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.
Light travel time across the solar system
One-way light time is the same as the radio signal delay for spacecraft, because radio waves also travel at the speed of light. These delays decide whether a probe can be steered in real time.
Light travel time on Earth
- Light travels about 30 cm (11.8 in) per nanosecond, a figure chip and network designers use constantly.
- Light could circle Earth's equator in about 0.134 seconds.
- In optical fiber light is slower, roughly two-thirds of c, so a New York to London fiber link has a one-way delay of around 28 milliseconds.
- The Apollo laser reflectors on the Moon return a pulse in about 2.5 seconds, which is how the Earth–Moon distance is measured to millimeters.
How to use this calculator
- Enter a distance and choose its unit: kilometers, miles, AU or light-years.
- Read the travel time, shown in the most readable unit (seconds, minutes, hours or years).
- For a round-trip signal, such as a command to a rover and its reply, double the result.
To go the other way and find how far light travels in a given time, use the light speed distance calculator.
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.
How long does a signal take to reach Mars?
Between about 3 and 22 minutes one way, depending on where Earth and Mars are in their orbits. A round trip takes double that.
How fast is the speed of light in mph?
About 670,616,629 miles per hour, or 186,282 miles per second.