Light Travel Time Calculator
How long light takes to cross a distance — or how far it travels in a given time.
Light covers 1 AU in 499 s.
Light travels 299,792.458 km every second, so its travel time is the distance divided by that speed. Sunlight crosses the 1 AU (≈1.496×10⁸ km) gap to Earth in about 499 seconds — roughly 8.3 minutes. Moonlight arrives in 1.28 seconds, while light from Proxima Centauri takes 4.24 years.
What light travel time measures
Because the speed of light in a vacuum, c = 299,792.458 km/s, is finite and constant, everything we see reaches us after a delay. Light travel time is simply the distance an object lies from us divided by c. Over everyday distances the delay is imperceptible, but across the Solar System it stretches to minutes and hours, and across interstellar space to years, centuries, and beyond.
c = 299,792.458 km/s. Rearranged, time = distance ÷ c — divide any distance by the speed of light to get how long its light has been travelling.
Worked example
How long does sunlight take to reach Earth? The average Earth–Sun distance is 1 astronomical unit (AU) = 1.495978707×10⁸ km.
- 1 Write the relationship. time = distance ÷ c, with c = 299,792.458 km/s.
- 2 Put the distance in kilometres. 1 AU = 1.495978707×10⁸ km.
- 3 Divide by the speed of light. 1.495978707×10⁸ ÷ 299,792.458 ≈ 499 s.
- 4 Scale to a friendly unit. 499 s ÷ 60 ≈ 8.32 minutes — the delay between the Sun and Earth.
Light travel times across the universe
Time for light to reach Earth, computed as distance ÷ 299,792.458 km/s.
| Source | Approximate distance | Light travel time |
|---|---|---|
| The Moon | 384,400 km | 1.28 seconds |
| The Sun | 1 AU (1.496×10⁸ km) | 8.32 minutes |
| Jupiter (mean) | 5.20 AU | 43.2 minutes |
| Proxima Centauri | 4.24 ly | 4.24 years |
| Andromeda Galaxy | ≈2.5 million ly | ≈2.5 million years |
A light-year is a distance, not a time
Despite the word “year”, a light-year measures distance: how far light travels in one year, about 9.4607×10¹² km. A parsec is another distance unit, equal to about 3.2616 light-years. Only when you divide such a distance by c do you get back a time.
The finite speed of light also means looking out into space is looking back in time. The Sun you see is 8.3 minutes old, and the light from Andromeda left it around 2.5 million years ago. No telescope shows the universe as it is now — only as it was when the light we catch first set out.