1838 Activity: Parallax
If we observe a field of stars when Earth is at one point in its orbit, and observe the same field again six months later when Earth has moved to the other side of its orbit (a distance of 186 million miles) closer stars will appear to have moved in relation to the more distant stars. The amount of movement, measured as an angular distance in the sky, is called the “parallax” of the star.
In order to appreciate the parallax effect it is helpful to start with your thumb. Close your fist, extend your arm and give the “thumbs-up” sign at eye level. Now as you look at your thumb close one eye (you can wink or place your other hand over one eye) and see where it appears against objects in the distance. Now open that eye and close the other. If you open and close alternate eyes you will see your thumb “jump” back and forth as it is viewed from one eye and then the other. Move your thumb closer to your eye and do this again. Notice that your thumb “jumps” even more.
Another way to think of the parallax effect is that it is like our normal depth perception. We can tell that some objects are closer and others further away because we have two eyes. If our eyes were set more widely apart we would be even better and perceiving depth. Observing a nearby star against the background from each side of Earth’s orbit is like expanding the distance between your eyes to the diameter of Earth’s orbit, 186 million miles!
For an animation of this effect, look here: http://www.astronexus.com/node/84