英語(yǔ)時(shí)差：奧爾特云與彗星2

Ancient people believed comets were omens. Sometimes they were thought to signal the fall of empires, other times the birth of kings. Even today, when we’re all supposed to be much more intelligent, people do crazy things when comets pass by. But even though they’re beautiful to look at, comets aren’t supernatural. In fact, they are big, dirty clumps of ice, that can be a mile wide or more. So Who’s Throwing Giant Snowballs At Us? Well, if you lived in the time of the Roman Empire you might have said the gods were doing it. But nowadays we know that comets come from the Oort cloud. The Oort cloud gets its name from the Dutch astronomer Jan H. Oort, who hypothesized its existence around the middle of the twentieth century.