Do you know what a comet is? 你知道彗星是什么吗？

　　A comet is an object in the solar system. 彗星是太阳系中的天体。

　　It travels around the sun like the planets. 它像其它行星一样绕着太阳转。

　　Then, why don't we call it a planet? 那么，为什么我们不把它叫做行星？

　　Because comets are usually very small and their orbits are not regular, they cannot be planets. 因为彗星通常很小，而且它们的轨道还不规则，所以它们不能成为行星。

　　Because their orbits are not regular, they sometimes crash into planets and disappear. 因为它们的轨道不规则，有时撞上行星后就消失了。

　　So, it is very hard to observe comet in the sky. 因此，很难在空中观测彗星。

　　However, there are some comets we can observe. 然而，有些彗星我们能观测到。

　　For example, Halley's comet is very famous. 例如，哈雷彗星就非常著名。

　　Every 75 or 76 years, we can see it in the sky. 每隔75或76年，我们就能在空中看到它。

　　So, if you live very long, you can see Halley's comet more than once. 因此，如果你的寿命很长，你就会不止一次能看到哈雷彗星了。