时差N小时:流星雨是怎样形成的?

If you find yourself away from a city, spend anhour looking up on any clear, moonless night,you'rebound to see a few brilliant "shooting stars." Theseactually have nothing to do with stars: They'remeteors, and if your viewing conditions are good, youcan see about seven per hour on any given night.During a "meteor shower" however, this rate mayincrease to over a hundred meteors an hour. Whatare meteors, and what makes them gather intoshowers?

如果在没有月亮的晴朗夜晚，你远离城市，花一个小时观察天空。你一定会看见许多绚丽的一划而过的星星。它们其实不是真正的星星。它们只是流星，如果观察条件允许，你每小时大约可看见七颗流星。然而，如果遇上一场流星雨，你流星出现的频率会增加，达到每小时100多颗。什么是流星?它们是怎么聚集从而形成流星雨的?

Meteors are caused by bits of rocky material that enter our atmosphere from space, thenburn up because of friction. They don't have to be big for you to see them. Surprisingly, mostvisible meteors are caused by debris no larger than a single grain of sand. They burn sobrightly because of tremendous friction when they hit our atmosphere at more than forty milesper second.

流星是由很小的外太空陨石进入大气层，与空气摩擦燃烧引起的。那些陨石很小，人类并不能用肉眼看见。出人意料的是，大多数可见的流星其实是由比沙粒还小的碎石屑形成的。陨石以每秒40多英里的速度闯入大气层，产生巨大的摩擦力，从而发出明亮的光芒。

Why would something as random as flying specks of space debris gather into showers? It'sbecause the Earth passes through the same regions of space each time it orbits the sun. Certainregions have more debris than others, so we have meteor showers on those nights.

为什么从外太空随机飞入的陨石屑会汇聚成流星雨呢?这是因为地球每次绕太阳轨道公转会经过同样的区域。某些区域比其它区域有更多的陨石屑，所以地球经过这些区域时的夜晚，我们就会看见流星雨。

Of course this raises the question of why certain parts of our orbit would be especially fullof 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 anddebris. 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 fromHalley's comet.

当然这又会引出另一个问题，为什么地球公转轨道上的某些区域会有特别多的陨石屑。答案与彗星有关。彗星就像是围绕太阳转的巨大的，脏兮兮的雪球。彗星及其彗尾扫过之后，会留下一道充满细小尘埃和碎屑的痕迹，并存在很长一段时间。如果有一颗彗星经过地球公转轨道，那么我们每年都会经过这道满是尘埃和碎屑的区域。例如，每年10月21号我们会经过猎户座流星雨，这就是哈雷彗星残留的陨石屑引起的。