Parallax is a phenomenon by which we can judge the distance to things just by looking at them. Astronomers use parallax to calculate the distance to stars.
Do you need a gigantic computer-based observatory to use it? Not at all. Your finger will do nicely.
Put one finger up in front of your face and close one eye, then the other. Your finger seems to jump back and forth between two positions.
That's because the perspective from each eye is slightly different, and your brain combines these images to give you a sense of three dimensional space.
Parallax Must Decrease With Distance
Notice, however, that the farther you move your finger away the less discrepancy there is between the images. Parallax must decrease with distance!
This is an exciting discovery, because we can now use parallax to judge the distance to objects out in space. Here's how.
Figuring Out The Distance
Because the earth moves around the sun in one year's time, every six months it is as far away from where it was six months ago as it can get. That distance is like the distance between your eyes.
Images taken of a far off object in January can be compared with images of the object in July. The apparent motion of the object due to parallax tells us how far away it is from earth.
But wait! How do you know that your telescope is pointed in exactly the right direction? By aligning it with the so-called "fixed stars" stars so far away they show no parallax motion whatsoever. Against that unmoving background, the closer stars can be picked out.