What is a light-year?
9.46 trillionkm in one light-year
A light-year is a measure of distance, not time: it is how far light travels in one year — about 9.46 trillion km (5.88 trillion miles), or 63,241 times the distance from the Earth to the Sun.
▶ See a light-year in the Cosmic Zoom
Distance, not time
Light is the fastest thing that can exist, travelling 299,792 km every second — fast enough to circle the Earth 7.5 times in one tick of a clock. In a full year it covers roughly 9.46 trillion kilometres, and astronomers use that enormous span as a ruler for the gaps between stars, because kilometres become unwieldy the moment you leave the solar system.
Because light takes time to arrive, looking far away is looking back in time. The nearest star is 4.24 light-years away, so you see it as it was 4.24 years ago. Look at the Andromeda galaxy and you see light that left it 2.5 million years ago — before our species existed.
A light-year, in other units
In kilometres
≈ 9,460,730,000,000 km
In miles
≈ 5,878,600,000,000 miles
In astronomical units
63,241 AU
Light-time across it
1 year (by definition)
To the nearest star
4.24 light-years
Across the Milky Way
≈ 100,000 light-years
The Cosmic Zoom reads out the real distance across your view as you travel — you cross your first light-year on the way to the nearest stars, then keep going until whole galaxies are the specks.
Keep going
✓ Facts verified July 2026
Every figure on this page is a real, rounded value checked against primary sources. Found something out of date? See how we keep it accurate.