# Light-crossing Times

$t = \frac{r}{c}$
where $r$ is a distance.
$t \sim 500 \left(\frac{r}{{\rm 1 AU}} \right) \ \ {\rm seconds}$
where $r$ is in astronomical units (AU).
$t= \ \left(\frac{r_{g}}{c}\right) \ \sim 500M_{8} \ {\rm seconds}$
where $r_{g} = 1.4822 \times 10^{13} \ M_{8} \ {\rm cm}$ is the gravitational radius, where $M_{8}$ is the black-hole mass in units of $10^{8}$ solar masses.