Why Does Earth Have Seasons?

Summer returns to North America. The earth has traveled another revolution about our star, the sun. The northern hemisphere receiving the direct light of the sun is heating up, and night is banished from the arctic circle.

As this is happening, the long night has descends upon the Antarctic continent. Winter ice is building in the southern seas of the Pacific, and snow may be falling in the Tierra del Fuego, Argentina.
The seasons have changed. But why? What causes the seasons?
There are several common misconceptions that people have about the seasons. The biggest misconception is that the earth is closer to the sun during the summer and farther during winter. This hypothesis is wrong because it can not explain why it is simultaneously summer in the northern hemisphere and winter in the southern. Coincidentally the earth is actually farthest from the sun during the northern hemisphere summer. This is a happy coincidence because if keeps our summers cooler than they might be.

In order to understand the cause of the seasons, it is helpful to take an outsiders view of the solar system. Looking from outside the solar system, the first thing that you see is the sun. The planets, including the earth, move around the sun on elliptical orbits. The Earth revolves around the sun once every year. In addition to orbiting the sun, planets rotate. The earth rotates on its axis once every day, causing the sun to appear to rise and set in our sky.

In the case of the earth, the axis of rotation is inclined with respect to the path of its orbit by 23 degrees. This special geometry is shown in the Figure attached to this article. This means that during the northern summer, the northern hemisphere is angled towards the sun, and the southern hemisphere is angled away. This is also illustrated in the Figure.

The second major misconception about the seasons is caused by a miss-interpretation of the geometry of the earth’s tilt. The northern hemisphere is slightly closer to the sun during the northern summer. But this is not why it is summer.

It is summer because the sun’s rays hit the northern hemisphere more directly and for a longer period of time. During the northern summer the sun’s rays hit the southern hemisphere indirectly, and for less time, producing winter in South America.

Hopefully you now understand the cause of the seasons. If you did not know how this works before, don’t worry you are in good company. Several studies have been done with recent graduates of such prestigious institutions as Harvard and Yale. After graduation, students were asked to explain the seasons. Many Ivy League grads were not able to explain this phenomena. It was determined that misconceptions, probably begun when they were kids, were the main impediment to learning the true cause of the seasons. Hopefully by outlining the common misconceptions, as well as explaining the true cause of the seasons, you are ready to explain this yearly phenomena to a new generation of students.

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