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 (日出 and sunset (日落).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. We have days and nights because of _____of the earth.