1. Seasons

3. Key Ideas

4. The axis points the same direction in space but it’s orientation with the sun changes.
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.

5. The angle the sun’s light makes with the surface of the earth results in either more or less direct light.

6. The angle the sun’s light makes with the surface also causes the Sun’s altitude to change.
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. The altitude of the sun in the sky affects how concentrated the light is.

8. Summary: 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.

9. Why distance doesn’t matter:
The earth is actually closer to the sun in winter!
The variation of the Earth-Sun distance is small — about 3%; this small variation is overwhelmed by the effects of axis tilt.

10. One more way the sky changes over time!
Precession – change in orientation of axis
It wobbles!
Precesses over about 26,000 years.
Polaris won’t always be the North Star.