How Far the Stars?

How do we know how far away the stars are? We cannot lay rulers down or drive through space. The most direct way we can use is the method of triangulation.

This is the basis for our binocular vision. When we look at something, our two eyes donıt both see exactly the same thing: because theyıre about two inches apart, nearby objects seem to be slightly displaced against the background. If you hold your finger in front of your face and close first one eye, then the other, the finger seems to shift compared with more distant objects.

Now letıs use the same idea to measure the distance to the stars. Instead of simply using our two eyes -- two inches apart -- weıll use the Earth at the opposite ends of its orbit -- places two hundred million miles apart. Nearby stars appear to shift when compared with the background of the more distant stars. Theres one problem, though. The shift is ever so slight. Even the nearest stars shift less than one second, or one thirty-six hundredth of a degree. And, as you can check with your fingertip again, as you move the finger away from you, youll see that the further away your finger is, the less it shifts. Stars are like that, too.

Early astronomers could not measure such small angles, and used this fact to show the Earth did not revolve around the Sun, but was, in fact, stationary.

But now we know the Earth really does revolve around the Sun, and can measure these angles to very small part of a second. We can determine the distance to almost every star we see in the nighttime sky.

For the more distant stars we have to use other, less direct methods. But this is how we start. And its a very important start.

(02/21/07)