Light travels at exactly 299,792.458 km/s in a vacuum. This tool divides any distance by that speed to tell you how long light takes to cross it — a clear way to grasp cosmic scale for students, astronomy fans and educators. Because light is not instantaneous, this travel time is also how far into the past you are looking when you observe a distant object.
How it works
The calculation is a single division:
time (seconds) = distance (km) ÷ 299,792.458
You enter a distance and a unit, and the tool first converts it to kilometres using these factors, then divides by the speed of light:
| Unit | Kilometres per unit |
|---|---|
| kilometre (km) | 1 |
| astronomical unit (AU) | 149,597,870.7 |
| light-year (ly) | 9.4607 × 10¹² |
| parsec (pc) | 3.0857 × 10¹³ |
The result is shown both as exact seconds and in the single most natural unit (years, days, hours, minutes or seconds), where one year is taken as 365.25 days.
Example
Sunlight from the Sun to Earth — a distance of 1 AU:
time = 149,597,870.7 ÷ 299,792.458 ≈ 499 seconds ≈ 8.3 minutes
That is why sunlight you see now left the Sun about 8 minutes and 20 seconds ago. Everything runs in your browser; nothing is uploaded.