A light year is a unit of distance that measures how far 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). Over the course of a year, this adds up to nearly 9.46 trillion kilometers (or about 5.88 trillion miles). Astronomers use light years to express distances between celestial objects, such as stars and galaxies. For example, the nearest star system, Proxima Centauri, is about 4.24 light years away from Earth. This helps us understand the vastness of the universe and the scale of space.