Big ideas · 6 minute read
What a light-year really is
Not a time but a distance: how far light travels in a year. Here is how to make the number mean something.
The speed of light
Light travels about 300,000 kilometres every second. In one second it could go round the Earth seven and a half times. In a year it covers about 9.5 million million kilometres, and that distance is a light-year.
Looking back in time
Because light takes time to arrive, everything you see is in the past. The Moon is 1.3 seconds ago. The Sun is eight minutes ago: if it vanished, you would not know for eight minutes.
The nearest star beyond the Sun is over four years ago. Some of the stars you can see on a dark night from our roof are thousands of years ago, and the Andromeda Galaxy, the most distant thing most people can see with their own eyes, is two and a half million years ago.
A way to picture it
If the Sun were a grapefruit in the middle of the planetarium dome, the Earth would be a grain of salt about eleven metres away. The nearest star would be another grapefruit about three thousand kilometres away, somewhere in Russia.
