A light-year is a measurement of distance and not time (as the name might suggest). A light-year is the distance a beam of light travels in a single Earth year, or 6 trillion miles (9.7 trillion kilometers). On the scale of the universe, measuring distances in miles or kilometers doesn't cut it.

What does a light year actually measure?

Light-year is the distance light travels in one year. Light zips through interstellar space at 186,000 miles (300,000 kilometers) per second and 5.88 trillion miles (9.46 trillion kilometers) per year.

Why do we measure in light years?

The main reason for using light years, however, is because the distances we deal with in space are immense. If we stick to miles or kilometers we quickly run into unwieldy numbers just measuring the distance to the nearest star: a dim red dwarf called Proxima Centauri that sits a mere 24,000,000,000,000 miles away!