Modeling the Rotation of the Earth's Axis - Part 2

This is the second in a two-part lesson demonstrating the rotation of the Earth on its imaginary axis. At the end of Lesson 2, the students will understand:

• The earth wobbles as it rotates
• This movement occurs because the outer shell motion of the earth causes the liquid parts of the mantle and outer core to slush around - throwing the earth off balance
• The earth's axis moves in a circular path as the earth wobbles
• The earth's wobbling is slight, and takes many years for a noticeable change

By demonstrating the movement of the Earth's axis, students will understand how the composition of the earth affects its motion.

Materials - Marking pen, raw egg, hard-boiled egg

• Allow the boiled and raw eggs to stand at room temperature for about 20 minutes.
• Mark numbers on each egg: Boiled #1, Raw #2
• Have students come up to table to observe.
• Place both eggs on a table, and try to spin each egg on its side.
• Have students report what they observed. (The hard-boiled egg spins easily, but the raw egg wobbles.)
• Have students hypothesize about why the raw egg wobbled. (Hopefully, they will realize that the cooked egg has a solid content that spins with the shell. The liquid inside the raw egg does not start spinning with the movement of its shell. The outer shell motion does cause the liquid to move, but slowly. The sluggish movement of the liquid causes the egg to wobble. You may, however, have to suggest to the students that the material inside each shell affects the way it spins if they are unable to formulate a hypothesis on their own.)
• Ask the students what this might tell us about the earth. (It might suggest that they earth has liquid inside of it, and that is why it wobbles.)
• Draw a diagram of the earth's layers (crust, mantle, outer core, inner core) on the board.
• Explain to the students that, since parts of the earth's mantle and outer core are liquid (and not solid like the hard-boiled egg), the earth wobbles during its rotations. Unlike the egg's wobbling, the earth's wobbling is very slight and takes many years for a noticeable change.

Explain to the students that running through the raw egg is an imaginary axis - just like the one running through the earth at the poles. Using what they have learned about precession, encourage them to make predictions about the imaginary axis running through the egg. The axis would move around in a circle, because the liquid inside the egg is causing the egg to wobble - thus causing the axis to move.

For additional activities demonstrating the rotation of the Earth, refer back to lesson one in this series.