We often don’t think about elementary questions such as “Why are there seasons?” But, when students are asked why, they think of the wrong answer. They will say that there is summer because the Earth is closer to the Sun and therefore is hotter, and they will also claim that there is winter because the Earth is farther from the Sun and is therefore colder. These seem to be correct, but they are not.
Seasons, first of all, are caused by the tilt of the Earth’s axis of rotation. The Earth not only revolves on its own axis of rotation, but it also revolves around the Sun. These two combined with the Earth’s 23.5 degree tilt results in a variation of seasons.
If we were to agree that there are seasons because the Earth is either closer or farther from the Sun, we won’t be able to explain why the southern hemisphere’s season is different from the northern hemisphere’s seasons. When it is winter in North America, it is summer in Australia. Why is this so? The Earth’s tilt explains this difference.
It is summer in Australia when it is winter in America because the Earth’s southern hemisphere may be tilted towards the Sun, while the northern hemisphere is tilted away from the Sun. This also accounts for the longer days and shorter nights in the summer, and the longer nights and shorter days in the winter.
The tilt is not only important for the seasons, but also for nature in general. The twelve hours of equal day light and darkness found in the equator results in plants that have longer flowering seasons. Those plants found in the northern hemisphere, on the other hand, must have shorter flowering seasons.
In the summer, the Sun is higher up in the sky, which results in stronger sunlight because the angles of its rays are directed more towards the ground. On the other hand, in the winter, the Sun is lower in the sky, which results in less concentrated sunlight because the angle of its rays is smaller.
The Earth’s orbit around the Sun is also not an exact circle. Its “elliptical” orbit (which is only considered elliptical because it is just a little bit not circular) shows that the Earth is never “more far away” or “closer to” the Sun during any one half of the year. Thus, we can’t argue that the seasons are caused by this variation in distance from the Earth to the Sun.
The seasons, therefore, are caused by the 23.5 degree tilt of the Earth’s axis of rotation.