How can we know how far away the sun is?
(Lansing State Journal, October 6,1993)
To find out what a parallax is, stretch your arm out and extend your thumb upwards. First, close your left eye and notice where your thumb appears with respect to an object on the other side of the room. Now, do the same using your right eye. You will notice that your thumb appears to move. This is the parallax effect. It occurs because your eyes are separated by a small distance, which causes your line of sight to change depending on which eye is open. If you were to draw a line from each eye, through your thumb, you would notice that the lines cross at the point of your thumb. The angle between these two lines is called the parallax angle. If you know the distance between your eyes and the parallax angle, you can calculate the distance to your thumb using trigonometry.
A similar technique can be used to measure the distance to the sun. Imagine two observers on opposite sides of the Earth. By analogy, the observers are your two eyes, the sun is your thumb, and a far off star is the object across the room. The parallax angle can be measured just like in the thumb analogy. Since we can determine the distance between the observers, we can calculate the distance to the sun.