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# Precession of Earth: Spinning on the Axis

By Pam Walker| Elaine Wood
John Wiley & Sons, Inc.

Just like a toy top that wobbles before it falls, the Earth wobbles as it rotates on its axis, the imaginary line that extends through the poles. This wobbling motion is due to the slight bulge at the equator. If you traced the Earth's axis out into space, you would see the extension of the axis slowly tracing a cone shape. The wobble is very slow; it takes the Earth 26,000 years to trace one complete conical shape. This wobbling motion of Earth on its axis is called precession. Because of precession, Polaris and Vega alternate as the North Star every 13,000 years. In this activity you will use a toy top to represent the Earth and observe how precession occurs as the Earth moves on its axis.

### Materials

Toy top
Flat table
Watch with second hand

### Activity

1. Spin the top on the table. Notice that when the top is moving quickly, its axis points straight up or perpendicular to the table. Watch the top continue to spin until you see it begin to slow down.
2. As the top slows, notice that the axis of the top is no longer perpendicular to the table. The weight of the top is beginning to pull it down. Notice that as the top continues to slow, it begins to wobble. If you look closely, you will see the upper axis of the top lean more to one side and then to the other side as the spin slows.
3. Observe the axis very carefully and you can see that it traces a cone shape as it goes around from one side to the other. Time how long it takes the axis to trace one complete cone before it falls.

### Follow-Up Questions

1. How long is the precession of the top? How long is the precession of Earth?
2. Explain why the star designated as the North Star changes over time.

1. Answers will vary; 26,000 years.
2. Due to precession, the northern axis of Earth does not always point to the same star. During part of precession, the axis points toward Polaris. During another part, it points toward Vega.

### Extension

Use a toothpick and a piece of clay to make a model of the Earth that can spin on its axis. Vary the shape of the clay to see how shape affects precession.

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