Back to our runner

Using simple geometry we can determine that for our runner moving at half the speed of light sqrt(1-0.5^2)=0.866 seconds pass for every second we experience.
So 4 seconds for us was only 3.464 seconds for our runner, so for him the light should have only moved 3.464 light seconds. But to us it appears that the light is 2 light-seconds from him. The change in time isn’t enough to explain the difference! Our runner’s perception of distance must be different as well.

Using simple geometry we can determine that for our runner moving at half the speed of light sqrt(1-0.5^2)=0.866 seconds pass for every second we experience.