Aristotle thought the stars were all exactly the same distance away, stuck to a dark shell that surrounded the Earth like the dome of a planetarium. Now we know that the farthest galaxy is over a billion times more distant than the nearest star. How do astronomers tell how far away the stars are?

One method works like the way your own eyes determine how far away earthbound objects are. Hold a finger upright at arm's length, then close an eye as if you were winking. Open that eye and close the other. As you alternate eyes, your finger will seem to change position.

Keep winking back and forth, then move your finger closer to your face. The closer your finger is, the more it appears to jump compared to the background.

This is the key to one method of measuring how distant the stars are. Closer stars seem to shift more than distant ones do when seen from different places. The farther apart your two observation points are, the more dramatic this shifting becomes. Your eyes are only inches apart, so no matter how much you wink at the stars, you won't be able to shift them with your eyes alone. Astronomers, however, can use telescopes in different parts of the world, thousands of miles apart.

Even better, they can photograph a star once, then again six months later. During six months, the Earth will have orbited to the other side of the sun, creating a pair of telescopic eyes that are almost two hundred million miles apart. With this distance, the shift becomes quite noticeable. This method has been used to map the location of thousands of near-by stars.