Field guide · Stars
StarLight-year
The distance light travels in a year — about 9.5 trillion km. It makes every look at the sky a look into the past.
- One light-year
- 9.46 trillion km
- Nearest star
- Proxima Centauri, 4.2 light-years
- Across the Milky Way
- About 100,000 light-years
- One parsec
- 3.26 light-years
Light is the fastest thing there is, and it still takes 1.3 seconds to reach us from the Moon, 8 minutes from the Sun and more than four years from the nearest star beyond it. A light-year is the distance it covers in a year: about 9.46 trillion kilometres.
So the sky is not a snapshot, it is a stack of them. You see Sirius as it was nearly nine years ago; Deneb as it was well over a thousand years ago; the Andromeda Galaxy as it was before our own species existed. The panel tells you when the light you are seeing set out.
Astronomers more often use the parsec, 3.26 light-years — the distance at which a star shifts by one arcsecond as the Earth goes round the Sun. Measuring that shift is how the nearest distances are known at all; Friedrich Bessel managed it first, for the star 61 Cygni, in 1838.
See it in the sky
Each one opens the sky map pointed at it, for tonight, from where you are.