A light-year is a unit of measurement used in astronomy to measure the vast distances between stars and galaxies in the universe. Simply put, a light-year is the distance that light travels in a vacuum in one year. Given that the speed of light in a vacuum is constant and approximately 299,792 kilometers per second (or about 186,282 miles per second), we can calculate the distance that light travels in one year using this speed. To understand this more clearly, let's do some simple calculations: