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What Is Parallax?

To measure the distances of stars, astronomers use a technique called parallax. You use the same technique to determine how far away things are by comparing the images from two eyes, separated by a few inches. Astronomers compare images of the sky taken at two positions of the Earth's orbit, separated by up to 300 million kilometers. In such pairs of images, nearby stars will seem to shift position relative to more distant stars. From measurement of the apparent shifts, the distances to nearby stars can be determined.

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