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1. We know how fast light travels, therefore in 1 year it must travel a fixed distance. For this purpose astronomers use the Julian year of 365.25 days, making a light year 9.460730473 times 10 to the power 15 metres..

2. It is used as a unit of distance to describe distances of stars and galaxies, mainly because to express these distances in kilometres would give unmanageably large numbers.

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10y ago

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