Shadows: How Can the Moon Cover the Sun?

Problem

How can the Moon cover the Sun?

Materials

• grape-size ball of modeling clay
• 2 sharpened pencils
• 3-inch (7.5-cm) Styrofoam ball

Procedure

1. Place the ball of clay on the point of one of the pencils and the Styrofoam ball on the other pencil's point.
2. Hold the pencil with the Styrofoam ball at arm's length in your left hand so that the ball is in front of your face.
3. Close one eye and hold the pencil with the clay ball in your right hand so that the ball is in front of but not touching your open eye. Slowly move the clay ball away from your face toward the Styrofoam ball. As you move the clay ball, observe how much of the Styrofoam ball is hidden by the clay ball at different distances.

Results

The closer the clay ball is to your face, the more it hides the Styrofoam ball.

Why?

The closer an object is to your eye, the bigger is its apparent size (the size an object at a distance appears to be). The small ball of clay can totally cover the larger Styrofoam ball, blocking it from view. In the same way, the Moon, with a diameter of 2,173 miles (3,476 km), can sometimes cover the much larger Sun, which has a diameter of 870,000 miles (1,392,000 km).

When the Moon passes directly between the Sun and Earth, and all three are in a straight line, the Moon eclipses (passes in front of and blocks the light of) the Sun. In this position, observers on Earth see a solar eclipse. The Sun is about 400 times larger than the Moon, but at times during the Moon's ellipse (oval) -shaped orbit, the Moon is about 400 times nearer Earth. It is in this position that the Moon and Sun appear to be the same size. In a solar eclipse in which the Moon appears as large as the Sun, the Moon completely covers the Sun. This event is called a total solar eclipse.

LET'S EXPLORE

When the Moon is far enough from Earth to appear smaller than the Sun, the Moon does not completely eclipse the Sun. An outer ring of the Sun's photosphere (bright visible surface of the Sun) is visible. This event is called an annular eclipse. Demonstrate an annular eclipse by repeating the experiment, slowly moving the clay ball away from your face until only a small outer ring of the Styrofoam ball is visible around the clay ball. Science Fair Hint: Make drawings to represent the Sun during the different types of solar eclipse.

show time!

1. Like all shadows, the Moon's shadow consists of a dark inner region called the umbra and a lighter outer region called the penumbra. Observe these parts of a shadow by positioning a desk lamp as high as possible above a sheet of paper on the desk. Hold your hand under the lamp about 1 inch (2.5 cm) above the paper. Observe the inner and outer parts of your hand's shadow.
2.  
   1. During a solar eclipse, only part of Earth is in the shadow of the Moon. Observers in the umbra see a total solar eclipse, while those in the penumbra see a partial solar eclipse, in which only part of the Sun is blocked by the Moon. Observers outside the Moon's shadow see no eclipse. Design a poster of a solar eclipse, similar to the one shown here, to show why a solar eclipse is not seen by all observers on the daytime side of Earth.
   2. During an annular eclipse, the umbra of the Moon's shadow does not reach Earth. Add a diagram to the poster designed in the previous experiment to represent an annular eclipse.
3. Never look at the Sun with your naked eye, even during an eclipse. One way to safely view an eclipse is by looking at a projected image of the Sun.

Check It Out!

Because Earth rotates, during a total solar eclipse the small umbra of the Moon's shadow sweeps across Earth's surface. Since the umbra is only about 167 miles (269 km) wide, a total eclipse of the Sun occurs rarely in anyone spot on Earth. Scientists can predict the location of future solar eclipses. For information about the dates and sites of future eclipses, see Philip S. Harrington's Eclipse! (New York: Wiley, 1997).

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Janice VanCleave's the Solar System
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