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Celestial Sphere: Sky Globe

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Author: Janice VanCleave

Stars look as if they are painted on a dome above Earth. To observers on Earth, this imaginary celestial sphere seems to move in a circle.

In this project, you will model the motions of Earth and the apparent motion of stars above Earth. You will also use the parts of a celestial sphere, such as its equator, lines of declination, and right ascension to determine the position of objects in the sky.

Getting Started

Purpose: To model the motion of the celestial sphere.

Materials

• lemon-size piece of modeling clay
• 12-inch (30-cm)-square piece of white poster board
• round toothpick
• paper clip
• narrow masking tape, 1/4 inch (0.63 cm) wide, or cut to that width
• 2-quart (2-L) transparent bowl
• 8 to 10 stick-on stars

Procedure

1. Shape the clay into a ball. Flatten one side until the ball looks like half a ball. This is your model Earth.
2. Place the model Earth on the poster board, flat side down. Stick the toothpick through the center of the clay from top to bottom. Stick the paper clip in one side.
3. Run a strip of tape around the outside rim of the bowl. Turn the bowl upside down over the model Earth. Position the model Earth under the center of the bowl.
4. Stick a star on the bowl at the spot where the paper clip points. Number this star 1.
5. Rotate the bowl clockwise about one-eighth of a turn, and repeat step 4, numbering the star 2.
6. Repeat steps 4 and 5, placing six to eight more stars on the bowl and numbering them in order (see Figure 22.1).

Results

The stars form a circle above the model Earth.

Why?

Astronomers use the celestial sphere to locate celestial bodies such as stars, suns, moons, and planets. Earth lies at the center of this hollow ball, and stars appear to circle across the sky.

A model of the celestial sphere is called a celestial globe. The bowl in this experiment serves as one-half of a celestial globe. The clay ball represents half of Earth. The toothpick represents Earth's axis, the paper clip an observer on Earth. The celestial equator is an imaginary circle that divides the celestial sphere in half, is perpendicular to the sphere's axis, and is in line with Earth's equator. In this investigation, the celestial equator is represented by tape around the rim of the bowl. The half of the celestial sphere above or north of the celestial equator is the northern hemisphere. The half of the celestial sphere below or south of the celestial equator is the southern hemisphere. (These regions of Earth are capitalized.)

To an observer on Earth (paper clip), the celestial sphere appears to rotate. The stars seem to move past the observer.

Try New Approaches

1. The celestial sphere is not a real object, nor does it rotate. Actually, it's Earth that does the rotating. If you could look down at Earth from above the North Pole, you would see Earth turn from west to east in a counterclockwise direction. Ask a helper to lift the bowl slightly while you rotate the model Earth, turning the paper counterclockwise. In what order do the stars move past the paper clip?
2. The imaginary axis of the celestial sphere aligns with Earth's axis. Thus, the north and south poles of the celestial sphere lie above the North and South Poles of Earth. Place a piece of masking tape at the north pole of your celestial globe and mark it with an X. Now place a stick-on star as near the X as you can, but not exactly on it. This represents Polaris, the star we cali the North Star, which is within 1° of the celestial north pole. Where would you place the tape if this were a model of the southern hemisphere? Science Fair Hint: Display a photo of your model. Label the different parts of your celestial globe.
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