The moon, I suspect, is a touchstone to every feeling person. As to me, its sicklied o’er pallor has made it an ideal companion on many an evening ramble given over to the inconcinnities of unrequited ardour. But though I often gazed, I never really observed. I didn’t notice, for example, that its path across the sky was erratic, shifting from quadrant to quadrant. I didn’t know what a quadrant was.
I remember certain facts and figures from childhood – those numbers that get stuck in your head and that no new data can quite replace [there’s a strange sense, for example, that China will always have 700 million inhabitants for me]. The moon is 283,000 miles away, compared to the sun’s 93 million. I’ve just written that without looking up references. A quick squiz informs me that the figure is closer to 238, 000 miles [an average distance between perigee, the closest, and apogee, the furthest]. Interestingly, this distance isn’t constant. The moon is edging away from us at a rate of just under 4cms per year.
I’m a late bloomer in most things. I didn’t get to closer quarters with the moon until my early twenties, when a friend encouraged me to look through his telescope. He told me that, if you looked closely, you could see some of the moons of Jupiter. Maybe it was my eyesight, or my laziness, but I could see nothing of that, and only had eyes for our scarred and potent satellite. This was a dozen years or so after the first moon walk, and certainly that was memorable, but there’s something about visual contemplation and intimacy. A woman’s face, for example. Really looking at it. It can also confer a dangerous fantasy of possession. Possession, at least, of some secret knowledge.
Where did the moon come from? I recall reading of that too in childhood. There were competing theories. When the earth was young and molten, a protrusion somehow formed, and the earth’s spin finally thrust it out of the main body to form its own orbit around us. Or there was a massive collision in the earth’s early life, which somehow caused a large chunk of matter to splinter off, while other matter was absorbed. This second theory, much elaborated of course, still persists, and is the most accepted theory in the discourse.
There’s a lot of debate though currently, and much exciting activity around the moon’s origin, as revealed in a recent issue of Cosmos [issue 22]. Some maths-physicists, for example, have posited the existence of two other moonlets, which may have formed as part of the putative collision some 4.4 billion years ago. In the early days, when the moon was much closer to us, these Trojan satellites, as they’re called, would have remained more or less stable in their orbits, a situation that may have persisted for a billion years, until our gravitational pull became too weak, and they were lost to space.
Other researchers have come up with a radical alternative to the top-dog theory. They suggest that in the period before the earth coalesced into its current more or less stable state, a gigantic bubble of very hot gas forced its way up through the mantle, massively impacting upon it and upon the earth’s crust, and forming our moon in the process. Their theory, they claim, solves a number of issues left unresolved by the more accepted alternative. I won’t try to go into too much detail – read Cosmos for that – but it seems that the existence of georeactors [sites within the earth of natural fission reactions] in the layer of the earth’s upper mantle are being postulated as evidence for the researchers’ claims. The moon’s density is much less than that of the earth, with the core taking up only 4% of its mass. The earth’s core constitutes 30% of its mass. How they’ve ascertained this is a mystery to me, but it’s generally considered that the reason for the difference is iron. The earth has lots of it at its centre, the moon doesn’t. Yet the similarity of oxygen isotope ratios in earthrock and moonrock prove that the two bodies were formed equidistant from the sun, and tend to suggest that they were once parts of the same body. Such differences and similarities have led, naturally, to some confusion and uncertainty.
In any case the new theory has some super-smart backers, though the impact theory is still very much top dog. It will be a field to keep an eye on in the future.
An interesting side-issue. The fission hypothesis, a brief version of which I gave at the start of this piece, with a molten, or rather volatile, portion of the earth being ripped off by centrifugal force, was originally proposed by George Darwin, son of Charles, in 1880. It is now discounted.