Edmund Halley was an English scientist who lived over 200 years ago. He studied the
observations of comets(彗星) which other scientists had made. The orbit of one
particular comet was a very difficult mathematical problem. He could not figure it out.
Neither could other scientists who dealt with such problems.
However, Halley had a friend named Isaac Newton, who was a brilliant mathematician. 
Newton thought he had already worked out that problem, but he could not find the
paper on which he had done it. He told Halley that the orbit of a comet had the shape
of an ellipse(椭圆形).
     Now Halley set to work. He figured out(觯决,计算出) the orbits of some of the
comets that had been observed by scientists. He made a surprising discovery. The
comets that had appeared in the years 1531, 1607 and 1682 all had the same orbit.
Yet their appearances had been 75 to 76 years apart.
     This seemed very strange to Halley. Three different comets followed the same orbit.
The more Halley thought about it, the more he thought that there had not been three
different comets, as people thought. He decided that they had simply seen the same
comet three times. The comet had gone away and had come back again.
     It was an astonislung idea! Halley felt certain enough to make a prediction(顶言 )
of what would happen in the future. He decided that this comet would appear in the
year 1758. There were 53 years to go before Halley's prediction could be tested.
     In 1758 the comet appeared in the sky. Halley did not see it, for he had died some
years before. Ever since then that comet had been called Halley's comet, in his honour.