A light-year is a unit of distance that represents how far light travels in one year in the vacuum of space. Since light moves at a speed of about 299,792 kilometers per second (or approximately 186,282 miles per second), a light-year is equivalent to about 9.46 trillion kilometers (or about 5.88 trillion miles).
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A light-year is the length travelled by light in vacuum during one Julian year of exactly 365.25 days. Using seconds per day and the exact vacuum speed of light givesIt is therefore approximately metres, or 9.46 trillion kilometres. The Julian-year convention matters because a calendar year need not have that duration.[ref]
For a stationary source four light-years away in an idealized flat spacetime, light takes four Julian years to reach the observer. The distance unit and the propagation time remain different physical quantities. In an expanding universe, present distance, lookback time multiplied by the speed of light, and luminosity distance generally differ, so this simple nearby-source interpretation cannot be applied to every cosmological distance.
