Parallax:
Because of the Earth's revolution around the Sun, nearby stars appear to move with respect to very distant stars.
The parallax of a star is the apparent angular size of the ellipse that a nearby star appears to trace against the background stars.
Because all parallaxes are small (the stars are very far away), we can use the small angle approximation: tan (x) ~ x, for small x. If we measure the distance to the star in A.U. (astronomical units), then the parallax is given by:
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