🌠 Light Travel Time & Distance Calculator
Enter a distance to see how long light takes to travel it — and the distance in km, AU, light-years and parsecs.
Light travel time
8.32 min
km
1.496e+8
AU
1
light-years
1.581e-5
parsecs
4.848e-6
Light travels 299,792.458 km every second. So you see the Moon as it was 1.3 seconds ago, the Sun 8.3 minutes ago, and the nearest star over 4 years ago. Units: 1 AU = Earth–Sun distance, 1 light-year ≈ 9.46 trillion km, 1 parsec ≈ 3.26 ly. 🔒 In your browser.
How the light travel time & distance calculator works
Light travels at a constant 299,792.458 km every second, so the travel time is simply the distance divided by that speed. The tool converts your distance (in km, astronomical units, light-years or parsecs) to a common basis, gives the light travel time in sensible units, and shows the distance in all four scales.
Because light takes time to reach us, we always see space in the past: the Moon 1.3 seconds ago, the Sun 8.3 minutes ago, the nearest star over 4 years ago, and distant galaxies as they were billions of years back. One light-year is about 9.46 trillion km.
Frequently asked questions
How long does light take to reach Earth from the Sun?
About 8 minutes and 20 seconds — the Sun is 1 astronomical unit (149.6 million km) away, and light covers that in roughly 500 seconds. So you always see the Sun as it was 8⅓ minutes ago.
What is a light-year?
The distance light travels in one year — about 9.46 trillion kilometres (9.4607×10¹² km). It's a distance, not a time, and it's how astronomers measure interstellar space.
How far away is the nearest star?
Proxima Centauri is about 4.24 light-years (1.30 parsecs) away, so its light takes over four years to reach us — we see it as it was more than four years ago.
What's the difference between a light-year and a parsec?
Both are distances. A parsec is about 3.26 light-years, defined from stellar parallax. Astronomers often use parsecs; light-years are more familiar in popular science.
Why do we see stars "in the past"?
Light takes time to travel, so the light entering your eye left the object long ago. The farther away something is, the further back in time you're seeing it — a form of cosmic time travel.