parallax

Hold your thumb at the tip of your nose. Look at your thumb with first your right eye and then your left. Your thumb appears to move because your eyes are not at exactly the same place, so each eye views the thumb from a different angle. Now hold your thumb at arm's length and repeat the experiment. Your thumb will still appear to shift, but will not appear to move as much as it did when it was closer. This phenomena is parallax. Parallax is simply the apparent change in the position of an object due to a change in the location of the observer, or in the above, change in position of the eye you are moving. Notice the things further away do not appear to move. Our brain uses this information to figure out how far away things are. Try this, Place a penny on a table, about an arm's lenth away. Then with both eyes open, touch the penny with one finger. Close one eye and try to touch the penny, you may miss by a little with one eye closed.

In order to measure the parallax of stars which are very far away, we must use the largest baseline possible. (The baseline is the distance between the two points where we take the measurements. For the experiment above with your thumb, the baseline is the distance between your eyes.) A larger baseline results in a larger shift, which means that we can measure the parallax of stars which are farther away.