1. Ask students why we have seasons, and record their answers on the board. Avoid correcting misconceptions at this point, and save their ideas for later analysis.

2. Have students work with the Earth in Motion: Seasons Flash Interactive , which introduces the basic concepts behind the seasons. As the students use the interactive activity, have them create a vocabulary list of relevant terms.

3. Review these vocabulary words as a class: Northern Hemisphere, Southern Hemisphere, equator, North Pole, South Pole, axis, rotation, day, night, orbit, year, summer, winter, autumn, spring, equinox.

4. Have students work in pairs to investigate how the angle of sunlight can vary. The angle of incidence — the angle at which the Sun's rays strike Earth — changes depending on the Sun's apparent location in the sky. Have students go outside to measure the Sun's current angle of incidence. The angle of incidence is the angle formed by the incoming light rays and the perpendicular to Earth's surface:

Distribute materials to students: masking tape, yardsticks, notebooks, rulers, protractors and calculators.

1. As one student stands with his or her back facing the Sun, have the second student use masking tape to mark the location of the standing student's heels.
2. Then have the second student mark the length of the standing student's shadow.
3. Students should then measure the length of the shadow as well as the height of the standing student. (Make sure to use the same units of measurement.)
4. Have students draw scale diagrams of the experiment in their notebooks.
5. For grades 9-12, have students calculate the Sun's angle of incidence: angle of incidence = arctangent (length of shadow/height of student). For grades 6-8, have students use a protractor to measure the angle of incidence on the diagram.
6. Have students compare their results with the rest of the class. Since all students took their measurements at the same time, they should find they have similar angles of incidence.
7. You may wish to repeat this activity a second time later in the day; just an hour or two later, students will be able to measure and feel a difference in the angle of incidence.

5. Ask students to think about how the direction of sunlight changes throughout the day. Discuss the following:

1. How does the Sun's position above the horizon affect the angle of incidence?
2. How does the angle of incidence affect the length of your shadow?
3. How does the angle of incidence compare between morning and midday? Between morning and evening?
4. What have you noticed about temperature changes from morning to evening?

6. When the angle of incidence is high (closer to 90 degrees), the Sun is directly overhead at that location. This causes sunlight to hit Earth there more directly, which warms it more efficiently. Have students discuss the following:

1. Why does direct sunlight have more energy than less direct sunlight?
2. What role do you think the angle of incidence plays in determining seasons?
3. If you compared the angle of incidence at midday during winter and summer, which season would have a higher angle of incidence? Why?
4. How does the angle of incidence change over the course of the year? How does your answer vary for different locations on Earth?
5. What role does the distance between the Sun and Earth play in determining seasons?

7. Have students create a model to better understand the changes in how sunlight strikes Earth throughout the year. Divide the class into groups of six students. Provide each group with a globe, flashlight, and four sets of sticker dots (each set a different color). Distribute the Modeling Sunlight on Earth Worksheet PDF Document . Note: Remind students that although Earth's orbit is elliptical, the variations in distance between Earth and the Sun are very small and the orbit can be thought of as nearly circular.

8. Discuss how seasonal changes affect Earth's environment.

1. What are some examples of seasonal changes in the environment?
2. Do all locations on Earth experience the same kinds of seasons?
3. How do seasonal changes affect plants and animals?
4. What kind of data could you gather if you wanted to study seasonal changes?

9. Have students explore the Global View of the Seasons Flash Interactive , which provides satellite images of the abundance of plant life on land and in the sea. Note: Students may have difficulty understanding what they are looking at in these images. These images are not photographs but are false-color representations of satellite data that measure subtle differences in the distribution of chlorophyll. Now ask:

1. What do the colors represent in these satellite images?
2. What patterns do you notice?
3. Why does the abundance of plant life vary?
4. How does the land vegetation change throughout the seasons? Give specific examples.
5. How does the abundance of phytoplankton change throughout the seasons? Give specific examples.
6. Compare the variation in seasons at the poles versus at the equator.
7. (Optional) Research why cold water has more nutrients than warm water.

10. Review why Earth has seasons. Look at the ideas recorded by the students at the beginning of the lesson. How have their ideas changed?

11. Now that it is clear that the 23.5-degree tilt of Earth's axis is responsible for the seasons (because it affects the angle of incidence of sunlight), ask students to consider what would happen if the tilt changed. Ask:

1. What would happen to the seasons if the tilt were less than 23.5 degrees?
2. What would happen if it were greater than 23.5 degrees?

12. Tell the class that just as a spinning top wobbles, so does Earth wobble on its axis over a period of about 26,000 years. It is not the angle of the 23.5-degree tilt that changes, but the direction in which it is tilted. Note: Using a globe to demonstrate how Earth's axis can change direction without changing the angle may clarify this point. Ask:

1. How would the change in direction affect what we call the North Star?
2. How might the change in direction affect the seasons?

13. Show the Natural Climate Change in Djibouti, Africa QuickTime Video , which shows that the slight changes in Earth's distance from the Sun due to Earth's slightly elliptical orbit, combined with the wobble of Earth's axis, can have drastic effects on climate. Ask:

1. What evidence is there to show that the climate of Djibouti was once very different from what it is today?
2. How does the wobble of Earth's axis provide a possible explanation for the climate change?
3. What does the narrator mean by "natural" climate change?
4. Do you think there will be more climate changes in the future? Why or why not?