A light-year is a unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46×10^12 km), or 5.88 trillion miles (5.88×10^12 mi) . It is the distance that light travels in one year in a vacuum. Light travels at a constant speed of 299,792,458 meters per second in a vacuum. To find the distance of a light-year, you multiply this speed by the number of seconds in a year (31,536,000 seconds) . The result is that one light-year equals 9.46 trillion kilometers or 5.88 trillion miles. Light-years are used to measure the vast distances of space, and they allow astronomers to determine how far back in time they are viewing since everything we view in the night sky has already happened.