英語聽力文摘 流星
If you find yourself away from a city, spend an hour looking up on any clear, moonless night.
You're bound to see a few brilliant "shooting stars." These actually have nothing to do with stars: They're meteors, and if your viewing conditions are good, you can see about seven per hour on any given night.
During a "meteor shower" however, this rate may increase to over a hundred meteors an hour. What are meteors, and what makes them gather into showers?
Meteors are caused by bits of rocky material that enter our atmosphere from space, then burn up because of friction. They don't have to be big for you to see them. Surprisingly, most visible meteors are caused by debris no larger than a single grain of sand. They burn so brightly because of tremendous friction when they hit our atmosphere at more than forty miles per second.
Why would something as random as flying specks of space debris gather into showers? It's because the Earth passes through the same regions of space each time it orbits the sun. Certain regions have more debris than others, so we have meteor showers on those nights.
Of course this raises the question of why certain parts of our orbit would be especially full of debris. The answer has to do with comets. Comets are big, dirty snowballs that orbit our sun. Long after a comet and it's tail have passed us by, it leaves behind a thin trail of dust and debris. If a comet crosses the Earth's orbit, we pass through this trail each year. For example, every October twenty-first we pass through the Orionid shower, which is debris left behind from Halley's comet.
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