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.
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?