If you read my post about Hubble’s Law, you may be asking “How did Hubble know the fastest-moving stars were farther away?”
That is a good question, and it means I am going to have to explain something called “parallax”. Parallax is the difference in the way an object looks from two different points of view. It is easy to observe parallax. Close one eye and look at something close to you, like the computer monitor or an object you can reach. Now quickly close that eye and open the other. Switch eyes like this a few times in rapid succession. The object you are looking at seems to jump back and forth. Of course it isn’t really moving – your eyes are a few centimeters apart, so each eye sees a slightly different view. Now look at something a little farther away, maybe across the room, and repeat the experiment. The object still seems to jump, but not as much. Next, go outside. Look at the farthest object you can see: trees on the horizon, mountains if you have any to look at, the moon if it is visible. Do the eye thing again. The object may not seem to move at all. You have just discovered parallax rangefinding.
With highly sensitive scientific instruments, we can detect parallax even with very distant objects. The difference in the appearance of a galaxy in spring and autumn (when Earth is at two different points separated by about 300,000,000 kilometers along its orbit) makes distant galaxies “jump” a little when the images taken are compared, just like the objects you experimented with. By analyzing the parallax, the galaxies’ distance can be calculated.
Together with Hubble’s Law, parallax allows us to describe the size and movement of our universe.