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The light-year represents the distance it takes for light to travel in a vacuum during one year.
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We know that the speed of light is 3 times 1010 centimetres per second.
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We must therefore multiply this by the number of seconds in a year – about 30 million seconds – to find the distance travelled by this light: the distance of a light-year.
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In professional astronomy, we prefer to use another unit called the parsec, which is equal to 3.26 light-years. Professional astronomers will speak of the parsec, the kiolparsec and the megaparsec more than they do of light-years, but think of it as a unit of length that gives us a typical scale for the distance between stars.