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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 0.0020 AU
1 Light-minute 0.12 AU
1 Light-year 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.


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