Astronomers have another unique way of scaling the universe. Sometimes, distances get so large that even scaling based upon the distance between the Earth and Sun, or even Pluto and the Sun becomes inconceivable. Instead, astronomers use light to measure distance. They can do this because light travels at a finite speed, not instantaneously as it may seems to us as we turn on the light in a room. It appears to us that light hits everywhere in a room at the same time, but in reality, it hits the far corners slightly after the front corners. However, the light moves so quickly, and the difference in time is so small between the lighting of the front and back corners, that we do not notice the discrepancy. When you make the "room" big enough, the difference in time between lighting the front and back gets larger and it takes a conceivable amount of time to travel from one end of the room to the other. If you make this "room" big enough, the light would have to travel a whole year before it lit the back wall. Astronomers call the length of this "room" a light-year because it is the distance that light can travel in one year. This distance is 9.4608 x 10¹²km. This distance provides a useful scale for an astronomer because not only is it large enough to measure the distances required by astronomers, but it also introduces time into the scale.