How Seasons Change

I was born and I grew up in a country near the equator. So, we didn’t experience any seasonal change. Nevertheless, in the school, we learned how seasons occur. And, may be my teacher clearly explained that seasons happen because of the 23.5⁰ inclination of the Earth. But I had forgotten it and I thought it was because the planet Earth travels on an elliptical course.

After some thought and playing with a torch and a tennis ball, I understood again how this inclination causes seasons. I know most of you already know it but I’m sharing it. Let’s try to understand it by a diagram.

• First, let’s find out how summer in northern hemisphere and winter in southern hemisphere occur.

• Figure 1 gives you Earth’s travel around the Sun and figure 2 describes when the Earth is at a position like A of figure 1.
• A particular country in the northern hemisphere is drawn with a blue cross and another country in the southern hemisphere is drawn with a red cross. The country’s location changes when the Earth rotates around its own tilted axis and that’s why we have several blue crosses (and red crosses). Grey filling tells you that the cross is in darkness.
• The blue cross is in daylight more than it is in darkness. Three blue crosses and the two blue-grey filled crosses give away this information. Remember again; this is the same cross we are talking about at different times of the day. So, the length of day and night is not equal. As a result, for a given 24 hours, the blue cross receives more heat than what it can receive if the axis is not inclined. This is summer for northern hemisphere. On the other hand, the exact opposite happens in the southern hemisphere, which results in winter.

• Now, let’s see how spring and autumn occur.

• This time, the rays of Sun come perpendicular to the sheet, as if you are the Sun, looking at Earth. As you can see, now the inclination of the Earth does not affect the length of day in the two hemispheres. Thus, day and night are equal in length for both hemispheres. (five blue crosses in daylight and another five at the back in darkness)
• From figure 3, the position D is in the middle of the Earth’s transition from C to A. For northern hemisphere, this is from winter to summer, and for southern hemisphere, this is from summer to winter.
• So, we can assume that these two transitions would make the trees respond differently, giving you the two seasons of spring and autumn.
• [winter-dead trees] -> gradually start to receive more sunlight so the blossoms will come out: This is spring.
• [summer-grown leaves]-> gradually start to lose sunlight which eventually result is low energy synthesization from sunlight by the green chlorophyll pigment in all the plants. As there is not enough sunlight, not much chlorophyll is needed by the trees and when this pigment vanishes, the leaves turn red and yellow. Small amounts of these colors have been in the leaves all along. We just can't see them in the summer, because they are covered up by the green chlorophyll: This is autumn.
  

• Finally, now it is easy to figure out why countries near the equator do not experience any seasonal change.

 

• Again, the inclination of Earth does not affect the length of day and night for countries near the equator.

Thank you for reading this. Hope you enjoyed it!