OBJECTIVES:

• Discussing what creates seasons.
• Analyzing evidence that the Earth rotates.

VOCABULARY:

• axis
• east
• north
• season
• south
• tilt
• west

MATERIALS:

• pictures of Earth from space
• chalk
• inflatable globe

Students use chalk on the playground to observe moving shadows.

BACKGROUND:

A season is one of the four periods of the year including spring, summer, autumn, and winter. Seasons are defined in two ways. Climatic seasons reflect changes in temperature, weather, and the length of daylight. The length of climatic seasons varies, for example at high northern latitudes winter is relatively long and summer is relatively short.

Summer in the Southern Hemisphere,  winter in the Northern Hemisphere

Astronomical seasons are defined by the position of the Sun with respect to the Earth. Because the Earth’s rotational axis is tilted 23.5⁰, the overhead position of the Sun changes throughout the year. The Sun appears to migrate more overhead in summer, and less so in winter. For the Northern hemisphere, the astronomical summer begins on the summer solstice, which is the day the Sun reaches its most northern, most overhead position. As the Sun moves back south, it comes directly over the equator. This marks the autumnal equinox, or the start of astronomical autumn. When the Sun reaches its southernmost point below the equator, it is the winter solstice, or the start of the winter season. Finally, as the Sun moves back north, it again crosses the equator. This is the vernal equinox, and the start of spring.

The climatic seasons correspond to the astronomical seasons because the Northern hemisphere is warmed when it is close to the summer solstice, because the Sun is more directly overhead. Likewise, the Northern Hemisphere grows cool as the Sun moves south, and the seasons transition through fall and into winter.

PROCEDURE:

1. Show the students pictures or slides of the Earth as seen from space. Point out the clouds, oceans, and land.
    
2. Show the inflatable globe to the class. The globe shows the oceans and land masses. Have them find the general area where they live.

   Point out that the globe is tilted, and explain that this is because the Earth is tilted on its axis relative to the Sun. Explain how this causes the seasons. Some students may notice that when the northern hemisphere tilts away from the Sun, the southern hemisphere tilts toward the Sun. Explain that our summer season is the winter season in the southern hemisphere.
    

3. In the following activity, the students will demonstrate that the Earth is rotating by watching changes in shadows cast by the Sun. Go outside to the playground, and have the students find the shadow cast by a pole. If many shadows are available, divide the class into groups. Have the students outline the shadow, or a portion of it, with chalk. Record the time. Come back in one or two hours. Have the students mark where the shadow is now located. Explain that the shadow has moved because the Earth has turned (rotated) while the Sun has stayed in the same place.

   Ask the students if they think shadows help us tell time. They may make the connection between the passage of time and the movement of the shadow, and answer yes, which is correct. Explain again that because the Earth is rotating, the shadows change position (this is how sundials work).
    

4. Ask the students where the Sun comes up every morning. They should have the sense that it always rises in the east. Ask them about sunset. Again, they may know that the Sun sinks in the same general area all the time. Explain that in reality, it is not the Sun that moves, but the Earth. As the Earth rotates toward the Sun, we experience sunrise, and as it rotates away, we experience sunset.

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