By Alexander Merz · Published on · Details checked on 14 September 2026
Short answer: A light-year is the distance light travels in one year: about 9.46 trillion kilometers (9,460,730,472,580.8 km). It’s a unit of distance – not a unit of time!
Why do astronomers measure in light-years?
Out in space, kilometers simply buckle under the size of the numbers: even the nearest star, Proxima Centauri, is about 40 trillion kilometers away. In light-years that turns into a handy 4.2. The light-year uses the most practical constant in the universe: the speed of light of 299,792.458 km/s. A year has a good 31.5 million seconds – multiply the two and you get those 9.46 trillion kilometers.
Light-years you can picture: examples
| Object | Distance |
|---|---|
| Moon | 1.3 light-seconds |
| Sun | 8.3 light-minutes |
| Neptune | about 4 light-hours |
| Proxima Centauri (nearest star) | 4.2 light-years |
| Sirius (brightest star) | 8.6 light-years |
| Center of the Milky Way | about 27,000 light-years |
| Andromeda Galaxy | about 2.5 million light-years |
A look into the past
The most fascinating consequence: when you look up at the night sky, you’re looking into the past. The light from Sirius spent 8.6 years on the road – you see the star as it looked 8.6 years ago. With the Andromeda Galaxy it’s 2.5 million years: its light set off when the first members of the genus Homo were living on Earth. That makes every telescope a time machine as well – our guide to telescopes for galaxies shows you which instrument lets you look millions of years back.
Converting light-years
1 light-year = 9.461 trillion km = 63,241 astronomical units (AU, the Earth–Sun distance) = 0.3066 parsecs. The pros among astronomers mostly work in parsecs, by the way – but for getting a feel for the scale, the light-year has won out.
Frequently asked questions
Is a light-year a time or a distance?
A distance. The name is misleading: what’s meant is the stretch light covers in one year – about 9.46 trillion kilometers.
How long would a spacecraft need for one light-year?
At the speed of the Voyager probes (about 61,000 km/h), roughly 17,700 years. To the nearest star (4.2 light-years) it would be more than 74,000 years.
Exactly how many kilometers is a light-year?
9,460,730,472,580.8 kilometers – defined via the Julian year of 365.25 days.
What’s the difference between a light-year and a parsec?
Both are units of distance. 1 parsec equals about 3.26 light-years and is defined via the parallax angle.