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Earth Shadows: How Does the Moon Move Through Earth's Shadow?

based on 7 ratings
Author: Janice VanCleave

PROBLEM

How does the Moon move through Earth's shadow?

Materials

  • 2 sharpened pencils
  • 3-inch (7.5-cm) Styrofoam ball
  • modeling clay
  • ruler
  • flashlight
  • 3 or 4 books
  • drawing compass
  • 3-inch (7.5-cm) square of white poster paper
  • scissors

Procedure

  1. Insert about 1 inch (2.5 cm) of the pointed end of one pencil in the Styrofoam ball. Use the clay to stand the pencil on the floor about 12 inches (30 cm) in front of a wall. The pencil should be vertical with the ball on top.
  2. Place the flashlight about 18 inches (45 cm) from the ball. Use the books to raise the flashlight so that its light is directed toward the ball.
  3. Use the compass and ruler to draw a 2-inch (5-cm)-diameter circle on the poster board.
  4. Cut out the paper circle and tape it to the pointed end of the other pencil.
  5. Hold the pencil so that the paper circle is about 2 inches (5 cm) in front of the wall and to the left of the ball's shadow on the wall.
  6. Slowly move the paper circle to the left so that it passes completely through the ball's shadow.
  7. Observe the shape of the ball's shadow on the paper circle as the paper circle moves through the shadow.

Earth Shadows

Results

First, only part of the paper circle is covered by the ball's shadow. Then all of the circle is covered. Eventually, the circle moves completely out of the shadow.

Why?

In this model, the flashlight (the Sun), the ball (Earth), and the paper circle (the Moon) are in line with each other. When this happens, the shadow of the ball falls on the paper circle, much like Earth's shadow falls on the Moon in an event called a lunar eclipse (the movement of the Moon into Earth's shadow). Like the model, when the Moon, Earth, and the Sun are in a straight line with Earth in the middle, the entire Moon moves into the shadow of Earth. This event is called a total lunar eclipse.

LET'S EXPLORE

The Moon's orbit in relation to Earth's is tilted about 5°. Thus, during the Moon's monthly revolution around Earth, it does not usually pass into Earth's shadow at full moon. Because of this, a lunar eclipse does not occur every month. In some eclipses, only part of the Moon passes into Earth's shadow, thus producing a partial lunar eclipse. Show the movement of the Moon without an eclipse by moving the paper circle so that it is above or below the shadow of the ball. Then repeat the experiment, moving the paper circle so that only about half of it moves through the ball's shadow as in a partial lunar eclipse. Science Fair Hint: Make sequence drawings of the paper circle as it moves to represent the results of each experiment. Label the different events Total Eclipse, No Eclipse, and Partial Eclipse.

SHOW TIME!

Demonstrate how particles in dishwashing liquid can scatter some colors of light but allow others to pass through. Fill a 10-ounce (300-ml) clear plastic glass with water and place the glass on a table about 6 inches (15 cm) in front of a white paper screen taped to a wall. Place a flashlight about 6 inches (15 cm) from the glass on the side opposite the screen. Use 2 or more books to raise the flashlight so that the light shines through the middle of the water and onto the screen. Add 1 teaspoon (5 ml) of milky-colored dishwashing liquid to the water and stir. Darken the room and observe the color of the light on the screen. Add 1 teaspoon (5 ml) of dishwashing liquid three more times, noting the color of the light after each addition. For more information about the red color of the Moon during a total lunar eclipse, see page 45 in Heather Couper and Nigel Henbest's How the Universe Works (pleasantville, NY: Reader's Digest Association, 1994).

Earth Shadows

  1. During a total lunar eclipse, the Moon can take on a reddish tint. This is caused by red light refracted (bent) by Earth's atmosphere (the blanket of gas surrounding a celestial body). White light, which is visible light such as that from the Sun or a flashlight, is made up of all the different colors of the spectrum (a band of colored lights produced when visible light is separated): red, orange, yellow, green, blue, indigo, and violet. The Moon's reddish color is produced by dust particles in Earth's atmosphere, which scatters the rays of most colors of visible light, but allows red light to pass through.
  2. Lunar eclipses only occur when the Moon is full. Design a poster of a total lunar eclipse, similar to the one here, to show why a total lunar eclipse is seen by all observers on the nighttime side of Earth.

CHECK IT OUT!

During a total lunar eclipse, how long does it take for the Moon to be totally covered by Earth's shadow? What is the general time required for a complete lunar eclipse? For information, see pages 108-109 in Jay M. Pasachoff's Peterson First Guides: Astronomy (Boston: Houghton Mifflin, 1988).

Earth Shadows

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