Light-second
= Light-second
{title2=$1\,\mathrm{ls}=c(1\,\mathrm s)$}
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 $ct$ coordinate of one <light-second>, making the slopes $x=\pm ct$ of the <light cone> particularly easy to read. The unit is a distance, rather than an elapsed time.