How seasons change?

          We know that the Earth revolves round the Sun and also rotates on its own axis. Days and nights are caused by the rotation of the Earth on its axis. The axis of the Earth makes an angle of 23° with the vertical. It is this inclination which causes changes in seasons. With its inclined axis, when the Earth revolves round the Sun, the Sun rays make different angles at the same place at different times. Due to the variations in angles, the distribution of the solar heat is not the same at the same place. This uneven distribution of solar heat on the Earth leads to the summer or winter season.

          If we look at the picture, we see that in June, when the northern hemisphere is tilted towards the Sun, it is summer in Europe, Asia and North America (northern hemisphere) and winter in the southern hemisphere. Six months later, in December, the southern hemisphere is tilted towards the Sun, so it is summer in southern hemisphere but winter in northern hemisphere.

          On 21st March and 23rd September every year, the Sun is exactly over the equator. On these two days, the duration of the day and the night is the same (12 hours) at every place on the Earth. From 21st March to 21st June, the Sun advances from the equator to the Tropic of Cancer. This results in hot season in the northern hemisphere, where days become longer and nights shorter. During this period, it is winter in the southern hemisphere. From 21st June to 22nd December, the Sun advances towards the tropic of capricon. This causes the summer season in the southern hemisphere and winter in the northern hemisphere. In the northern hemisphere, the days are shorter and the nights are longer during this period. After 22nd December, the Sun again starts moving towards the north and reaches the equator again on 21st March. During this period, the days in northern hemisphere start getting longer and the nights shorter.

          In March and September the Sun is overhead at the equator. Both hemispheres are enjoying either autumn or spring.

          Thus, the revolution of the Earth round the Sun and its rotation on its own inclined axis changes the seasons as well as the duration of the days and the nights.