2.2 The Reason for Seasons Our goals for learning: What causes the seasons? How do we mark the progression of the seasons? Does the orientation of Earth’s axis change with time?

Thought Question TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter. A good way to begin discussion of seasons is by posing this question about the most common season misconception.

Hint: When it is summer in the U.S., it is winter in Australia. TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter. Hint: When it is summer in the U.S., it is winter in Australia. This hint should make students realize that distance from the Sun CANNOT be the explanation for seasons: if it were, the entire Earth should experience the same seasons at the same time. Note: Some students think the axis tilt makes one hemisphere closer to the Sun than the other; you can show them why this is not the case by revisiting the 1-to-10 billion scale model solar system from Ch. 1. When you remind them that the ball point size Earth orbits 15 meters from the grapefruit size Sun, it’s immediately obvious that the 2 hemispheres cannot have any significant difference in distance.

The real reason for seasons involves Earth’s axis tilt. TRUE OR FALSE! Earth is closer to the Sun in summer and farther from the Sun in winter. • Seasons are opposite in the N and S hemispheres, so distance cannot be the reason. The real reason for seasons involves Earth’s axis tilt. Now that you’ve answered the T/F question, we can go on to explore the real reason for seasons. Note: You might optionally mention that, in fact, Earth is closest to the Sun during N. hemisphere winter…

What causes the seasons ? Misconceptions about the cause of the seasons are so common that you may wish to go over the idea in more than one way. We therefore include several slides on this topic. This slide uses the interactive version of the figure that appears in the book; the following slides use frames from the Seasons tutorial on the Astronomy Place web site.

Axis tilt causes uneven heating by sunlight throughout the year. This tool is taken from the Seasons tutorial on the Astronomy Place web site. You can use it to reinforce the ideas from the previous slide. As usual, please encourage your students to try the tutorial for themselves.

Summary: The Real Reason for Seasons Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun. Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct. AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.

Why doesn’t distance matter? Only small variation of Earth-Sun distance — about 3%; this small variation overwhelmed by effects of axis tilt. Note: more ocean, less land means less extreme seasons in the southern hemisphere. Note: Some planets (major and minor) have greater distance variation that DOES affect their seasons — notably Mars, Pluto. The two notes should be considered optional. If you cover the first note, you might point out that since Earth is closer to the Sun in S. hemisphere summer and farther in S. hemisphere winter, we might expect that the S. hemisphere would have the more extreme seasons, but it does not because the distance effect is overwhelmed by the geographical effect due to the distribution of oceans.

How do we mark the progression of the seasons? We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinox Here we focus in on just part of Figure 2.13 to see the four special points in Earth’s orbit, which also correspond to moments in time when Earth is at these points.

We can recognize solstices and equinoxes by Sun’s path across sky: Summer solstice: highest path, rise and set at most extreme north of due east. Winter solstice: lowest path, rise and set at most extreme south of due east. Equinoxes: Sun rises precisely due east and sets precisely due west. Of course, the notes here are true for a N. hemisphere sky. You might ask students which part written above changes for S. hemisphere. (Answer: highest and lowest reverse above, but all the rest is still the same for the S. hemisphere; and remind students that we use names for the N. hemisphere, so that S. hemisphere summer actually begins on the winter solstice…)

Seasonal changes are more extreme at high latitudes Other points worth mentioning: Length of daylight/darkness becomes more extreme at higher latitudes. The four seasons are characteristic of temperate latitudes; tropics typically have rainy and dry seasons (rainy seasons when Sun is higher in sky). Equator has highest Sun on the equinoxes. Optional: explain Tropics and Arctic/Antarctic Circles. Path of the Sun on the summer solstice at the Arctic Circle

Does the orientation of Earth’s axis change with time? Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years. Polaris isn’t always the North Star. Positions of equinoxes shift around orbit; e.g., spring equinox, once in Aries, is now in Pisces! Precession can be demonstrated in class in a variety of ways. E.g., bring a top or gyroscope to class, or do the standard physics demonstration with a bicycle wheel and rotating platform. You may wish to go further with precession of the equinoxes, as in the Common Misconceptions box on “Sun Signs” on p. 37 --- this always surprises students, and helps them begin to see why astrology is questionable (to say the least!). Can also mention how Tropics of Cancer/Capricorn got their names from constellations of the solstices, even though the summer/winter solstices are now in Gemini/Sagittarius. © 2005 Pearson Education Inc., publishing as Addison-Wesley

© 2005 Pearson Education Inc., publishing as Addison-Wesley What have we learned? What causes the seasons? The tilt of the Earth’s axis causes sunlight to hit different parts of the Earth more directly during the summer and less directly during the winter. How do we mark the progression of the seasons? The summer and winter solstices are when the Northern Hemisphere gets its most and least direct sunlight, respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight. © 2005 Pearson Education Inc., publishing as Addison-Wesley

© 2005 Pearson Education Inc., publishing as Addison-Wesley What have we learned? Does the orientation of the Earth’s axis change with time? Yes. The tilt remains about 23.5 degrees (so the season pattern is not affected), but Earth has a 26,000 year precession cycle that slowly and subtly changes the orientation of the Earth’s axis. © 2005 Pearson Education Inc., publishing as Addison-Wesley