­So to measure really long distances, people use a unit called a light year. Light travels at 186,000 miles per second (300,000 kilometers per second). Therefore, a light second is 186,000 miles (300,000 kilometers). A light year is the distance that light can travel in a year, or: 186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year = 5,865,696,000,000 miles/year. A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers). That's a long way! Using a light year as a distance measurement has another advantage -- it helps you determine age.