Brighter: Why are Planets and the Moon so Bright in the Sky?

PROBLEM

Why are planets and the Moon so bright in the sky?

Materials

• transparent tape
• sheet of typing paper
• sheet of 9-by-12-inch (22.5-by-30 cm) white construction paper
• medium-size box with one side at least 9 by 12 inches (22.5 by 30 cm)
• yardstick (meterstick)
• 3 or 4 books
• flashlight

Procedure

1. Tape the typing paper to a wall so that its bottom edge rests on the floor. The paper represents the screen in a photometer (an instrument that measures the brightness of light) located at a point on Earth.
2. Tape the white construction paper to one side of the box. The paper represents the surface or atmosphere of a planet.
3. Stand the box 12 inches (30 cm) from the screen, with the paper-covered side of the box facing the screen.
4. Set the books near the wall to one side of the screen. Set the flashlight on top of the books so that its bulb is at an angle to the paper on the box and its light shines on the center of the construction paper.
5. Turn the flashlight on, then darken the room. Observe the brightness of the photometer screen.
6. Turn the flashlight off and again observe the brightness of the screen.

Results

The screen is bright only when the flashlight is on.

Why?

The Sun and other stars are luminous (giving off light). But the Moon and planets, even though they shine, are not luminous. These celestial bodies reflect (bounce back) light from the Sun to Earth, the same way the construction paper reflects the light from the flashlight to the screen. Without the Sun, the Moon and planets would not shine.

LET'S EXPLORE

1. Different planets and other celestial bodies, such as moons, reflect different of a reflecting material affect the amount of light reflected? Repeat the experiment, placing the white construction paper on the box as before. Observe the brightness of the light reflected on the screen. Then, tape a different color paper, such as brown, over half of the white paper on the box. First shine the light on the white paper, then on the brown paper, and compare the brightness of the light reflected on the screen by each paper. Repeat using various colors, such as red, green, and black. From the results, determine how the surface or atmospheric color of a celestial body affects its brightness in the sky.
2. How does distance from the Sun affect a celestial body's apparent brightness (how bright a celestial body appears to be as observed from Earth)? Repeat the original experiment, placing the box at different distances from the screen. Place it at these distances: 6 inches (15 cm), 12 inches (30 cm), and 18 inches (45 cm). Adjust the angle of the flashlight at each distance so that its light strikes the center of the construction paper on the box each time. Notice the screen's brightness at each distance. From the results, determine how distance from the Sun can affect a planet's apparent brightness. Can being too close to the Sun affect the visibility of a planet? For information, see chapter 15, "Blinding."

This article is brought to you by: