— Background Lessons

Background
Many people have misconceptions about why we have seasons. The real reason for seasons lies in the fact that the Earth is tilted on its axis. The classic diagram of the Earth's tilt, can be found in textbooks for elementary students and graduate students alike.

The tilt of the Earth affects the length of days and also the amount of direct sunlight received during the seasons. As you can see, days are shorter in the Northern Hemisphere during the winter because the North Pole is tilted away from the sun and the Northern Hemisphere receives less direct sunlight. Just the opposite is true in the summer, when the North Pole is tilted toward the sun. As the Earth orbits the sun, the Northern and Southern Hemispheres are put into and out of direct sunlight. The part of the Earth tilted toward the sun gets more direct sunlight than the part tilted away from the sun.

In equatorial regions, the length of days and the directness of sunlight don't change as much. This is why the four seasons of winter, spring, summer, and fall don't occur there. (This lesson is also available in the Printed Teacher's Manual)

Materials Needed
Pie Tins
Black Paint
Clay
Balls of Various sizes
String
Construction paper
Dowels

Activity

1. In groups of three-four, ask students to develop a demonstration to show why the Northern and Southern Hemispheres have four seasons. Using rubber balls or clay they should make models of the Earth and the sun. Have other materials available, such as string, construction paper, dowels, etc. They will need to account for the tilting of the Earth, the position of the poles, the elliptical path of the Earth around the sun, the spinning of the Earth on its axis and the effects of direct and indirect light. They will need to use some means to mark the months of the year. Have the student groups perform their demonstrations for one another.

2. This activity will demonstrate how the sun's position in the sky changes with the seasons. On the school grounds, have student pairs find a shadow cast by something permanent, like a fence post or piece of playground equipment. Ask them to measure and record its length and position using a yardstick and compass. Have them predict what will happen if they record length and position at various times of the year. Have students measure the shadow each week at the same time of day and graph their results.

3. This activity will demonstrate the difference between the heat from direct and indirect sunlight. When the sun shines directly on something it heats it up more than if it shines on it at an angle. Students can prove this through an experiment that uses two black pieces of tin. Painted pie tins will work well. Set both tins in a piece of clay out in the sun. Place one lying flat so that the sun shines directly on it. Place the other standing vertically so that it receives very little sunshine. After 10 minutes, ask students to feel which is hotter. If possible, place a thermometer in the face of each tin and measure the difference in temperature.

Try This! Journaling and Discussion Questions

1. What causes shadows to get long and short? During what season will shadows be the longest? Why?

2. Why is there no "winter" or "summer" at the equator?

3. Some people think that we have summer because the Earth moves closer to the sun. How would you explain that this is not true?

4. How do the experiments with direct sunlight help to show why we have spring and summer? Why do you think snow melts more quickly on land that slopes to the south?

5. What would need to happen for the United States and Canada to experience
only summer? What would have to happen to give a "winter" to those living near the equator?

6. What other seasons are there besides winter, summer, spring, and fall? Where do they occur?

7. What would happen if the Earth's axis was not tilted?

National Science Education Standards

Science as Inquiry
Develop descriptions, explanations, predictions, and models using evidence. (5-8)

Think critically and logically to make relationship between evidence and explanations. (5-8)

Physical Science
An object's motion can be described by tracing and measuring its position over time. (K-4)

The sun is a major source of energy for changes on the earth's surface. (5-8)

Earth Science
The sun, moon, stars, clouds, birds, and airplanes all have properties, locations, and movements that can be observed and described. (K-4)

The sun provides light and heat necessary to maintain the temperature of the earth. (K-4)

Objects in the sky have patterns of movement. The sun, for example, appears to move across the sky in the same way every day, but its path changes slowly over the seasons. The moon moves across the sky on a daily basis much like the sun. The observable shape of the moon changes from day to day in a cycle that lasts about a month. (K-4)

The sun is the major source of energy for phenomena on the earth's surface, such as growth of plants, winds, ocean currents, and the water cycle. Seasons result from variations in the amount of sun's energy hitting the surface, due to the tilt of the earth's rotation on its axis and the length of the day. (5-8)