon because it has no ‘goldilocks’ zone. The other three are highly unlikely to have moons because of the narrowness of the ‘goldilocks’ zone. And indeed, Mercury and Venus do not have satellites, and Mars’s Phobos and Demos are overlarge potatoes which are expected to disintegrate.

I see you jumping up and down. What about us? Earth and that thing up there?

Ah, Asimov argues that the Earth/Moon pair is not a true planet/moon relationship because the Moon is so much larger in comparison with its primary than any other such relationship in the solar system. By a long way. He suggests that the Earth/Moon combo is really a binary planet, waltzing its way around the sun. Of course, all planets with moons have a wobble in their orbit but the Earth/Moon shimmy must be quite pronounced. Certainly I don’t think there’s much disagreement these days that our Moon was derived from the same stuff as the Earth. This article suggests accepted theory is that a Mars-sized object collided with the Earth, aggregating the material and spewing off a portion which later formed the Moon.

The next thing you have to wonder is – how important is that massive moon to life on Earth? But that’s another topic, isn’t it?

Isn’t science wonderful?

6 Responses

  1. Paul Trembling

    Asimov was one of the great writers of my teen years. Along with some others (Clark, Heinlein, Simax, Leinster), he had a lasting influence on my daydreams! Nice to be reminded of that.

    You left a bit of a cliff-hanger there, Greta! How important is the moon to life on Earth? I read somewhere, once, that without the moon’s influence our atmosphere might have become too thick to breath! I don’t know how true it is, but it could have some implications for the frequency of life on other planets. And what about the effect of tides? Not to mention the real problem – without a moon, what would wolves howl at?!

    • Greta van der Rol

      Oh yes, and what about the were wolves? Yes, the influence of the Moon on life on Earth is, I felt, a topic worthy of discussion in its own right. I’ll get around to it.

      I was and am a huge Asimov fan. He was also very interested in history and that influence is clear in his Foundation series. I guess he’s been a huge influence for me.

  2. livinginaquantumworld

    Excellent article, I thoroughly enjoyed the references to Asimov. It’s astounding what a time we live in, about as good as it gets for astronomers & physicists amateur or professional. At least we can view distant solar objects, as at some point in the future the universe will have expanded so much that our distant relatives will only know of their solar system.

    • Greta van der Rol

      I’m with you. The deep space probes are wonderful, they tell us so much. There’s a thought – expansion to the point of not having anybody to contact.

  3. juliabarrett

    I so love science, Asimov and astronomy. Although I have a difficult time imagining a Mars-sized object striking the earth and not destroying the earth altogether. Hmmmm. Now I have to put on my thinking cap.

    • Greta van der Rol

      There has been speculation along those lines for years. Remember the name Velikovsky “When worlds collide”? This would have happened millions of years ago, before the Earth was fully formed. It would have been blasted into pieces (so would the other Mars-sized bit) and then gravity would have drawn it all back together again. It’s said there’s evidence that of Saturn’s or Jupiter’s moons suffered the same treatment and coalesced back into a ball. I’ll see if I can find the story.

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