Galaxy OriginsLight is fast, but space is larger. A light-year is the distance light travels in a year, about 9.5 trillion kilometres. Galaxies are millions of light-years away, so their light takes millions of years to arrive.
Looking back in time
The light that reaches a telescope tonight left its galaxy long ago. For a galaxy whose light travels 280 million years, that light set out when the continents were joined into Pangaea, and arrived tonight.
How the distance is measured
For most galaxies, astronomers measure how much their light is stretched by the expansion of the universe, the redshift, and convert it to a distance. Catalogues such as HyperLEDA and the 2MASS Redshift Survey record these measurements. For very nearby galaxies, other methods give more precise distances.
On a Galaxy Origins print
Every print carries the light travel time of its galaxy, and the record card tells you what was happening on Earth when that light set out. How it works.