Here's a quick experiment you can do right now. Look at something that's at least a few meters away from you. For the sake of a concrete example, I'll say a lightswitch that's on the far side of the room, but it could be anything. Now, close one eye and hold up your thumb so it covers the thing you're looking at. Then, close your open eye and open your closed eye.

Suddenly, the lightswitch (or whatever) is no longer covered by your thumb! Why?

Your thumb is directly between your first eye and the lightswitch, thus blocking its view of the lightswitch. But your second eye is in a slightly different place, and the line that goes from it to the lightswitch is not blocked by your thumb, so it can see the lightswitch.

This is an example of a phenomenon called parallax. It shows up in many situations and has practical applications too, so let's look into it in more depth.

Let's start with the basics. You know how when an object is close to you, it looks bigger than when it's far away? That's because we don't directly perceive an object's size or distance. We can perceive the direction to an object and the difference between different directions. Here, I'll show you what that means.

Click or tap and drag to move the red circle.

The dot on the left represents your eye, and the circle on the right is an object. The marked angle is the subtended angle, or the angular size of the object. The direction from the eye to the top of the circle is different than the direction from the eye to the bottom of the circle. The closer the circle is to the eye, the bigger that difference is, and hence the bigger the circle appears to be to the eye.

And of course, this works exactly the same when you're looking at the distance between two objects. The direction from the eye to the first object is different than the direction from the eye to the second object. The closer the two object are to the eye, the bigger that difference is, and so the apparent distance between the objects gets larger.

This is also why things that are far away appear to move more slowly. The farther away they are, the smaller the distance they move appears to be, but they still take the same amount of time to move that distance.

You can flip it around so instead of the object moving, you're the one that's moving. It's exactly the same situation, just from the opposite point of view. That's why when you're driving in a car nearby things like houses and trees go by very quickly, but distant things like mountains go by much more slowly.

Instead of one observer that's moving, you can have two observers in different places, or just two eyes. The direction from the first eye to the object is different than the direction from the second eye to the object, and that difference gets bigger the closer the object is to the eyes.

This is one of the ways you can tell how far away something is. When you look at something very far away, your eyes are nearly parallel, but when you look at something very close to you, you have to cross your eyes.

That's how stereograms work. You show one picture to one eye, and a picture that was taken from a slightly different position to the other eye. Here's an example. Cross your eyes until the two images are on top of each other. When they line up, you should be able to see the trees and houses "pop out" in front of the skyscrapers.

This is actually how astronomers can measure the distance to nearby stars. The Earth circles around the Sun, so in six months it will be about 300,000,000 km (186,000,000 miles) from where it is now. That's a very, very large distance, but even then the parallax deviation of the nearest star is less than 0.001 degrees. This was actually used as an argument in favor of geocentrism, because that was too small for anyone to measure until the 1800s. They thought it was more likely that the Earth was stationary than that the stars were millions of times farther away from the Earth than the Sun.