In this traditional investigation of the causes of Earth's seasons, children model Earth's movement around our Sun using Styrofoam balls and flashlights and make observations about the changing distribution of light through our year.
Begin the activity by asking why we have seasons (information can be found in “About Seasons”). Note correct and incorrect responses to revisit at the close of the activity.
- Do all places on Earth experience the same season at the same time?
- Does Earth's distance from our Sun affect seasons?
- Why or why not?
Provide the children with the materials and invite them to explore the causes of Earth's seasons. The children may not be familiar with Earth's axial tilt. The axis is the imaginary line going through Earth, around which Earth spins. Refer to the globe as needed.
- What does the flashlight represent? (Our Sun)
- The Styrofoam ball? (Our Earth)
- What will the toothpicks be? (Earth's axes)
- Do the axes of our Earth point directly up and down?
- What does the globe show us? (Our Earth is tilted approximately 23.5 degrees. The tilt is measured relative to the orbital plane, the plane defined by Earth's orbit around the Sun)
Begin the experiment by having the groups use the materials to revisit what causes day and night. Have the children note their observations on a sheet of poster paper.
- What are the motions of Earth and our Sun that create the 24-hour day/night pattern we know on Earth? (Earth spinning, or rotating on its axis)
Invite the children to experiment with their Styrofoam Earth and flashlight Sun to “create” the seasons in a way that explains what they know about Earth's seasons. One child will hold the Sun and another will move Earth. The other children in the group will evaluate the results and suggest model changes.
- How long does it take for Earth to go through a cycle of seasons or to go from one winter to the next? (One year)
- What changes as we go from summer to winter? (Day length decreases, temperatures get colder)
- Does winter in the northern hemisphere occur at the same time as winter in the southern hemisphere? (No, seasons are “opposite” in the northern and southern hemispheres; note that children can arrive at this conclusion in the activity)
- What motions of our Sun and Earth can the children propose that will explain these observations about the seasons on Earth?
Are the children remembering to keep the Earth axis tilted?
- Does the direction of tilt change? (No, the axis always points in the same direction — in the northern hemisphere, it currently points toward Polaris, the North Star)
Draw the children's attention to the north polar region, where seasonal and day/night differences through the year are extreme.
- Do the children's models show the northern hemisphere getting sunlight “full time” during the northern hemisphere winter?
They may be familiar with the fact that the north pole experiences months of darkness in the winter and months of daylight in the summer. Simply spinning on our axis does not explain months of darkness or light at the poles. The long polar night and day cycles also can help the children fix the direction of the axis so that it points consistently in a single direction for the movement of Earth around the Sun.
Once the polar seasons are established, assist the children in noting other changes over the year.
- What is the relationship between our Sun and Earth in the northern hemisphere summer?
- Do the children's models explain the “opposite” seasons in the northern and southern hemispheres?
- What do the children observe about the direction of incoming “solar rays” in the winter and summer (They are in the same direction as the Earth's axis; the incoming rays are parallel to the plane in which Earth's axis lies)
- Where are the Sun's rays striking most directly in the northern hemisphere summer? (In the northern hemisphere)
- In the northern hemisphere winter? (In the southern hemisphere, during the southern hemisphere summer)
Invite the children to model the slightly elliptical orbit. Ask them to describe what is happening to the seasons at the south and north polar regions.
- Does distance from our Sun influence Earth's seasons? [No. Earth's orbit is not a perfect circle, it is an ellipse, and our Sun is not exactly at the center of the ellipse. This means that sometimes Earth is farther from our Sun than at other times. Earth is closest to our Sun in January (147.5 million kilometers at perihelion) and the farthest away in July (152.5 million kilometers at aphelion). However, the difference in distances is so small, that it makes little difference in the winter and summer temperatures]
Share with the children that on Earth's orbital path around our Sun, we are farthest from the Sun in July and closest in December, or just the opposite of what we might expect. Revisit the common misconception that distance from our Sun causes Earth's seasons, to make certain the children understand why this is not the case.
- Does distance alone — without axial tilt — explain the seasons? (NO! Without tilt, there are no seasons, and the northern and southern hemispheres would experience the same conditions at the same time.)
If distance was the main factor then the northern hemisphere summer would occur when Earth is closest to our Sun (the opposite is true) and the northern and southern hemisphere winters and summers would not occur at different times.
Invite the groups to demonstrate their models of the seasons and assess that each child understands that the annual pattern of seasons on Earth is caused by Earth's annual revolution around our Sun with its axis pointing to a fixed position in space.
Explore equinoxes and solstices.
Invite the children to model the solstices and equinoxes to check for comprehension about the seasons. Challenge them to illustrate the seasons for the southern hemisphere.
- When does the longest day of the year occur in the northern hemisphere? (In the summer)
- What do we call it? (Summer solstice, around June 21)
- What is the relationship between our Sun and Earth that makes this happen? (Our northern axis is tilted directly toward the Sun on that day of Earth's journey around the Sun)
- When does winter solstice occur in the northern hemisphere and what happens? (Around December 21 or 22 the northern hemisphere experiences the shortest day)
- Where is our northern hemisphere axis pointing? (Directly away from our Sun)
- The equinoxes are times of “equal night” (March 21 and 22 for northern hemisphere spring and September 22 and 23 for fall).
- Can the children show where equinoxes occur in their model?
- What happens with our Sun's rays in the spring and fall? (Incoming rays are perpendicular to the plane in which Earth's axis lies; all locations on Earth experience equal duration of daylight)
January 4, 2007
For each group of children:
• 1 flashlight
• 1 Styrofoam ball
• 1 colored marker
• 2 toothpicks
• Globe of Earth
Connections to the National Science Standard(s)
Standards A&D (grades K–4): Identify questions about seasonal changes using their own observations. Conduct an experiment using a model to answer questions about patterns of movement of objects in the sky. Understand why the sun, for example, appears to change its path over the seasons. Identify weather changes over the seasons. Critique and communicate explanations.
Standards A&D (grades 5–8): Identify questions about seasonal changes using their own observations. Conduct an investigation using a model and evidence to explain and predict observations about how objects in the solar system have regular and predictable motion that explains such phenomena as seasons. Understand that seasons result from variations in the amount of the sun's energy hitting the surface, due to the tilt of the earth's rotation on its axis and the length of the day. Critique and communicate explanations.