Ly is a unit of measurement used in astronomy to describe vast distances in space. It stands for "light-year," which is the distance that light travels in one year. Light moves at an incredible speed of about 299,792 kilometers per second (or about 186,282 miles per second), so a light-year is approximately 9.46 trillion kilometers (or about 5.88 trillion miles). Astronomers use light-years to express distances between stars and galaxies, making it easier to comprehend the immense scale of the universe. For example, the nearest star system, Proxima Centauri, is about 4.24 light-years away from Earth. This measurement helps scientists understand the structure and expansion of the universe.