Seasons

• How Earth's curvature produces latitudinal differences in insolation.
• How the tilt of Earth's axis produces seasonal changes.
• Why the southern and northern hemispheres experience opposite seasons.
• How changes in the axial tilt angle affect seasonal temperatures.
• How Earth's axis does not change orientation during one orbital revolution.
• How Earth's yearly orbit produces the observed seasonal changes.

• One 4-inch white styrofoam ball
• One 12-inch bamboo skewer (used for kabobs)
• One round toothpick
• One small, flashlight

• One 12-inch white Styrofoam ball
• One half-inch wooden dowel rod approximately 36 inches long
• One large, bright, light source. (We found that a large flashlight or lamp works better than the light from a projector.)

During the lecture, have the student groups work through each of the following tasks. After groups have completed a task, demonstrate the correct answer using the instructor's demonstration model.

Student Task #1

• Use flashlight and ball-on-stick to represent the Sun and Earth respectively. (For this demonstration, keep the stick vertical.)
  • How does the distribution of incoming sunlight vary between the Equator and the Poles? (Can you explain why the equator is hotter than the poles?)

Student Task #2

• Use flashlight and ball-on-stick to represent the Sun and Earth respectively. (For this demonstration, tilt the stick.)
  • What effect does tilting the stick (Earth's axis) have on the distribution of incoming solar radiation (insolation)?
  • Use the model to show the relative positions of Earth and Sun for our winter and summer.
  • What happens when you rotate Earth on its axis? What are you demonstrating?

Student Task #3

• Use flashlight and ball-on-stick to represent the Sun and Earth respectively. (For this demonstration, tilt the stick.)
  • Demonstrate the annual orbit of Earth around the Sun and use the model to explain:
    • Why it can be winter in the U.S. when it is summer in Australia;
    • The contrasting positions of the Earth and Sun for each season in the U.S.
    • What happens to seasonal temperatures at various locations if the tilt angle increases or decreases?

• Students may hold the flashlight too close to the globe.
• Students must hold the flashlight steady when using the model and not move the flashlight vertically.
• Students must use the brightest center of the light beam as a reference point when manipulating the model.
• Students will find it difficult to constantly point the axis in one direction while rotating the globe through one orbit. This must be done, otherwise the misconception that the orientation of the Earth's rotation axis with respect to the plane of the ecliptic changes throughout one orbital revolution yearly may be reinforced.

• If you have a personal response system, you can ask conceptest questions that focus on the sun-earth relationship. (See References and Resources for sample conceptest questions.)
• By walking around the class, the instructor can observe how well the groups comprehend the underlying concepts and ask individual students to explain their conceptual understanding
• A short quiz could be given at the end of the class.
• Students could complete a worksheet containing questions about subduction zone earthquakes.
• Students could write a 'minute paper' explaining the distribution of subduction zone earthquakes.