A light-year, alternatively spelled light year (ly or lyr [2]), is a unit of length used to express astronomical distances and is equal to exactly, which is approximately 5.88 trillion mi. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Learn what a light year is, how to measure it, and how it differs from other units of length in astronomy. Find out the distance to various astronomical objects in light years and see a map of stars near Earth. 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. A light-year is the distance light travels in a vacuum over the course of one year. Multiply the speed of light by the number of seconds in a year, and you get the magic number: about 9.46 trillion kilometers, or roughly 5.88 trillion miles.
