Скачать или смотреть Teach Astronomy - Proper Motion

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The component of a star's motion on the plane of the sky is called the tangential velocity, and it's typically harder to measure than a radial velocity. We need the distance to the star, typically given by parallax, and the rate of angular motion across the plane of the sky. This is called the proper motion. For a typical stellar space velocity of 20 kilometers per second a star moves about 600 million kilometers in a year. This is a large amount of motion, but at a distance of a parsec it only corresponds to a half an arcsecond per year on the plane of the sky, barely detectable. For more distant stars it would be hard to detect. On the other hand we can be patient and make observations over more than one year. In this way proper motions have been detected for thousands of stars.