12月大学英语六级考试快速阅读实战演练（4）

　　Our galaxy is a gigantic agglomeration of stars and planets whose numbers will probably never been known. Currently we estimate this number to be about thirty billion. Scientists have estimated that the diameter of our galaxy, if it were to be traveled, would take us about fifty thousand light years and the thickness to be about fifteen to twenty light years.

　　We live in small part of the Milky Way Galaxy, which is referred to as a solar system. Our solar system is made up of nine planets and 31 moons, which orbit the center of galaxy. At the center of our galaxy is our Sun, which is approximately twenty-five thousand light years from our solar system.

　　These nine major planets in order from the center are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

　　Mercury is the planet nearest to the Sun. As it orbits the Sun, it does not rotate, keeping the same face of the planet toward the Sun at all times. This means that one side of the planet has a continual burning day of 900 F, and the other side a continual night and a deadly cold of 450F below zero. Mercury is the fastest traveling of the nine planets making one full orbit around the Sun in only eight days. Life on mercury would be impossible. If you could live where the night meets the day and survive the extreme conditions, you would need dark goggles to protect you eyes from the extreme light. Mercury has little or no atmosphere to diffuse the sunlight. You would also weigh considerable less due to the lack of gravity. A person weighting 100 pounds on Earth would weigh 27 pounds on Mercury.

　　Looking into the night sky you will notice Venus, the brightest star in the sky, and the second planet from the Sun. Venus can only be seen at certain times of the day either at dawn or at Sunset. In physical characteristics, Venus is most like earth with a diameter of seven thousand six hundred miles, and gravity the same as earth. It is located a distance of sixty-seven million miles from the Sun and makes one complete orbit around the Sun in two hundred twenty five days. A single day on Venus can last up to two hundred and twenty five earth days due to its slow rotation. The atmosphere appears to consist of mostly nitrogen and carbon dioxide. This is due to the lack of life on the planet.

　　The best time to view Venus comes every one hundred and twenty years as it passes between the Earth and the Sun. The last time to best view it was on June 7, 2004.

　　Earth, the third planet form the Sun is our home planet. Earth is the only planet in our solar system known to sustain life. Under the layer of atmosphere that surrounds the planet we are able to provide all the necessary components to make life sustainable. Earth is the fifth larges planet in the solar system. Its diameter is just a few hundred kilometers larger than Venus. Earth rotates on its axis one complete revolution in a twenty-four hour period and orbits the Sun in three hundred and sixty five days.

　　Mars is the fourth planet from the Sun and is the closest neighbor to earth. Every seventeen years Mars reaches its closest point to earth in its orbit around the Sun at thirty-five million miles.

　　Mars is a small planet that has a red glow. It makes one complete orbit around the Sun in six hundred and eighty seven days. Oxygen and water vapor have been detected in the atmosphere of Mars, and a yellowish fog and clouds move around the planet indicating that the planet has different types of weather and seasons like the earth. A single day on Mars is very much like a day on earth lasting twenty-four hours and thirty-seven minutes, however, a year on Mars is twice as long as one year on earth.

　　Jupiter the fifth planet from the Sun gets its name from the Father of the Greek Gods. Jupiter, which orbits four hundred eighty-three million miles from the Sun, is the biggest of all the planets in our solar system. It has a diameter of eighty-nine thousand miles and a volume of thirteen times that of earth. Jupiter orbits the Sun every twelve years. Its atmosphere is thousands of miles thick and is made up of mainly hydrogen. The surface temperature is about -205F. There are no seasons on Jupiter, but it has floating clouds of ammonia and methane gas.

　　Jupiter has a bright red distinctive spot that is more than twice as wide as Earth. This mark has astronomers puzzled because there is no reason for the bright spot due to the fact that it does not receive much Sunlight. This spot enables us to see that Jupiter has a very fast rotation. One rotation is completed every ten hours. The forces placed on the planet due to this quick rotation cause the clouds at the equator to bulge like a tire, but due to the gravitational forces of the planet keep the clouds from being cast into space.