Change In Seasons - Why Season Changes?

We experience different types of seasons round a year. Chiefly, they are summer, autumn, winter and spring. As you have experienced, it is quite hot in summer (June-August) but it is quite cold in winter (December-February). But it is neither hot nor cold in spring (March-May) and in autumn (September-November). We need layers of very warm clothes in winter but we get sweat pouring off even with very light cotton clothes in summer. Everyone
may wonder why this actually happens.

The seasons occur due to the revolution of the earth on its oval orbit on which it is always tilted at 23.5 degrees. Therefore, two poles are inclined towards the sun for half of the year. The hemisphere that is tilted towards the sun has a longer day and receives more heat of the sun. But the hemisphere that is tilted away from the sun has a shorter day and receives less heat of the sun. And as you know already, seasons are just opposite in two hemispheres. For example, Christmas that is observed on 25 December falls in winter in the northern hemisphere and in summer in the southern hemisphere. However, there is no seasonal change on the equator which always receives direct rays of the sun and lies at the same distance from the sun.

Seasons keep changing due to the revolution of the inclined earth on its oval orbit. As shown in the adjacent figure, on 22 December, the earth is at the farthest point of the orbit (point A) on the right. The sun is just over the Tropic of Capricorn. Therefore it is summer in the southern hemisphere and winter in the northern hemisphere. 22 December is the winter solstice for the northern hemisphere; it is the shortest day there. The North Pole has a complete darkness as no sunlight falls on it. Then onwards, the northern hemisphere gradually tilts to the sun. After three months on 21 March, the earth reaches point B in the figure, where the sun falls directly over the equator. As both the hemispheres are at the same distance, day and night are equal round the world on this day. This is called equinox i.e. time of equal day and night. This is the beginning of spring in the northern hemisphere.

The northern hemisphere gradually gets closer to the sun. After another 3 months on 21 June, the earth reaches the other farthest point (point C) on the orbit on the left. At this point, the sun is just over the Tropic of Cancer. It is the longest day of the northern hemisphere and is called the summer solstice there. At this time, the South Pole has a complete darkness. Then onwards, the northern hemisphere gradually tilts away and the southern hemisphere tilts to the sun. After 3 months on 23 September, the sun again falls directly on the equator so both poles are at the same distance (point D). It is another day of equal length of day and night or equinox.

For the rest part of the year, the southern hemisphere further tilts to the sun. After another 3 months, the earth is at point A again. By then, it has completed one revolution. The sun is over the Tropic of Capricorn again. So it is the longest day in the southern hemisphere but the shortest one in the northern hemisphere. In this way, seasons keep changing round the globe.