Reasons for the Seasons

Reasons for the Seasons

Guiding Question: How does the tilt of Earth’s axis affect the light received by Earth as it revolves around the sun?

Skills Focus: making a model, observing, inferring, predicting

Materials:

• books
• flashlight
• paper
• pencil
• protractor
• toothpick
• acetate sheet with thick grid lines drawn on it
• plastic foam ball marked with poles and equator

Producer:

1. Make a pile of books about 15 cm high.
2. Tape the acetate sheet to the head of the flashlight. Place the flashlight on the pile of books.
3. Carefully push a pencil into the South Pole of the plastic foam ball, which represents Earth.
4. Use the protractor to measure a 23.5° tilt of the axis of your Earth away from your ‘flashlight sun.’ This position represents winter in the Northern Hemisphere.
5. Hold the pencil so that Earth is steady at this 23.5° angle and about 15 cm from the flashlight on. Dim the room light.
6. The squares on the acetate should show up on your model Earth. Move the ball closer if necessary or dim the room lights more. Observe and record the shape of the squares at the equator and at the poles.
7. Carefully stick the toothpick straight into your model Earth about halfway between the equator and the North Pole. Observe and record the length of the shadow.
8. Without changing the tilt, turn the pencil to rotate the model Earth once on its axis. Observe and record how the shadow of the toothpick changes.
9. Tilt your Earth 23.5° toward the flashlight. This is summer in the Northern Hemisphere. Observe and record the shape of the square at the equator and at the poles. Observe how the toothpick’s shadow changes.
10. Rotate the model Earth and note the shadow pattern.

Analyze and Conclude

1. Observing: When it is winter in the Northern Hemisphere, which areas on Earth get the most concentrated light? Which areas get the most concentrated light when it is summer in the North Hemisphere?  The
2. Observing: Compare your observations of how the light hits the area halfway between the equator and the North Pole during winter (Step 6) and during summer (Step 9). 
3. Inferring: If the squares projected on the ball from the acetate become larger, what can you infer about the amount of heat distributed in each square?
4. Inferring: According to your observations, which areas on Earth are consistently coolest? Which areas are consistently warmest? Why?
5. Predicting: What time of year will the toothpick’s shadow be longest? When will the shadow be shortest?
6. Drawing Conclusions: How are the amounts of heat and light received in a square related to the angle of the sun’s rays?
7. Communicating: Use your observations of an Earth-sun model to write an explanation of what causes the seasons.