Demonstration of How the Earth Moves on its Axis: A Lesson of the Earth's Rotation
Content Knowledge Objectives:
Students often have a difficult time visualizing how the axis influences the Earth rotation. At the end of Part I of the lesson, students will understand:
- The earth axis wobble occurs as it rotates
- The earth's wobbling is very slight, and takes many years for a noticeable change
- The earth's axis moves in a circular path as the earth wobbles. This movement is called precession.
- It takes 26,000 years for Earth to wobble enough for the axis to make one complete turn
Students will demonstration this knowledge by completing a demonstration of Earth rotation patterns.
Students will reject the idea that the earth's axis remains stationary and fixed.
1. What Do We Already Know About the Earth and Its Axis?
- Ask the class to brainstorm. What do they know about the Earth and its axis?
- List comments on the board.
Look for the following ideas to surface:
- The Earth's axis is an imaginary line. (If students do not mention this one, or seem confused about it, you may wish to show them an aerial photograph of the planet earth. Then discuss why you cannot see the axis in the picture.)
- Our planet spins like a top while it circles around the sun.
- Earth's rotation pattern around its axis is in a counterclockwise motion.
- It takes 24 hours for the earth to rotate completely around the axis.
- The Earth stays level as it rotates.
- The Earth is always tilted in the same way.
2. What Do We Know About the Axis Itself? Does It Move, or Does It Remain Stationary?
- Allow students to formulate a hypothesis (or several)
- Write their hypothesis on the board
3. Model Building
Materials: Modeling Clay, Toothpicks
- As a demonstration of Earth rotation patterns, ask students to use the materials given to make a model of the Earth on its axis by shaping a piece of clay into a ball about the size of a marble and inserting the toothpick through the center of the clay so that just the tip of the toothpick sticks out on one side.
4. Testing Our Hypothesis
- Remind students of their hypothesis.
- Allow students to make predictions about what will happen to the toothpick when you spin the model. (You may wish to write these predictions on the board as well.)
- Have students "rotate" their model by spinning it like a top.
- Have students observe the way the top of the toothpick moves.
(Note: The ball will spin poorly if the toothpick is not through the center, or if the clay is not round.)
- Ask students to report what they observed. If they are unsure, have them spin the model a few more times.
- Write their observations on the board.
- Discuss - were the predictions right or wrong? Do they need to modify their hypothesis about the movement of the earth's axis?
- Explain to the students that in fact the earth's axis does move in a circular path. This movement is called precession. The top of the toothpick makes many revolutions as the clay ball spins, but it takes 26,000 years for the axis to make a complete turn.
5. Why Does the Axis Move?
- First, look at the model.
- Have the students spin the model again - but this time, ask them to watch the clay ball.
- Ask the students to report what they observed. (Hopefully, they will notice that the clay ball wobbles when it spins. If not, you may need to directly ask, "What is the clay ball doing? Is it wobbling?")
- Ask the students what they think this might tell us about the earth. Might it wobble as well?
- Now ask students why they think the clay ball wobbles.
- Allow them to make hypothesis. (Hopefully, they will realize the ball wobbles because of its composition - that there is a shifting of weight because it is not perfectly round. If not, you may have to suggest it.)
- Pose the following question to the students - "What about the Earth's composition? Could that make the earth axis wobble?" Do not take responses at this time.
In Part II of this lesson, we will learn more about the axis of the Earth, and how Earth's composition affects its rotation.