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Frank Schlesinger: The Twenty-Fourth Bruce Medalist

"How far away is that star?" The question comes up often at public viewing nights. It has taken an enormous amount of work to answer that question, even for the nearest stars. Look at a nearby object against a distant background from two different positions. It will appear to move through an angle which increases with the separation between the observing positions and decreases with the object's distance. half this angle is called the parallax of the object, and it is not difficult to show that fit he parallax is one second arc, then the distance to the object is 206,265 times half the separation distance. (The number is the number of arcseconds in a radian.)

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