Light-time

Light-time is the product of the speed of light in a vacuum and time it takes light to travel a certain distance in space. It is most commonly used in light-years to calculate interstellar distances.

For example, the Earth can be said to be 8.3 light minutes from the sun, since it takes 8.3 minutes for light to cross that distance. Mars is 13.4 light minutes from the sun.

Common units of light-time

The light-second, light-minute, and light-year (and to a lesser extent light-hour) are the most common light-time units used.

Common units of light-time

Unit	Meters	Miles	Astronomical Units
1 Light-second	$2.9979\times 10^{8}{\mathsf {m}}$	$1.8628\times 10^{5}{\mathsf {mi}}$	0.0020 AU
1 Light-minute	$1.7987\times 10^{10}{\mathsf {m}}$	$1.1177\times 10^{7}{\mathsf {mi}}$	0.12 AU
1 Light-hour	$1.079\times 10^{12}{\mathsf {m}}$	$6.706\times 10^{8}{\mathsf {mi}}$	7.214 AU
1 Light-year	$9.4607\times 10^{15}{\mathsf {m}}$	$5.8786\times 10^{12}{\mathsf {mi}}$	63000 AU

For Mars, light time translates into communication times. For example, it could be said that at its closest Mars is about 4 minutes away from Earth, while at maximum separation it is about 20 minutes away.