Transcript: The vast distances to the nearest stars encourage astronomers to use a new unit of distance. Whereas meters and kilometers work well on the Earth, and the astronomical unit is the appropriate unit for scales within the solar system, the distance scale to the stars is given as a lightyear. A lightyear is the distance that light travels in one year. It’s equal to about 6-million-million miles or 1016 meters. It’s defined as the speed of light, three hundred thousand kilometers per second, times the number of seconds in a year. Notice two things. The lightyear is not a metric unit. Occasionally astronomers use none metric units when they are convenient for the scales they’re dealing with, and also a lightyear is a unit of distance, not a unit of time.