Light-year ... A light-year, alternatively spelled light year (ly or lyr[3]), is a unit of length used to express astronomical distances and is equal to exactly 9 460 730 472 580.8 km, which is approximately 9.46 trillion km or 5.88 trillion mi. A light year is the distance that light travels in a vacuum in the span of one Earth year. Since light is the fastest thing in the universe, moving at a blistering pace of approximately 299,792 kilometers per second (or about 186,282 miles per second), it covers a truly vast distance in just one year. light-year, in astronomy, the distance traveled by light moving in a vacuum in the course of one year, at its accepted velocity of 299,792,458 metres per second (186,282 miles per second). A light-year is the distance light travels in one Earth year, about 6 trillion miles. Learn how we use light-years to measure the distance of objects in space and how they relate to time and space.
