Variations in the amount of sunshine received are the basis for the occurrence of different seasons. The reasons for the occurrence of different seasons are,
- Revolution of the earth
- Tilt of the earth’ axis
- Apparent movement of the sun
- Revolution of the earth: The earth revolves around the sun in an elliptical orbit. This motion is known as revolution. The earth takes 365\(\frac{1}4\) days to complete one revolution.
- Tilt of the earth’s axis: The axis of the earth is tilted at an angle of 66 \(\frac{1}2\)° from the orbital plane and 23 \(\frac{1}2\)° from the vertical plane.
The earth maintains this tilt throughout its revolution. This is known as parallelism ofthe earth’s axis. Since the parallelism is maintained throughout the revolution, the position of the sun in relation to the earth varies.
Apparent movement of the sun : The sun shifts apparently between Tropic of Cancer and Tropic of Capricorn. This is known as the apparent movement of the sun. There will be variation in the amount of solar energy received on earth due to the apparent movement of the sun.
The sun rays fall vertically over one hemisphere during one half of the year and on the other hemisphere during the other half. Temperature will be higher over places where the vertical rays ofthe sun fall. The temperature will be low at places where the sun rays are slanting.
