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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Hold your finger out at arms length and close one eye. Now switch eyes. Your finger appears to move a little. Switch eyes back and forth quickly. That’s parallax. Now, since we know the distance between your eyes, and the distance your finger appears to move, we can use that to math out how far your finger is.
So you can take a picture of a distant star, then wait 6 months for the Earth to be on the other side of our orbit and take the picture again. Now we can compare how much that star appears to move on the picture. Since we know how far apart the Earth was at each point in orbit we can math out how far away the star is.
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.
Imagine that there is an object in front of you about 10 metres away. When you stand directly in line with that object, it is right in front of you.
However, let’s say you start walking in a circle. The centre point of this circle is in that original spot 10 metres in front of the object. As you walk in a circle, you will see the position of this object change. When you are on one side of the circle, the object will appear to the left of you. When on the opposite side, it will appear to the right. This change in perception is basically what parallax is. The object never moved, but since we moved, it appears in different positions.
Parallax is important in astronomy because we are trying to observe objects while we are constantly moving. On some days, the object might appear to the left, and other days, it might appear to the right. Understanding parallax helps us determine where the object on average should be, and we can also predict where it will appear at any given time. We can also use the principle of parallax to determine how far objects are by using geometry and trigonometry.
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