Mars’ apparent motion has been a mystery to astronomers for hundreds of years. Most early theories of tilt solar system argued that the Sun and planets turned around the Earth. But the sudden westward motion of Mars presented problem. Why would Mars move west for two months when it nears “opposition”?

　　In 1543, a Polish church worker named Nicolas Copernicus published a different theory. His theory said the Earth and planets moved around the Sun in perfectly circular orbits. Copernicus’ theory was simpler. But his Sun-centered system still did not explain the observed motion of Mars very well.

　　Finally, a German mathematician named Johannes Kepler published a complete theory of the motion of the planets in 1619. He had carefully studied the motion of Mars for many years.

　　Johannes Kepler discovered that the planets do not move in circular orbits around the Sun. Instead, they travel in flattened orbits called ellipses(椭圆). Mars’ elliptical orbit is the cause of its unusual brightening this year and its apparent large size.

　　During the next several weeks, you can see for yourself why the mysterious motion of Mars has caused so many people to wonder.

　　9.From the passage we can learn that _________.

　　A. the sun and planets generally travel westward

　　B. Kepler studied the orbit of the planets in 1619

　　C. Copernicus’ theory well explained the motion of Mars

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