1. Seasons

2. Review: The sky varies as Earth orbits the Sun
Use this interactive figure to explain how the constellations change with the time of year.

3. 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.
Seasons depend on how Earth’s axis affects the directness of sunlight

5. Sun’s altitude changes with seasons
Sun at noon in summer: higher altitude means more direct sunlight.
Sun at noon in winter: lower altitude means less direct sunlight.
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.

7. Key idea: Earth’s axis points to Polaris all year
so its orientation relative to the Sun changes
Summer occurs when sunlight hits a hemisphere more directly
Winter occurs when the sunlight is less direct.
AXIS TILT is the key to seasons; without it, we would not have them.

8. Why distance doesn’t matter:
Variation of Earth-Sun distance is small — about 3%; this small variation is overwhelmed by the effects of axis tilt.
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.

9. Orientation of Earth’s
Precession – change in orientation of axis
It wobbles!
Precesses over about 26,000 years.
Polaris won’t always be the North Star.
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” --- 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.