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Hold a finger in front of yourself, and look at it with one eye and then the other. It will appear to move. That is parallax.
Similarly, stars appear to move when looked at from one end of the earth's orbit versus half a year later on the other side of the sun. There's more of a motion the closer the star is.
One parsec is the distance to a star that will move one arcsecond on the sky (1/3600 degree) over the earth's orbit.
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