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A light-year, alternatively spelled lightyear, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46 × 1012 km), or 5.88 trillion miles (5.88 × 1012 mi). [note 1] As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year (365.25 days).
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