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Earth's seasons — the annual climate changes that different locations experience — result from a combination of Earth's orbit around the Sun and the tilt of Earth's axis. Understanding why the Earth has seasons is one of the most difficult concepts for students to understand, and it is often a battle to get them to abandon their preconceived ideas. Two popular misconceptions are as follows:
In this lesson, students use class discussion, interactive activities, hands-on activities, and videos to learn about the seasons, Earth's motion, and the role of its tilted axis. Students also study satellite data showing seasonal changes of plant life and explore an example of long-term natural climate change.
Three class periods
Arrange to start the lesson on a sunny day. If possible, arrange computer access for all students to work in pairs. Designate a corner (where two walls and the ceiling meet) as the North Star. Make copies of the Modeling Sunlight on Earth Worksheet PDF Document for all students.
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.
5. Ask students to think about how the direction of sunlight changes throughout the day. Discuss the following:
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:
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.
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:
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:
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:
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:
Have students discuss the following:
The Digital Library for Earth System Education (www.dlese.org) offers access to additional resources on this topic.