The light travel time formula
Light moves at a fixed speed, so the time it needs to cross a distance is simply the distance divided by the speed of light. Because space is so vast, that time can range from a fraction of a second (the Moon) to thousands of years (distant stars), which is why we always see the sky as it was in the past.
time = distance ÷ speed of light
c = 299,792.458 km/s = 186,282 miles/s
Worked example
The Sun is about 1 AU, or 149.6 million km, from Earth. Divide by the speed of light: 149,600,000 ÷ 299,792 = 499 seconds = 8 minutes 19 seconds. That is why a sunset happens about 8 minutes after the Sun actually dips below the horizon.
Frequently asked questions
How long does light take to reach Earth from the Sun?
About 8 minutes 20 seconds, crossing roughly 93 million miles (1 AU) at light speed.
How do you calculate light travel time?
Divide the distance by the speed of light: time = distance ÷ c. Light crosses 1 million km in about 3.34 seconds.
How long does light take to reach Earth from the Moon?
About 1.28 seconds, since the Moon is roughly 384,400 km away.
How long does light from Proxima Centauri take to reach us?
About 4.24 years, because it is 4.24 light-years away.
Why do we see stars as they were in the past?
Light takes time to travel, so a star 100 light-years away is seen as it looked 100 years ago. Looking out into space is looking back in time.
How long does light take to travel one light-year?
Exactly one year, by definition.