The word “day” has two meanings. When we talk about the number of days in a year, we are using “day” to mean 24 hours. But when we talk about day and night, we are using “day” to mean the time between sunrise and sunset. Since the earth looks like a ball, the sun can shine on only half of it at a time. Always one half of the earth is having day and the other half night. A place is moved from day into night and from night into day over and over by the spinning（旋转）of the earth. At the equator（赤道）day and night are sometimes the same length（长度）. They are each twelve hours long. The sun rises at 6 o'clock in the morning and sets at 6 o'clock in the evening. For six months the North Pole（北极）is tilted（倾斜）toward the sun. In those months the Northern Hemisphere（半球）gets more hours of sunlight than the Southern Hemisphere（半球）. Days are longer than nights. South of the equator nights are longer than days. For the other six months the North Pole is tilted away from the sun. Then the Southern Hemisphere gets more sunlight. Days are longer than nights. North of the equator nights are longer than days. Winter is the season of long nights. Summer is the season of long days.

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    Jupiter(土星) is the fifth planet from the sun and it is the largest one in the solar system. It is about 11 times bigger than the earth. Jupiter's strangest thing is its huge red spot. Pictures taken from space tell us that the red spot is probably a hurricane(飓风). It is so big that the earth would fit right in the middle of it. Sometimes the hurricane looks like a huge oven with orange-red waves (波浪). Some other time the color almost disappears.

    On the earth, hurricanes move. We can watch them travel across water to land. Jupiter's red spot also moves. Scientists say it “walks”. It is carried along by air in Jupiter's atmosphere(大气层).

    So far, Jupiter has been visited by four spacecrafts. All of them flew by Jupiter very quickly. On the next trip to Jupiter, a spacecraft will stay near the planet much longer. New information will be sent back to the earth. Soon scientists will know much more about this strange red spot in the atmosphere.

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