Lesson 1: There Is A Season

Spring, summer, fall, and winter. Each year we go through the changes of seasons. Cold winter weather transforms into a mild spring, that progresses into a warm summer, that finally cools off into fall. Why do we go through these changes? It all has to do with the angle that the sun's rays hit the earth's surface.

Think of the earth as a spinning top. Instead of spinning straight up and down though, it is tilted to one side. Because the earth is tilted to one side, the angle of the sun's rays changes as the earth travels around the sun. For example, during the winter, the Northern Hemisphere is tilted away from the sun, so the sun is not very high in the sky. In the summer, the Earth is tilted toward the sun, and the sun is higher in the sky. The higher in the sky the sun is, the more direct the sun's rays hit the earth. This makes the earth heat up more.

Watching the sunrise throughout the year, you will notice that the sun does not always rise in the same place. This is also related to the earth's tilt. In the winter, the sun will rise in the southeast. As it gets closer to spring the sunrise will move to the north. On the first day of spring (The Spring or Vernal Equinox), the sun will rise directly in the east. During the spring, the sunrise will continue to move north, and will be rising in the northeast by the first day of summer. Then, it will start to move south again, and will be rising directly in the east on the first day of fall (The Fall or Autumnal Equinox). The southward motion will continue until the first day of winter, and the cycle will start over.

Activity One – I See the Light

Materials: Globe, flashlight or lamp

Setting Up the Activity: Have someone hold the globe about five feet from the light source, with the north pole tilted away from the light source. If using a flashlight, you may need to change this distance to get good coverage. Spin the earth and notice how the light hits the globe.

Keeping the light source in the same location, have the earth "orbit" the sun in a circle. After travelling halfway around the circle, spin the earth (the North Pole should now be tilted toward the light source) and see how the light hits the globe. Note areas near the poles.

Questions:

Q. How does the position of the earth change the way the light hits the globe?

A. When the Northern Hemisphere is tilted toward the light, the light is hitting the surface more directly. This would coincide with our summer. When the Northern Hemisphere is tilted away from the sun, the light hits the surface less directly. This coincides with our winter.

Q. What happens at the North Pole at these positions?

A. During our summer, the North Pole is receiving light all the time. In winter, the North Pole is receiving no light.

Q. Is there any time when both north and south poles receive light?

Yes, at the equinoxes. One pole is at "sunset" and the other is at "sunrise."

Activity Two – Grab Some Rays

Materials: Black marker, pencil, clear jar at least six inches tall, white poster board, glue, used matchstick, thread, aluminum foil, scotch tape

Setting Up the Activity: This activity will make a radiometer, a simple device that gives a measure of solar energy. Cut the poster board so you have four pieces of 1/2 inch by 1-inch rectangles. Color one side of the poster board with the black marker. Glue aluminum foil to the white side of the poster board pieces. Next, glue one edge of the poster board to a side of the matchstick. Do this with each piece of poster board, making sure the foil sides do not face each other. It should look like a paddlewheel when you're done, with the paddle at one end of the matchstick. With a five-inch length of thread, glue one end of the thread to the top of the matchstick (end away from the poster board pieces). Tape the other end of the thread to the middle of the pencil. Suspend the paddlewheel in the jar. Place the jar in a sunny location.

Questions:

Q. What happens when the radiometer is placed in a sunny location?

A. It starts spinning.

Q. Why does this happen?

A. The black side of the paddles absorbs energy, while the foil side reflects energy.

Q. What happens when the sun's rays are not hitting the radiometer?

A. It stops spinning.

Q. Are there changes in the way the radiometer works throughout the day?

A. As the sun gets higher in the sky on a clear day, the radiometer should spin faster.