When you look up at the night sky, what do you see? There are other (26) heavenly bodies out there besides the moon and stars. One of the most (27) fascinating of these is a comet.
Comets were formed around the same time the Earth was formed. They are (28) made up of ice and other frozen liquids and gases. (29) Now and then these “dirty snowballs” begin to orbit the sun, just as the planets do.
As a comet gets closer to the sun, some gases in it begin to unfreeze. They (30) combine with dust particles from the comet to form a huge cloud. As the comet gets even nearer to the sun, a solar wind blows the cloud behind the comet, thus forming its tail. The tail and the (31) generally fuzzy atmosphere around a comet are (32) characteristics that can help identify this (33) phenomenon in the night sky.
In any given year, about a dozen known comets come close to the sun in their orbits. The average person can’t see them all, of course. Usually there is only one or two a year bright enough to be seen with the (34) naked eye. Comet Hale-Bopp, discovered in 1995, was an unusually bright comet. Its orbit brought it (35) relatively close to the Earth, within 122 million miles of it. But Hale-Bopp came a long way on its earthly visit. It won’t be back for another four thousand years or so.
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发表于 2016-7-30 15:35:37