How long is a light-year?

The story behind the answer

A light-year is the distance light travels in a vacuum in one year. Light moves at about 299,792 km/s; multiplied by roughly 31,557,600 seconds in a year, that equals about 9.461 trillion kilometres (≈5.88 trillion miles). So the correct answer is 9.461 Trillion Kilometres.

Astronomers use light-years to describe immense interstellar distances (Proxima Centauri is about 4.24 light-years away). Despite the name, it’s a unit of distance, not time. For scale, 1 astronomical unit (AU)—the average Earth–Sun distance—is about 149.6 million km, so 1 light-year is roughly 63,241 AU. Memory tip: think “how far light goes in a year,” not “how long a year is.”