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.^[1]

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.

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	$1.079\times 10^{9}{\mathsf {mi}}$	$6.706\times 10^{8}{\mathsf {m}}$	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.