What units are light years? light-year, in astronomy, the distance traveled by light moving in a vacuum in the course of one year, at its accepted velocity of 299,792,458 metres per second (186,282 miles per second). A light-year equals about 9.46073 × 1012 km (5.87863 × 1012 miles), or 63,241 astronomical units.
Why is light-year not a unit of Time? As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). Because it includes the time-measurement word “year”, the term light-year is sometimes misinterpreted as a unit of time.
What is the speed of a light-year? 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.
How fast is a light second? Light is fast! It can reach the universal speed limit — 186,000 miles per second. (If you could travel as fast as light, the universe would look very different.) Because it moves so quickly, light can seem to appear instantaneously.