Parallax
Parallax is the apparent change in position of an object when viewed from two different positions. What does that mean? Parallax is something that you have to SEE in order to UNDERSTAND. Look at this cool 3D view of a distant nebula. Can you tell which parts are closer to you and which are further away? I bet you guessed correctly, but do you know HOW you knew? Without realizing it, you were using the foundations of parallax to help you brain estimate distances.
Watch the brief YouTube clip below to help you make sense of parallax.
Watch the brief YouTube clip below to help you make sense of parallax.
Did you try the trick of putting two fingers in front of you eyes, and then closing each eye separately? If you did, then you experienced parallax. So what, I hear you saying. Parallax is actually amazingly helpful. You can use math (hello, Geometry - I'm looking at you!) to calculate the distance between you and a star using parallax. The more it seems to shift, the closer it is to us. See the diagram below.
This diagram above could be a homework problem in Geometry. Parallax can tell us how far the stars are from us.