Seasons are the most wonderful thing that nature has gifted to Earth. Without seasons, life on earth would have ceased long back. But seasons have kept life alive on this Earth. Changing landscape according to the seasons is the greatest beauty of nature. Each living being on Earth can sense these changes and they adapt themselves to these changes to sustain their life on Earth. So, seasons play a crucial role in making Earth as a habitable zone for life in our solar system.
One of the age old questions is “How these seasons occur on Earth?” Well, a fifth grade school child may answer this question. And the most celebrated answer among children is this, “Earth revolves around the Sun in an elliptical orbit with Sun at one focus. So, when Earth is very close to Sun we have summer and when Earth is far away from Sun, we have winter.” The below image depicts what is said in the sentence.
At first stroke, this answer looks evident. When the Earth is closest to Sun, Earth receives more sun light than at other position. The hemisphere which is facing the Sun will have summer and the other hemisphere which is away from Sun will have winter. These conclusions can be derived from the above statement. I studied this in my school days. I firmly believed this explanation until I found that, this is totally wrong. Yes, the most obvious statement that we make about seasons is wrong. Some teachers still follow the same logic and teach wrong concept to their students!!
Let us have a closer look on how these seasons occur?
There are two motions of Earth which are evident to us. Earth revolves around the Sun and Earth spins about its axis. Earth takes nearly a year (365 days) to go around the Sun in an elliptical orbit and it takes a day (24 hour) to spin about its axis. But there is a weird and wonderful thing about this axis of rotation of the Earth. (A body which rotates about its axis is called axis of rotation). It is tilted at an angle of 23.5 degree from the vertical line as shown in the right side image. This angle or tilt is constant throughout the elliptical orbit. And also the axis of rotation of the Earth points in the same direction at all positions of the orbit as you can see in the below image. This direction is always towards the north.
Now let us come to our core topic seasons. We have four seasons on Earth, summer, winter, spring and fall.
In the month of December Earth will be at position A. At this position Earth is close to sun. But northern hemisphere will receive less sun light than the southern hemisphere due to the tilt of the earth. Because of this reason northern hemisphere will observe winter and southern hemisphere will observe summer!
But in the month of June, that is at position C the season reverses. At this position Earth is far away from Sun compare to position A. But now northern hemisphere will receive more sun light than southern hemisphere due to the same reason tilt of the axis of rotation of the Earth. So, summer season at northern hemisphere and winter season at southern hemisphere. But observe one thing; direction of the axis of rotation of the Earth is not changed. It is still pointing in the same direction.
And the same explanation goes on for position B and D. At position B, northern hemisphere will have spring and southern hemisphere will have fall. Exactly reverse happens at position D.
We have got the answer! The seasons occur on earth is because of the tilt of the axis of rotation of Earth (or Inclination of axis of rotation).
But we did not talk about the distance between Earth and Sun. Because distance is not the cause of seasons! Let me talk with numbers. On Jan 4, Earth is closest to Sun with a distance of 148,000,000 km. On July 4, Earth is at farthest point from Sun with a distance of 152,000,000 km. The variation in distance is only 2%. This small variation in distance cannot cause seasons on earth.
So we have seasons on Earth because, the axis of rotation of Earth is tilted. (23.5 degree). If there is no tilt in the axis, then there are no seasons on Earth!!!
What I have learnt in school and wrote in exam is totally wrong.
Viswa Keerthy S
(*Title is with respect to northern hemisphere of Earth where I live. )