Understanding light travel time
Discover how long light takes to cross the solar system and the distances between stars.
The idea behind it
Even light has a finite speed. A light-year is the distance light travels in a Julian year, not a unit of time. Light travel makes remote observations a view of the past.
The relationship to remember
1.50e+8 km ÷ 299,792.458 km/s = 499 seconds (0.01 days).
Make your own discovery
Switch the distance unit to light-years and enter 4.24 for an approximate light travel time from Proxima Centauri.
Try the interactive lab ↗What this model leaves out
Travel in a vacuum at constant distance. This simple model excludes cosmic expansion, changing orbital positions, and gravitational delays.
Check your understanding
Is a light-year a time unit?
No. It is a distance: about 9.46 trillion kilometers, traveled by light in one Julian year.
Is this a spacecraft travel calculator?
No. These times apply to light in a vacuum. Spacecraft travel far below light speed.
Continue with the formula reference or test yourself in space practice.