We often come across the term “light-year” in the context of space travel. But what does it actually mean?
A light-year is the distance light travels in one year. At a speed of 186,000 miles/sec (300,000km/sec), light travels 5.88 trillion miles (9.46 trillion km) in a year—a distance well beyond immediate comprehension.
Scientists created the term light-year to measure astronomical distances beyond the confines of the Earth. And in the vastness of space, light photons, which can go around the Earth 7.5 times in just one second, seem slow.
The above animation from planetary scientist Dr.James O’Donoghue helps put the speed of light into a broader perspective while highlighting the vast distances between celestial bodies.
Light Speed: Fast, but Slow
The Moon is the nearest celestial body to Earth at 239,000 miles (384,400 km) away. A light photon emitted from Earth would get to the Moon in a mere 1.25 seconds.
But how does this compare to other celestial bodies in our solar system?
| Celestial Body | Distance at Closest Approach | Light Travel Time from Earth |
| Moon | 0.38 million km | 1.25 sec |
| Mars | 54.6 million km | 3 min |
| Sun | 150 million km | 8 min |
| Jupiter | 588 million km | 33 min |
| Saturn | 1.2 billion km | 67 min |
| Pluto | 4.3 billion km | 4 hrs |
If you watched the entire length of the above video (click here to see video), you probably saw how “slow” light is. The same photon of light that reached the Moon in a little over a second took three long minutes to reach Mars, the next planet beyond Earth in our solar system.
It takes light just over eight minutes to get from the Sun to Earth. This means that when we look at the Sun, we see it as it was eight minutes ago, and if it were to disappear suddenly, we wouldn’t realize it for eight whole minutes.
Therefore, how “fast” or “slow” light is depends on your perspective. To us Earth-dwelling humans, it feels instantaneous. But the vastness of the universe makes even light seem slow—and it travels at a speed that our spacecraft aren’t even close to matching.
Written by James O’Donoghue
February 1 2022
