How do astronomers know the distances of planets, stars, and galaxies? They use a visual phenomenon called parallax to measure distances. Parallax is the way an object appears to move, looking like it changed its position, when it is seen from two different locations, or perspectives.
To see parallax for yourself, hold out your arm and stick up your thumb. Closing one eye, line up your thumb with an object across the room. Now quickly switch your eyes (while keeping your thumb in the same position) and you will notice that the object you were looking at is no longer lined up with your thumb — the two objects appear to have moved away from each other. (Note that the movement of the objects is only an apparent movement — they look like they moved, but they actually stayed right where they were.)
This optical illusion is because of parallax. The difference in distance between your two eyes makes your thumb (a relatively nearby object) line up differently with the object that is across the room (a relatively distant object).
In astronomy, parallax is used along with the Earth’s orbit around the sun to measure stellar distances. See Figure 1, below, for an idea of how this works. Basically, when a person looks at the same stars when the Earth is at different places in its orbit around the sun, the closer stars will appear to move position relative to the more distant stars. This apparent movement, or parallax, can be used to figure out distances between Earth and specific, nearby stars.
In the example you did using your…
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