What causes the earth to experience different seasons?

Let's see what effect the tilt of the earth's axis has on our seasons. First, we have to keep in mind a very important scientific principle:

A beam of light (heat) that is concentrated will provide more light (heat) than one that is more dispersed.

Look at the following image of a flashlight to see this principle in action.

Illustration of a dispersed beam of light using a flashlight

When the flashlight is directly overhead, isn't the beam of light (and heat) more concentrated than when the flashlight is at an angle? The image on the left will receive more light (heat) than the image on the right.

Now lets look at a picture showing the tilt of the earth's axis as the earth orbits the Sun.

Illustration of sun light hitting Earth in different seasons

Notice that during the Northern Hemisphere Summer, the Sun is more directly overhead than at any other time of year. However, during Winter, the Sun is at a definite angle with respect to the Northern Hemisphere and the rays of the Sun are not concentrated. Therefore, it follows that the more concentrated Sun's rays will warm the land and water surfaces during Summer. In Winter when the rays are more dispersed, the land and water surfaces will not get as warm. During Fall and Spring, the Sun's rays are not as concentrated as they are during Summer but are more concentrated than they are during Winter. Therefore, during Fall and Spring we experience temperatures that are cooler than during Summer but are warmer than during Winter.

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