Looking far is looking back
Light takes time to travel. The view of a distant object arrives after the event that produced it.
In 30 seconds
- A light-year is a distance, not a duration.
- It is about 9.46 trillion kilometres.
- For nearby examples, travel time equals distance divided by light speed; cosmological distances need more care.
Make it concrete
Sunlight takes roughly eight minutes to reach Earth. Move the distance slider in the lab and compare an ordinary message with a space signal.
SourceDestination
1 AU = 149,597,870.7 km exactly, approximately the mean Earth–Sun distance. 2 AU = 299,195,741.4 km. Light needs about 8 min 19 s per AU.
A 0.6 s pause at the destination separates each pulse; it is not part of light travel time.
- One-way light time
- 1.282221 s
- Time on screen
- Simulated time
- 0
- Ready
- Ready
Straight-line travel in vacuum: t = d / c. The line always represents the selected distance; objects are not drawn to scale. Radio routing and processing add delay.
Check your understanding
Is a light-year a unit of distance?
- A light-year is a distance, not a duration.
- It is about 9.46 trillion kilometres.
- For nearby examples, travel time equals distance divided by light speed; cosmological distances need more care.
Go to the source
Sources reviewed: 13.09.2026