题目

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 day and night are sometimes the same length. They are each twelve hours long. T若中心在原点、焦点在坐标轴上的双曲线的一条渐近线方程为，则此双曲线的离心率为