Science
What is a light-year?
A light-year is the distance light travels in one year — about 9.5 trillion kilometers. Despite the word 'year,' it measures distance, not time, and it's used because cosmic distances are far too vast for everyday units.
See it, don’t just read it.
Watch a 2-minute lesson with voice + animation that explains a light-year.
Key things to understand
- 1It's a unit of distance, not time.
- 2Light travels about 9.5 trillion km (5.9 trillion miles) in a year.
- 3It exists because distances in space are enormous.
- 4The nearest star is about 4.2 light-years away.
- 5Seeing a star 100 light-years away shows it as it was 100 years ago.
Frequently asked questions
- Is a light-year a measure of time?
- No — despite the name, it measures distance: how far light travels in one year, about 9.5 trillion kilometers.
- How far is one light-year?
- Roughly 9.5 trillion kilometers (5.9 trillion miles) — light moving at about 300,000 km per second for a full year.
- Why do astronomers use light-years?
- Cosmic distances are so vast that kilometers become unwieldy; light-years (and parsecs) keep the numbers manageable.