A light-second is a unit of distance that measures how far light travels in one second in a vacuum. Since the speed of light is approximately \(299,792,458\) meters per second, a light-second is equivalent to about \(299,792,458\) meters, or approximately \(300,000\) kilometers (or about \(186,282\) miles).
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A light-second is a unit of length: the distance vacuum light travels in one second. It is useful in a spacetime diagram because a time of one second corresponds to a coordinate of one light-second, making the slopes of the light cone particularly easy to read. The unit is a distance, rather than an elapsed time.