What Is a Light-Year? A light-year is the distance light travels in one year. Light moves incredibly fast — about 186,000 miles (300,000 kilometers) per second, covering 5.88 trillion miles (9.46 trillion kilometers) per year. To measure the vastness of space, we use light-time — the distance light travels in a specific time frame. Since … Continue reading Astronomy & Astrophysics 101: What Is a Light-Year?