Materials List: 1 set per student group (polystyrene foam ball, pencils, lamp or overhead projector, string, bamboo skewer, graph paper, clip boards, masking tape, flashlight, color pencils), map of United States, globe, science learning logs
Prior to this activity, be sure that the students understand that the rotation of Earth on its axis causes night and day. Ask students to contrast the usual temperature differences that occur during the day with those at night. Ask them why the temperatures drop after the sun sets. (The sunlight provides us with energy.) Have students model Earth’s rotation using a polystyrene foam ball held in front of a lamp or overhead projector. The lighted side should be identified by the student as daytime and the side without direct light should be identified as nighttime.
Ask students to think of questions that they may have regarding the seasons. (Students may wonder why seasons happen, if all places have the same seasons, etc.) Ask students to name the seasons and to describe the changes in the weather and the environment that they have experienced during each season. Ask students if anyone has lived in a different part of the country or visited relatives in another state. Locate those places on the map. Have them share any information about the seasons in those locations.
Inform the students that the seasons are related to Earth’s revolution, or trip around the Sun that occurs in a year, and its tilt. Since we cannot go into outer space to observe the movement of Earth, ask students what would be a good way to set up a science inquiry to observe the event within a classroom (make a model). Students will use the polystyrene foam balls found at craft centers as a model of the Earth. Attach a string around the center of the ball to represent the equator and insert a bamboo skewer through the center of the foam ball to represent the north and south poles. Discuss the meaning of the equator and the meaning of an axis. Position the ball on the pencil to represent the planet titled on its axis. Help the students to tilt the ball at a 23-degree angle. Use a globe as a model. First, instruct the students to hold the Earth model with the axis perpendicular to the floor in front of a lamp to represent the Sun. Move it around the lamp as if in orbit. Guide students to note that the light strikes Earth’s different hemispheres equally.
Ask students to predict what would happen to the area if the planet were tilted on its axis.
To model how the angle of a surface affects how light spreads out, have students complete the following activity in small groups. Attach a sheet of graph paper to a clip board with masking tape. Hold the board perpendicular to the floor and shine the flashlight directly onto the graph paper about two feet away. (Be sure the flashlight is parallel to the floor, and, therefore, perpendicular to the paper. Trace the outline of the flashlight’s beam on the graph paper. Keeping the same distance from the paper to the flashlight, rock the board towards and then away from the flashlight. Observations should be recorded in their science learning logs (view literacy strategy descriptions). Tilt the board at a large angle like 45° or 60° and trace the new outline of the beam with a different color. Try a couple of other angles, marking the outlines with different colors. Count the number of squares colored or partially colored for each angle. Have students draw conclusions about how the angle of the surface affects how the light spreads out. (Students should conclude that more area is covered by the beam when the board is tilted at larger angles.) Explain to students that when the same amount of light is spread over a larger area then the intensity of the light decreases.
Instruct the students to hold up their pencil models so that Earth is tilted on its axis. Place the model so that the students will observe that the light is now slanted as it reaches the surface. Have students move the Earth model into positions that would represent the beginning of the four seasons, pausing in each position to observe which hemispheres are receiving more direct sunlight and those that are receiving less direct sunlight. Carefully observe the students while they move the Earth models through the seasons to make sure that the tilt of the axis remains parallel to its starting position. Have students note when the poles are in darkness. Instruct the students to rotate the pencil to observe day and night. Have them observe whether the rotation affects the areas that are in total darkness or light. Ask students how the amount of light an area gets could influence its weather and climate. (Sunlit hours would be affected which would affect the temperatures.) As the students move their models, name the seasons that the students are observing on the model. The students will also note that the illumination of the Earth forms equal parts on the model at the beginning of the fall and spring. (The equinoxes are periods of equal day and night across the globe.) The students should be instructed to observe that the illuminated parts of the Earth change as the Earth’s position in its orbit changes. Ask them to recall the illumination at the beginning of the activity when the Earth was not tilted. Ask the students if the area of light would change from season to season if the Earth were not tilted. (Allow them to model this again if needed. They should conclude that the Earth’s tilt influences the seasons.)
Discuss Earth’s revolution around the Sun, the tilt of Earth, and their relationship to seasons. Aid the students in diagramming the Earth’s orbit including the Earth illustrated in four locations to represent the illumination areas for the beginning of the four seasons. The students will label the diagram with the dates of the beginning of each season and the names of each season for both hemispheres.