Last night, we walked out of our front door; five minutes up the path to Pipeland Hill brought us to a dark spot above the city lights. I lay down in the field and looked at the stars appearing as the daylight faded.

It was the time of the maximum in the Perseids meteor shower. Not a dramatic display, but I saw three brilliant meteors in half an hour; a satellite, possibly the International Space Station, passed across, as did a plane leaving a very clear vapour trail. Altogether an uplifting experience.

And one not possible in London. (I did once watch a lunar eclipse from Parliament Hill on Hampstead Heath, among a noisy and friendly crowd; but the lights are inescapable.)

This led me to ask myself a stupid question. Why do the Perseids happen at the same time every year? It can’t be that a cloud of debris floats there in space waiting for the earth to pass through it once a year: the fact that the radiant is in a fixed direction indicates that the debris is moving faster than the earth. I was going to pose the question here in the knowledge that someone would be able to give me the answer; but on the BBC website I found a diagram that suggested a solution. The debris is actually distributed over the whole orbit of the comet (Swift–Tuttle) which produced it, circulating round the orbit like a ring around the Sun; the orbit is more or less fixed even though the debris moves fast.

But this raises further questions. The comet (which has an orbital period of just 133 years) must have been shedding debris for a considerable time for it to have become distributed over the entire orbit like this. What is the mechanism for this? I would naively expect the debris to move with the comet, or, if expelled from the comet at high speed, to move in a whole range of orbits.

About Peter Cameron

I count all the things that need to be counted.
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