The length of the day determines the duration of sunlight which affects the amount of solar radiation received by the earth’s surface. The longer period of sunshine, greater the quantity of solar radiation will be received by a portion of earth.For example, at the equator the length of days and nights is 12 hours in all the months but the tropics of Arctic and Antarctic sunshine duration varies between 0 and 24. On the autumn and springe quinoxes (September 23and March 21 respectively),the sun is overhead at the equatorat noon.The night and day all over the earth are equal on these days and maximum amount of insolation is received at the equator, and the amount of insolation decreasing towards the poles.It is caused by vertical sunshine at equator but with increasing latitudes, the rays become more and more slanting. Therefore, poleward, the received energy keeps on declining.