Humans’ binocular vision is what allows us to judge distance, meaning that Odysseus’ battle with the Cyclops should have been a much easier victory for the Greek hero. Parallax, one of the useful effects of binocular vision—and a fun word to say—is critically important for scientists determining distances of celestial objects like stars and planets. To give your students a better understanding of parallax, and to give you an excuse to say it, we present this handy dandy Demo Science science demo.
Parallax, Parallax, Parallax!
This is a great student-participation experiment. Make sure each of the little buggers in your classroom has a pencil or pen at their disposal, and provide them with a larger, “fixed” object like a lamp or a globe set on your desk or something like that. That’s all the supplies you need—the rest is up to your smelly students.
Have the kids hold their pencils at about half-arm’s length, right in front of their eyes, with your background object directly behind the pencil. Ask them to close their left eye and note what they see; then, have them close their right eye and open the left, again noting what they see. Have them repeat the eye-switch four or five thousand times, modulating the distance the pencil is from their eyeballs, so they really get a good look at the parallax effect.
If they’re not screwing around (lookin’ at you, Kevin!) and actually followed your instructions, your students will notice that their pencils appear to jump back and forth when they switch viewing eyes, while the background object stays in the essentially same location. This, friendo, is parallax.
An Optical Illusion (Kind Of)
Of course, the pencils didn’t move; instead, the angle from which your students viewed their writing utensils did. The closer an object is to a viewer, the more pronounced the parallax effect will be. Hence, the up-close pencil seems to be moving, while the more distant lamp (or whatever) stays still.
Similarly, parallax makes closer stars in the sky appear to move, while those farther away appear essentially fixed in place. To calculate a star’s distance from Earth, astronomers measure its “different positions” multiple times throughout the year, as our planet orbits the sun. By measuring multiple times and considering the effect of parallax, scientists can better pinpoint the location of a star.