# 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]

## Common units of light-time

The light-second, light-minute, and light-year are the most common light-time units used.

Common units of light-time
Unit Meters Miles Astronomical Units
1 Light-second ${\displaystyle 2.9979\times 10^{8}{\mathsf {m}}}$ ${\displaystyle 1.8628\times 10^{5}{\mathsf {mi}}}$ 0.0020 AU
1 Light-minute ${\displaystyle 1.7987\times 10^{10}{\mathsf {m}}}$ ${\displaystyle 1.1177\times 10^{7}{\mathsf {mi}}}$ 0.12 AU
1 Light-year ${\displaystyle 9.4607\times 10^{15}{\mathsf {m}}}$ ${\displaystyle 5.8786\times 10^{12}{\mathsf {mi}}}$ 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.

## References

1. “Light-Time.” n.d. In Marriam-Webster Dictionary.