Keep in mind, the speed of light is not really constant. The speed of light is constant for a stationary body, i.e. the speed of light will always appear to be 186,292 miles per second to the casual observer. Time is the stray factor here. The faster one's velocity, the more time slows down. This means 186,292 miles / 1 second becomes 186,292 miles /.999999.... second and the greater the velocity the closer that .999999.... gets to zero. This is why a ship traveling at what appears to be the speed of light for it, can travel for one year, yet when it returns to where it started, a much greater amount of time has passed.