The reasons for the seasons

Here’s the quick and dirty answer. The earth has seasons because its geographic poles are tipped 23.5 degrees relative to its orbit around the sun. When the northern hemisphere is leaning toward the sun, it is summer in the northern hemisphere and winter in the southern hemisphere. The reverse is true when the southern hemisphere is leaning toward the sun. In between those two points, it is fall and spring. See this picture to see what I mean. Graphic (not to scale) of earth's orbit

The seasons do NOT change because of how close the sun is to the earth. In fact, in January we are closer to the sun by a few hundred thousand miles than in July, since the orbit is an ellipse and the sun is off center.

Now this view is from outer space. From earth, the situation seems much different. As the seasons progress, it seems as if the sun’s daily motion slowly drifts northward and southward.

When the north pole is leaning toward the sun, from our point of view it looks as if the sun rises in the northeast, crosses high in the sky, and sets in the northwest. When the sun reaches its northernmost rising and setting points, it has reached the summer solstice.

Path of sun in the summer

Likewise, in the winter, when the south pole is leaning toward the sun, it appears as if the sun rises in the southeast, crosses low in the sky, and sets in the southwest. The extreme here is the winter solstice. (NOTE: Reverse the seasons for locations in the southern hemisphere.)

Sun's path in winter

When the sun rises due east and sets due west, we call these times the equinoxes. We have 12 hours each of day and night, and the sun shines evenly on both hemispheres.

Sun's path at the equinoxes

Ancient civilizations paid attention to these dates, and many structures have wall openings or alignments that mark the solstices and equinoxes. Frequently, there would be major feasts and holidays to coincide with the seasonal changes. The Roman Saturnalia, held around Dec. 25, was one example.

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