How Far is That Planet?

This experiment comes from my new "How Do You Know That?" show. How do we know how far away the planets are? Did someone take a tape measure and stretch it from the Earth to each planet? Did someone get in their car and drive there, to check the mileage? No, but we still know how far away they are.

To see how this is done, we will need:

  • a thumb
  • 2 eyes
  • something to look at

Look at something that is far away. It might be a tree or a building. Be sure it is something that is sitting still, not a car going down the street. Now, close one eye or cover it up with your hand. Stretch your hand out in front of you and stick up your thumb. Move your hand until your thumb covers up the distant object. Now, change eyes. Open the closed eye and close the open one. Look at your thumb and the distant object. It has moved!!!! Did that tree really move? No. Did your thumb move? Not unless you moved it. What changed was the path you were looking along, because your eyes are an inch or two apart.

Holding your thumb in front of the object again, (good exercise isn't it), switch back and forth with your eyes. Right eye, left eye, right eye, left eye, etc. It will seem that your thumb is jumping back and forth. Notice how far it jumps. Now move your thumb close to your nose. Start switching your eyes again. Right eye, left eye, right eye, left eye. How far does your thumb seem to jump now? Much farther. The closer an object is to your eyes, the more it jumps. The farther away it is, the less it jumps. This can be used to tell how far away something is.

But what if something is VERY far away? Then it would move so little, we would not be able to tell. For that we need to move our eyes farther apart. How can we do that? Wouldn't that hurt? No, instead of taking out our eyes and moving them apart (ouch!), we use one eye from two different people. One person looks at the planet in one city, and another person in another city far away looks at the same planet, at exactly the same time. By comparing the planet’s position from the two spots, we have moved our eyes very far apart.

This is fine for measuring the distances to planets in our solar system, but stars are so far away that we have to move our eyes much farther apart. How can we do that? Imagine the Earth going around the sun. During summer, we are on one side of the sun, and in winter we are on the other side. If you compare the position of a star in summer and winter, that moves your eyes a couple of hundred million miles apart. And even that is not far enough for distant stars. If the star is over 200 light years, then we have to use other methods to estimate its distance.