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# Heliocentric: The Movement of the Earth Around the Sun

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Author: Janice VanCleave

Our solar system is heliocentric, which means there is a sun in the center and the celestial bodies of the solar system move around the sun. The Earth is one of the nine planets in our solar system.

In this project, you will study the relative position of the Sun in the sky as seen by an observer on Earth. You will map the apparent path of the Sun during the day and at different times of the year. You will also examine the history of the theories of the movement of the celestial bodies in our solar system.

### Getting Started

Purpose: To demonstrate the apparent movement of the Sun across the sky using a heliocentric model.

### Materials

• Sheet of typing paper
• Pencil
• 2-quart (2-liter) clear glass bowl
• Gooseneck table lamp
• 5-6 3/4-inch (1.9-cm) round yellow labels

### Procedure

1. Place the paper on a table and mark an X in the center of the paper.
2. Turn the bowl upside down on the paper with the X in the center ofthe bowl.
3. Position the lamp so that the light bulb is about 12 inches (30 cm) above the tabletop and about 12 inches (30 cm) from the rim of the bowl.
4. On the lamp side of the bowl, hold one of the labels near, but not touching, the bowl.
5. Move the label until its shadow falls on the X mark, then stick it to the bowl.
6. Rotate the bowl slightly in a counterclockwise direction.
7. Repeat steps 4 and 5 twice, using the remaining labels (see Figure 4.1).

### Results

The labels are placed in a line around the bowl with their shadows falling on the X.

### Why?

In this heliocentric (sun-centered) model, the lightbulb represents the Sun. The labels represent the changing apparent position of the Sun in the sky during the day as seen from the Earth. In this experiment, to an observer on Earth, represented by the X, the Sun appears to move around the sky at the same altitude. This view of the Sun would be made at the Earth's Poles during the summer. At this time, the Sun is above the horizon all day and moves in a very tight spiral as it rises to a maximum altitude of 231/2° on or about December 21, then slowly starts its descent. (At the Poles, the Sun is above the horizon for half the year and below the horizon for the other half.) Actually, the Sun is not moving. Instead, the Earth is rotating on its axis, giving the illusion that the Sun is moving across the sky. Since the axis of the Earth is tilted in relationship to the Sun, the maximum height of the Sun in the sky during the day changes as the Earth revolves (moves in a curved path about an object) around the Sun. During part of the trip around the Sun, the north axis (North Pole) is tilted toward the Sun, and during part of the trip, the south axis (South Pole) is tilted toward the Sun.

The Earth or any celestial body that revolves around another celestial body is called a satellite. The curved path that a satellite traces is called an orbit.

### Try New Approaches

At latitudes between the polar regions, 661/2° N and 661/2° S, the Sun rises and sets each day. The height of the Sun depends on whether the Earth's axis is tilted toward or away from the Sun. Repeat the experiment twice. First, represent the Earth's axis tilted away from the Sun by raising the bowl on the side facing the lamp. Keep the bowl raised by placing a book under the bowl on that side. Repeat again, raising the bowl on the side away from the lamp to represent the tilt of the axis toward the Sun.

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