Light travel time calculator
How old is the light you see in the sky?
Light is fast but not infinitely fast. When you look at the sky you see objects not as they are now but as they were when their light set out. Pick an object and see how many years — or millennia — its light has travelled to reach you. You are literally looking into the past.
How old is the starlight?
Pick an object — see how long its light traveled to reach you.
Light travel time
4 years
You see Proxima Centauri as it looked in the year 2,022.
A light-year is a distance, not a time
A light-year is the distance light travels in one year: about 9.46 trillion kilometres. If a star is "100 light-years away", its light travelled for 100 years — you see it as it looked a century ago.
A window into the past
Sunlight is 8 minutes old, the light of the nearest star four years. With distant galaxies we look back millions or billions of years — some stars we see no longer exist.
Frequently asked questions
How old is sunlight?
About 8 minutes and 20 seconds — that is how long light takes from the Sun to Earth.
Do we see stars that no longer exist?
Possibly. For very distant stars the light can be so old that the star has since died.
What is the nearest star?
Proxima Centauri, about 4.24 light-years away — its light is over four years old when it reaches us.
How far is the Andromeda Galaxy?
About 2.5 million light-years. Its light left long before modern humans existed.
Why is light not infinitely fast?
The speed of light (about 300,000 km/s) is a fundamental cosmic limit — nothing can travel faster.
Can I measure the distance myself?
Astronomers use parallax and standard candles. For us it is enough: distance in light-years = age of the light in years.
The cosmos in your inbox
Once a week: the best of the universe, made simple.
Welcome gift: a cheat sheet of 50 cosmic facts. Preview the 50 facts →
- Every week
- Always free
- No spam