4、If the earth were spinning just like a rubber ball， there would only be day and night on earth， but no seasons， and days would always be the same length as nights --- 12 hours each.But this is not how the earth spins.It spins with its axis tilted(倾斜).Its axis is always at the angle(角度)to the plane of its orbit(地球的轨道平面)--- an angle of about 231/2 degrees.It is this tilting that accounts for(说明，解释清楚)our four seasons and the lengthening and shortening of days and nights.For this reason also， the Equator(赤道)(an imaginary line (一条想象出来的线)drawn around the earth at equal distance from the two poles)is not always directly under the sun’s rays.For six months the earth is tilted towards the sun， and the Northern Hemisphere(北半球)gets more than its share(份量)of sunlight every day.Days are longer than nights， and what is more， the sun’s rays come down more perpendicularly(垂直地，直射地)instead of slanting down(斜射).