Friday, October 15, 1999

What kind of shower don't you need an umbrella for?

Sometimes out in West Texas it actually gets too hot to rain. Raindrops do fall, but they evaporate before they can reach the ground. If you were standing outside during this rain shower, you wouldn't need an umbrella!

There's another kind of shower you could stand in without getting wet. In fact, you would only know about this kind of shower if the sky were clear: a meteor shower.

What is a meteor shower? On a normal night of stargazing, if you watch carefully, you can expect to see a few streaks of light. During meteor showers, however, you can see dozens of meteors in an hour, or even dozens per minute! The meteors all appear to come from the same place in the sky, as if someone were pointing a "meteor thrower" at Earth. Most of the meteors vanish quickly, but some can flare brightly, even leaving trails in the sky.

In the past, meteor showers caused great fear and superstition. People actually thought the stars were falling from the sky!

Today we can put aside those fears and analyze the properties of a meteor shower. For example, we know that very few, if any, of the meteors in a meteor shower reach the ground. This tells us that the particles in a meteor shower are very small. When these dust-sized particles hit the Earth's atmosphere, air friction heats up the particles. They glow brightly for a moment, and then, like those West Texas raindrops, they evaporate.

Meteor showers also occur regularly. Meteors shoot out of the constellation Orion, for example, every year on October 20. If only rain showers were so predictable!

As comets go around the Sun, some of their dust leaks away. Solar wind blows more dust off the moving comet, just as the wind on Earth blows dirt off a moving truck. The comet leaves this dust behind, marking its orbit around the Sun. If the orbit of the Earth crosses the orbit of the comet, then the Earth can capture some of this "comet litter," causing a meteor shower. Since the Earth always intersects the comet's orbit at one or maybe two points every year, the meteor showers only occur at that time. For example, on October 20, the Earth crosses the orbit of Halley's Comet, creating the Orionid meteor shower. So each year Earthlings can sit back and enjoy the meteor shower--that is, if other showers will stay away for the day!

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