Projector
"Star" slide (See Heat Currents
activity.)
150 or 200 watt light bulb and
uncovered light fixture
Projection screen
Dark room
Procedure:
1. Place the light bulb and fixture near the projection screen.
2. Turn out the room lights and project the star slide on to the screen.
3. When everyone's eyes adjust to the dark light, turn on the light bulb.
4. Observe what happens to the star images on the screen.
Discussion
In this demonstration, the light bulb represents the Sun. Turning on the bulb causes many of the star images on the screen to disappear. Images toward the outside of the screen will probably still be visible. Note: You can also do this demonstration by simply turning on the room lights. Using a bright light bulb, however, more closely simulates what happens in the sky.
This demonstration shows that stars do not go away during the daylight. Instead, light from the Sun is scattered by the gas molecules, water, and dust particles in our atmosphere. This scattered light masks the far dimmer light of stars more distant than our Sun.
Day/night cycles greatly reduce the observing time available to astronomers employing optical telescopes based on Earth. Furthermore, on nights when the Moon is full or near full, the Moon's scattered light greatly interferes with the quality of pictures terrestrial telescopes can gather. You can simulate the Moon's effect on nighttime observation with a dimmer switch set to a low setting.
For Further Research:
- Are there any regions of the electromag-netic spectrum that are not affected by day/night cycles?
- Compare the number of stars visible on a clear moonless night with the number visible when the Moon is full. One way to do this would be to hold up a hoop, such as an embroidery hoop, at arms length in the direction of a familiar constellation. Count the stars visible in this location on a moonless night and on a night when the Moon is full.