Astronomy Ch02.9: Seasons 1: What Is the Cause of the Seasons?

Part C Based on what you have learned from your rankings in Parts A and B, which statement best describes how variation in the Earth-Sun distance affects the seasons? - The variation in Earth-Sun distance over the course of each year has virtually no effect on the seasons

As you've already seen in Parts A and B, seasons on Earth are not affected by the variation in Earth's orbital distance over the course of each year. This is probably not too surprising when you realize that Earth always stays within about 3 percent of its average distance from the Sun. Other factors are much more important in creating the seasons.

Part A Refer to the data in the introduction. Rank the seasons for the Northern Hemisphere based on Earth's distance from the Sun when each season begins, from closest to farthest. - winter - spring - fall - summer

Many people guess that seasons are caused by variations in Earth's distance from the Sun, but if that were the case, we'd expect it to be warmest (summer) when Earth is closest to the Sun. As you can see in the rankings, Earth is actually farthest from the Sun when it is summer in the Northern Hemisphere. We conclude that variations in the Earth-Sun distance from are not the major cause of our seasons.

Part B Refer to the data in the introduction. Rank the seasons for the Southern Hemisphere based on Earth's distance from the Sun when each season begins, from closest to farthest. - summer - fall - spring - winter

The fact that the two hemispheres have opposite seasons is further proof that the seasons are not caused by Earth's varying distance from the Sun. Keep in mind that Earth is so small in size (diameter 12,800 km) compared to its distance from the Sun (about 150 million km) that there is no significant difference in the distances of the two hemispheres from the Sun; therefore, if the seasons were caused by Earth's varying distance from the Sun, the entire Earth would have summer at the same time.

Part D You've now seen that Earth's varying distance cannot be the cause of our seasons. So what is the cause of the seasons? - The tilt of Earth's axis causes different portions of the Earth to receive more or less direct sunlight at different times of year

Without axis tilt, we would not have seasons on Earth. Seasons on other planets occur for the same reason, which is why planets with axis tilts (such as Mars and Saturn) have seasons and planets with very little axis tilt (such Jupiter or the "upside-down" axis of Venus) do not.