I was recently watching a documentary about outer space and this was constantly brought up. I have a general understanding but it always seems so confusing because there is many definitions on the internet.
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Parallax is when the same object appears to be in different place compared to some background when viewed from different positions. Here’s a good example that you can try right now. Hold your arm out all the way and hold up your thumb so that’s it’s in front of and blocking something that’s on the other side of the room, maybe a window or a lamp. Now close one eye. Now close that eye and keep the other eye opened. You’ll notice that your thumb appears to jump from side to side in front of the object in the background. That’s parallax.
Parallax is useful in astronomy because it helps us measure to distance to things outside of our solar system like other stars( but not things super far away). Over the course of 6 months, the Earth travels from one side of the sun to the other. That’s the equivalent of looking in one and and then the other. We can look at distance objects at both points in Earth’s orbit and see how much they appear to shift side to side compared to the stars in the background. Because we know the distance between the two observation points and the angle from the center point, we can do some basic trigonometry to find the distance of the object we are observing.
Note that parallax is not useful for things inside our own solar system, those objects have too much apparent motion of their own and there are easier ways to find the distance to objects that close.
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