# How Do We Know the Distance to Far-Away Galaxies?

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.

# How Do We Know That the Universe Is Expanding?

The answer to this question is connected to Hubble’s Law: all the things we can observe in deep space – stars, nebulae, galaxies, and everything else – show a redshift, or Doppler shift, proportional to their distance from us (or from each other). Since you are reading this article, I am going to assume you do not know about Hubble’s Law, or the Doppler Effect, so I will do my best to explain those concepts.

The Doppler Effect is what you hear when a fast-moving, noisy object passes by. If a car went by at 160 kilometers (100 miles) per hour and the driver was leaning on the horn the whole time, you would notice that the horn would suddenly start to drop in tone as the car passed you. This is because sound waves do not travel that much faster than a speeding car – only about 1100 kilometers (720 miles) per hour. As the car speeds away from you, the sound waves get “stretched out”, and longer wavelengths make a lower tone. The first time this was explained scientifically was in 1842 by an Austrian scientist named Christian Doppler, which is why we call it the Doppler Effect.

The Doppler Effect works for light waves too, but we don’t notice it all around us on Earth because light travels so fast (300,000 kilometers / 186,000 miles per SECOND) that nothing on Earth is far enough or fast enough to make it change colors by the Doppler Effect. Distant stars and galaxies are a different matter. They are very far away, and moving very fast. The farther away they are, the more their light is “stretched” to a longer wavelength, which makes them appear redder (this is called “redshift). The American astronomer Edwin Hubble observed this by watching many stars through a telescope. Other scientists had predicted that the universe is expanding, but Hubble was the one who proved it. Before Hubble, we didn’t even know there was a universe beyond the Milky Way galaxy. Even astronomers thought that the blurry objects in the telescope were nebulae, clouds of space gas. After Hubble’s discovery, they started to realize that there were many other galaxies quite like the Milky Way, deep out into a universe that was far bigger than anyone had imagined.

Maybe the best model for the expanding universe is to take a light-colored balloon and make a few small dots on it with a Sharpie. Ask someone to blow it up, and watch what happens to the dots as the balloon inflates. The dots that are farthest apart will move away from each other faster than the ones that are closest together.

Our country honored Edwin Hubble by naming the first orbiting telescope after him. Since 1990, the Hubble Space Telescope has given us the clearest and most beautiful pictures of distant galaxies and other objects in deep space.