Bad Astronomy: Misconceptions and Misuses Revealed, From Astrology to the Moon Landing 'Hoax' by Philip C. Plait

Author:Philip C. Plait [Plait, Philip C.]
Language: eng
Format: epub, mobi
Tags: Miscellanea, Science, Popular works, Errors; Scientific, Astronomy, Physics, History
ISBN: 9780471409762
Publisher: Wiley
Published: 2002-01-02T07:00:00+00:00

So the meteor shower I mentioned above not only recurs in time but in space, too. Every August those meteors appear, and they seem to flash out of the sky from the direction of the constellation Perseus. Showers are named after their radiant, so this one is called the Perseids.

One of the most famous showers comes from the direction of Leo every November. The Leonids are interesting for two reasons: One is that, relative to us, the parent comet orbits the Sun backwards. That means we slam into the meteoroid stream head-on. The meteoroids' velocity adds to ours, and we see the meteors flash across our sky particularly quickly.

The second interesting thing is that the meteoroid stream is clumpy. The comet undergoes bursts of activity every time it gets near the Sun (every 33 years or so), and this ejects lots of bits of debris. When we pass through these concentrated regions, we see not just dozens or hundreds of meteors an hour but sometimes thousands or even tens of thousands. This is called a meteor storm. The celebrated storm of 1966 had hundreds of thousands of meteors an hour, which means, had you been watching, you would have seen many meteors whizzing by every second. It must have really seemed as if the sky were falling.

So that's why we get meteors. But why are they so bright? Almost everyone thinks it's friction-our atmosphere heating them up, causing them to glow. Surprise! That answer is wrong.

When the meteoroid enters the upper reaches of the Earth's atmosphere, it compresses the air in front of it. When a gas is compressed it heats up, and the high speed-perhaps as high as 100 kilometers per second-of the meteoroid violently shocks the air in its path. The air is compressed so much that it gets really hot, hot enough to melt the meteoroid. The front side of the meteoroidthe side facing this blast of heated air-begins to melt. It releases different chemicals, and it's been found that some of these emit very bright light when heated. The meteoroid glows as its surface melts, and we see it on the ground as a luminous object flashing across the sky. The meteoroid is now glowing as a meteor.

Here I am guilty of a bit of bad astronomy myself. In the past, I've told people that friction with the air heats the meteoroid and, as I said above, this is the usual explanation given in books and on TV. However, it's wrong. In reality, there is actually very little friction between the meteoroid and the air. The highly heated, compressed air stays somewhat in front of the meteoroid, in what physicists call a standoff shock. This hot air stays far enough in front of the actual surface of the rock that there is a small pocket of relatively slow-moving air directly in contact with it. The heat from the compressed air melts the meteoroid, and the slow-moving air blows off the melted parts. This is called ablation.

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