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# Twirlers: How Does the Period of Rotation of Different Parts of Earth Compare?

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Source:
Author: Janice VanCleave

### PROBLEM

How does the period of rotation of different parts of Earth compare?

### Materials

• one-hole paper punch
• 2 sheets of different colored paper
• pen
• paper plate
• school glue
• sharpened pencil

### Procedure

1. Use the paper punch to cut 6 circles from each color of paper.
2. Use the pen to mark a dot in the center of the plate. The plate represents Earth.
3. Glue the paper circles of one color around the edge of the plate and glue the other circles near the center of the plate. The paper circles represent different areas of Earth.
4. When the glue dries, ask an adult to insert the point of the pencil through the center of the plate so that the point exits through the dot.
5. Hold the pencil so that the side of the plate with the paper circles faces up. Rotate the pencil so that the plate turns one full turn in a counterclockwise direction. Compare the movement of the paper circles on the edge of the plate with the movement of those in the center.

### Results

The plate and all the paper circles move together, rotating in the same amount of time.

### Why?

The paper plate, like the terrestrial planets (Earth-like planets that are small, dense, rocky worlds)—Mercury, Venus, Earth, and Mars—is solid, so all of its parts move together when it rotates. Therefore, the period of rotation (the time it takes for an object to make one complete rotation on its axis) is the same for all parts of Earth.

### LET'S EXPLORE

Some celestial bodies, such as the Sun, Pluto, and the Jovian planets Gupiter-like planets that are large, gaseous, low-density worlds)—Jupiter, Saturn, Uranus, and Neptune—are mostly spheres of gas. (Note that Pluto is gaseous but is not large like the Jovian planets, so it doesn't fit in either the terrestrial or Jovian category.) On Pluto and the Jovian planets, the period of rotation at the equator is different from that at the poles or points between. Demonstrate how different parts of a fluid can have different periods of rotation by replacing the paper plate with a small bowl of water. Drop one color of paper circles on the surface of the water around the bowl's edge and drop the other color of paper circles in the center of the bowl. Use your finger to stir the water in a counterclockwise direction. Compare the movement of the paper circles in the center with the movement of those on the outside. How do the periods of rotation of the two groups of paper circles compare?

### SHOW TIME!

1. How do the periods of rotation of the other planets compare to that of Earth? Prepare a table similar to the one shown here, with the period of rotation of each of the nine planets in hours and in Earth days. (Use the periods of rotation of the planets in the appendix.) Since the period of rotation is given in hours, divide this by 24 to calculate Earth days. For example, Mercury's period of rotation is 1,407.5 hours. In Earth days, this would be 1,407.5 hours ÷ 24 hours = 58.6 Earth days.
2. How does the direction of rotation of the planets compare? All the planets and the Sun rotate in the same west-to-east direction as Earth, except for Venus and Uranus. Venus rotates in an east-to-west direction, and Uranus rotates east to west on its side. Model the direction of the rotation of Earth, Venus, and Uranus by following these steps to make three clay models:
• Shape a walnut-size piece of modeling clay into a ball.
• Insert a pencil representing the axis through the middle of the clay ball.
• Repeat these steps twice to make two more models.
• Use a pencil to label the three models E, V, and U.
3. Lay 2 index cards on a table. With the aid of two helpers, stand the models of Earth (E) and Venus (V) vertically on the cards. Tilt the Earth model (E) about 23° as shown. Hold the model for Uranus (U) sideways so that the eraser end (the north pole) is pointing toward the other models. As seen from the north pole of each model, turn Earth counterclockwise and Venus and Uranus clockwise. Use a photograph and/or diagrams of the models as part of your display to show the direction of rotation of these three planets.

### CHECK IT OUT!

French physicist Jean-Bernard-Léon Foucault (1819-1868) used a pendulum made from a 223-foot (67-m) wire with a suspended sphere weighing 62 pounds (28 kg) to demonstrate that Earth rotates. Find out more about Foucault's pendulum. Where did Foucault perform his experiment? For information, see pages 25–30 in Janice VanCleave's A+ Earth Science (New York: Wiley, 1999).