How the Sky-Stone Becomes Visible

The Sky-Stone Viewing Test

At the North Star Planetarium, a class built a small viewing box for a model of a sky-stone. A lamp stood on one side of the box. The sky-stone was a gray, painted clay ball in the middle. A narrow opening on the other side let an observer look toward the ball. The class changed one part of the setup at a time and recorded whether the observer could see the ball.

The class could not see light traveling through the box. They used arrows in a model to show a path. First, light traveled from the lamp to the sky-stone. Then light bounced off the sky-stone and traveled into the observer's eye. The sky-stone did not make its own light. The observer needed a complete path from the lamp to the ball and from the ball to the eye.

The class kept the lamp, sky-stone, box, and observer in the same places. In Test A, the lamp was on, the viewing opening was clear, and the ball was visible. In Test B, the lamp was off, and the ball was not visible. In Test C, the lamp was on, but a dark card covered the space between the lamp and the ball. In Test D, the lamp was on and light reached the ball, but a dark card covered the space between the ball and the observer's eye. In Test E, a clear sheet was placed between the ball and the eye instead of the dark card.

The class recorded these results: Test A—lamp on, no card, ball visible. Test B—lamp off, no card, ball not visible. Test C—lamp on, card between lamp and ball, ball not visible. Test D—lamp on, card between ball and eye, ball not visible. Test E—lamp on, clear sheet between ball and eye, ball visible. The teacher reminded the class that a model should show the direction of the light, not just draw lines between the parts.