- Ruler, Pencil, or even a finger.
- Pick an object which is some distance in front of you. Twenty to thirty feet is a good distance.
- Hold the ruler/pencil/finger pointing up at arms distance in front of you, as you face the distant object you chose.
- Close your left eye and note where your ruler/pencil/finger stands in front of the distant object.
- Without moving your body, head, or hand, open your right eye and close your left eye.
- See how the ruler/pencil/finger has apparently changed positions. This is called Parallax.
Astronomers use this to determine the distances to stars. They note a star's apparent position when the Earth is on one side of the sun, then measure the distance to its apparent position six months later, when the earth is on the other side of the sun. The more distance between the two points, the closer the star is to Earth.
