What Is a Light Year
Last updated: March 31, 2026
Quick Answer: A light year is a distance unit (not time) equal to how far light travels in one year: 9.46 trillion kilometers (5.88 trillion miles). Used because cosmic distances are too vast for kilometers.
Key Facts
- One light year = 9.46 trillion km
- Light speed = 299,792 km/s
- Nearest star (Alpha Centauri) = 4.37 light years away
- Milky Way = ~100,000 light years across
- Seeing distant stars = looking back in time
Why Light Years?
Writing 40,000,000,000,000 km is unwieldy. "4.24 light years" is clean.
Distances
- Moon: 1.3 light seconds
- Sun: 8.3 light minutes
- Proxima Centauri: 4.24 light years
- Milky Way center: ~26,000 light years
- Andromeda Galaxy: 2.5 million light years
Looking Back in Time
Light from a galaxy 10 million light years away shows it as it was 10 million years ago. James Webb Telescope sees 13+ billion years back.
Other Units
- AU: Earth-Sun distance (~150 million km)
- Parsec: ~3.26 light years
Sources
- Wikipedia — Light-year CC-BY-SA-4.0
- NASA — Light Year public_domain