The speed of light
Light and radio waves travel through space at exactly 299,792.458 km per second, about 186,282 miles per second. That is fast on Earth, but slow on cosmic scales, which is why astronomers measure distances in light-seconds, light-minutes and light-years.
Travel time = distance ÷ speed of light
Familiar delays
| From | Light travel time |
|---|---|
| Around the Earth | 0.13 s |
| The Moon | 1.28 s |
| The Sun (1 AU) | 8 min 19 s |
| Mars | 3 to 22 minutes, depending on positions |
| Jupiter | 33 to 53 minutes |
| Voyager 1 (about 165 AU, 2025) | about 23 hours |
| Proxima Centauri | 4.24 years |
| Center of the Milky Way | about 26,000 years |
| Andromeda galaxy | about 2.5 million years |
Example
1 astronomical unit (149,597,871 km) ÷ 299,792.458 km/s ≈ 499 seconds, or 8 minutes 19 seconds. When you see the Sun, you see it as it was 8 minutes ago. A radio signal to a spacecraft near the Sun–Earth distance takes almost 17 minutes to go and come back.
Why it matters
- Space missions: Mars rovers cannot be driven in real time; commands are sent in batches, and landings run autonomously.
- Looking back in time: distant galaxies are seen as they were millions or billions of years ago.
- Internet latency: light in optical fiber travels about one third slower, around 200,000 km/s, so a transatlantic round trip takes at least about 60 ms. Geostationary satellites add about half a second of round-trip delay.
- GPS: positioning depends on measuring signal travel times to within nanoseconds; 1 ns of error is 30 cm (1 ft) of position error.
Units of distance
- 1 light-year = 9.461 × 10¹² km, about 5.88 trillion miles.
- 1 parsec = 3.26 light-years.
- 1 AU = 149,597,870.7 km, the average Earth–Sun distance.
The calculator converts any of these into travel time and the round-trip delay for radio communication.