Light Travel Time Calculator
Light is the fastest thing there is, but space is big. Enter a distance to see how long light, or a radio message, takes to cross it.
Light travel time
How long light or a radio signal takes to cross a distance in space.
Light travel time
8 min 19 s
One way, at the speed of light in a vacuum.
- Seconds
- 499.005
- Minutes
- 8.31675
- Hours
- 0.138612
- Days
- 0.00577552
- Years
- 0.0000158125
- Distance in km
- 149,598,000
- Distance in AU
- 1
- Distance in light years
- 0.0000158125
A radio message sent now arrives in 8 min 19 s, and the quickest possible reply gets back 16 min 38 s after you sent it.
The light you see from it today set off 8 min 19 s ago, so you see it as it was then.
Planet distances change through the year as the planets move round their orbits. Presets use typical or rounded figures.
How this is calculated
- 1
Convert the distance to kilometres
Miles, astronomical units (1 AU = 149,597,870.7 km), light years and parsecs are all converted to kilometres first.
- 2
Divide by the speed of light
Time = distance ÷ c, where c = 299,792.458 km/s exactly. One AU, the Earth to Sun distance, gives 499 seconds, or 8 minutes 19 seconds.
- 3
Express it in useful units
The seconds are converted to minutes, hours, days and Julian years of 365.25 days. A light year is defined with that same year, so a star 4.2465 light years away is exactly 4.2465 years away in light travel time.
- 4
Double it for a reply
Radio waves travel at the speed of light too, so the one-way time is the delay before a spacecraft hears a command. A round trip, command and answer, takes twice as long.
Frequently asked questions
How long does sunlight take to reach Earth?
About 8 minutes 20 seconds on average. It varies from about 8 minutes 10 seconds in early January, when Earth is closest to the Sun, to about 8 minutes 27 seconds in early July.
How long does it take light to travel from the Moon?
About 1.28 seconds at the Moon's average distance of 384,400 km. Radio conversations with the Apollo astronauts had a round-trip delay of roughly 2.6 seconds.
How far is a light year?
The distance light covers in one Julian year of 365.25 days: 9,460,730,472,580.8 km, or about 9.46 trillion km. A parsec is about 3.26 light years.
How long does a signal take to reach Voyager 1?
Voyager 1 was roughly 165 AU from the Sun in 2025, so a command takes about 23 hours to arrive and the reply another 23 hours. The delay grows by about 3.6 AU, or half an hour, each year.