parallax


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The apparent shift in position of an object relative to background objects when observed from two different locations. The distance between the two measurement points is called the baseline, and the angle formed by measuring the position of the object this way is called the parallactic angle. An everyday demonstration of parallax is easy to try: hold a finger in front of your face, and without moving it, wink one eye and then the other. When you see the position of your finger change with respect to the background, you are seeing parallax.

Astronomers measure the parallax of a stars close to the Earth using Earth's orbital diameter as a baseline. The astronomer observes a star at a six-month interval, when the Earth is at opposite ends of an imaginary baseline defined by the width of its orbit around the Sun. Astronomers look for a parallax shift in the two images of the star, and if they find one, they can use simple geometry to estimate the star's distance from Earth. The greater the shift with respect to background stars, the closer the star. This sort of measurement is applicable to only nearby stars, no more than a few hundred light-years away. At 300 light-years, the angular shift is equivalent to a U.S. quarter seen at a distance of 300 miles, or 500 kilometers.