PHENOMENON OF SEASONS, Solstice & Equinoxes - Explained

THE PHENOMENON OF SEASONS

The phenomenon or change of seasons is caused chiefly by the revolution of the earth round the sun and the inclination of the earth's axis at an angle of 66 1/2° to the plane of its orbit which constantly points to the same direction. It can be understood from the diagram on the article which shows four positions of the earth during its revolution round the sun.

The Position on 21 June : The Summer Solstice

(1) North Pole is inclined towards the sun and the South Pole is away from it.

(2) The rays of the sun fall vertically at the Tropic of Cancer (23 1/2° N).

(3) Major parts of the Northern Hemisphere are lit by the sun-rays. As a result, the days are long and hot and it is summer in the Northern Hemisphere. This position on 21 June, when the sun shines vertically on the Tropic of Cancer, is called the Summer Solstice.

The Position on 22 December : The Winter Solstice

(1) The South Pole is inclined towards the sun and the North Pole is away from it.

(2) The rays of the sun fall vertically on the Tropic of Capricorn (23 1/2° S). As a result, the days are long and hot there. It is summer in the Southern Hemisphere. Thus, on 25th December every year, when Christmas is being celebrated, it is winter in the Northern Hemisphere as in England, the U.S.A., India etc. but it is summer in the Southern Hemisphere as in Australia, South America, etc. Remember, when the sun shines vertically on the Tropic of Capricorn it is called the Winter Solstice.

The Position on 21 March and 23 September : The Spring and Autumn Equinoxes

(1) Both the North Pole and the South Pole are neither inclined towards the sun nor are they away from it.

(2) The rays of the sun fall vertically on the Equator. As a result, days and nights are of equal duration throughout the world on these two days (i.e., 21st March and 23rd September). The seasons in both the Northern Hemisphere and the Southern Hemisphere are similar. It is neither very hot nor very cold. On 21 March, it is spring in the Northern Hemisphere. It is called the Spring Equinox. On 23rd September, it is autumn in the Northern Hemisphere. It is called the Autumn Equinox. In the Southern Hemisphere, the seasons are opposite to the seasons in the Northern Hemisphere.

On the Equator, the sun always shines vertically overhead. Therefore, there is only one season on the Equator. It is a long and hot summer.