I guess you could flag this as biology and physics both at the same time, but it only let me pick one, so I went with other. I understand that parallax is how the landscape moves in the background, but I’m unable to explain it to someone else.
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Parallax describes the apparent movement of something when observed from two points not on a straight line with the object. Imagine if you were to look at an object which is a foot away, and then you moved a foot in any direction but directly towards or away from the object. It would move a fair amount in your vision, right? But if you repeated that same trial with something which was a mile away, it wouldn’t appear to move very much at all. That movement as your perspective changes is parallax. (Note that if you moved directly towards or away from it, it wouldn’t appear to move at all).
The reason it exists is because you effectively create a triangle with the object you are looking at and the two points you observe it from. The closer the object is to the points, the larger the angle created in the triangle at the object you are looking at it – meaning it appears to move more. We can use the apparent difference in where an object appears when observed from two points to calculate how far it is, so long as we know the distance between the two points. This was useful for making maps before satellites existed, and it’s still useful for calculating how far away nearby stars are.
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