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 is tilted（倾斜）toward the sun. In those months the Northern Hemisphere（半球）gets more hours of sunlight than the Southern Hemisphere. 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.

    It was Sunday yesterday. Michael sat on the sofa and watched TV. He was very relaxed. His mother was doing the dishes. Suddenly, she felt terrible. She had to stop. Michael came to his mother and asked, "Are you all right, Mom?" "Yes. I'm OK, but I need to take a rest," said his mother.

    Michael felt sorry for his mother. He thought he should help his mother do the dishes. She felt terrible because she was too tired. So he did the dishes and cleaned the room when his mother was lying in bed. Michael felt happy because the room looked clean.

    When Michael's mother felt better, she went to do the dishes. But she was surprised to see the clean room. "You are really a great boy," she said happily. Michael was happy, too.