A few evenings ago, a small group of friends came to my house to watch the Geminidsmeteor shower. We put a lot of effort into preparing our for the experience by chopping down trees which obscured the view, stocking up on charcoal and meat for the braai, setting out comfortable garden chairs and inviting the right people. By all accounts, the shower exceeded the most optimistic predictions and would have been a marvellous show had we not been clouded out. What was to be an edifying evening of astronomy turned into a pleasant social gathering.
Had the weather cooperated, though, what would we have seen? First of all, the Geminids are pretty average meteors. If you’ve ever seen a shooting star flit across the sky like a faint spark of light, then you have an idea of what the average Geminid looks like (Other showers are predominantly very slow bright meteors, others are dense swarms. Each is unique). The best reports showed in excess of 140 meteors per hour, which means that in a single minute of viewing you could expect to see at least two meteors.
If you watched the shower for long enough, you would begin top notice a pattern: They all seem to come from the same point in the sky – near one of the fainter stars of the constellation Gemini, in this case, which is why this shower is called the Geminids. Now this is only an illusion – they are all travelling in straight lines parallel to each other. In reality, the earth is ploughing through a region of space where these meteoroids (debris left behind from an extinct comet) are present, and it’s only an effect of perspective that makes them appear to radiate out from that point.