Mission 6 · Galaxies
How big is space?
Scale comparisons and simple space maths: light travels 300,000 km every second.
How big is space? 🌌
Space is so big that kilometres stop being useful. Astronomers measure it with light instead.
The fastest thing there is
Light travels 300,000 km every second. That is fast enough to go around the Earth 7½ times in one second!
Let's do some space maths
How long does moonlight take to reach us? The Moon is about 384,000 km away.
So when you look at the Moon, you see it as it was 1.3 seconds ago.
And sunlight? The Sun is about 150,000,000 km away.
If the Sun switched off (don't worry, it won't!), we would not know for 8 minutes.
Light-years
A light-year is how far light travels in one year: about 9.5 million million km. It is a distance, not a time!
| Object | Distance | Light takes… |
|---|---|---|
| Moon | 384,000 km | 1.3 seconds |
| Sun | 150 million km | 8 minutes |
| Neptune | 4.5 billion km | 4 hours |
| Proxima Centauri (nearest star) | 40 million million km | 4.2 years |
| Centre of our Milky Way | — | 26,000 years |
| Andromeda galaxy | — | 2.5 million years |
💡 Fun fact: On a dark night you can see the Andromeda galaxy with your own eyes, as a faint smudge near Cassiopeia. The light entering your eye left Andromeda 2.5 million years ago, when the first humans were just starting to make stone tools.
Our galaxy, the Milky Way
The Sun is one of about 100–400 billion stars in the Milky Way. Our galaxy is a giant spinning spiral, and we live in one of its arms. And the Milky Way is just one of trillions of galaxies in the universe.
Try it yourself
- Sound travels about 340 metres every second. How many times faster is light? (about 880,000 times!)
- A spaceship flies at 40,000 km per hour. How many hours to reach the Moon? (about 9.6 hours)