The Shadows of the Moon

Model a total solar eclipse using the real sun!

Materials

• a barbeque skewer


• a round object, approximately 1" (2.5 cm) in diameter, that you can stick the skewer into (e.g., a Styrofoam ball; a small potato)


• a round, light-colored object with a smooth surface, about 4" (10 cm) in diameter (e.g., a rubber ball )


• a sunny day


• a friend

To Do and Notice

1. Find a sunny outdoor spot. Stick the skewer into the small object to make a handle. This will represent the moon.


2. One person should hold the larger object, which represents the earth, near the ground. The other person should pace off ten feet (3 m) from the model earth in the direction of the sun, and then position the model moon directly in the path of the sunlight shining on the model earth. You should see a small dark shadow on the model earth surrounded by a larger, lighter, somewhat fuzzy shadow.


3. Slowly move the moon so that the shadow travels across the model earth. The moon's shadow moves across the earth from west to east.


4. Slowly move your model moon farther away from the model earth. Notice that at some point the dark inner shadow disappears.

What's Going On?

The ratio of the large object to the small object and the distance between them is a fairly accurate model of the earth-moon system.

Anyone on earth within the path of the small dark shadow, called the umbra, will experience a total solar eclipse. The path of totality is quite narrow, however, often something like a hundred miles (160 km) across, which explains why most eclipse chasers have to travel significant distances. Within the lighter, larger shadow, called the penumbra, the sun is only partially blocked, and anyone within the penumbral path will see a partial eclipse.