3、Now we know that all this is caused by the movement of the earth around the sun.As the earth journeys around the sun， it spins on its own axis.This can be illustrated (解释，说明)by a simple experiment.If you push a sharp stick(尖头棒)through a rubber ball and twirl(使旋转)it with two fingers， the ball spins around in much the same way the earth is spinning at this moment.The points where the stick comes through the ball correspond to(等同于)the North and South Poles.If you twirl this ball at night directly in front of a bright light， you will notice that half of the ball is lighted up while the other half is in the shade(阴暗处).That is just like our night and day.If you keep the stick strictly vertical(垂直的)to the light and twirl it at an even speed， any spot on the ball’s surface will be in the light and in the shade the same length of time.