University of Edinburgh

BSL Physics Glossary - light year - definition

It is a measure of the distance travelled by a beam of light in one year. It's a useful measurement for measuring very long distances between stars. If we didn't use this measure we would find a lot of zeros on the end of all the measurements.

The speed of light is 3 x 108 metres per second. That's really fast. Imagine that over one year. A year is 365 days. Multiply that by 24 hours. Multiply that by 60 minutes. And finally multiply by 60 seconds. The answer you get after doing this massive calculation is 9.46 x 1015 metres. It is such a long number that scientists prefer to use the light year unit instead. It's a suitable unit for measuring massive distances in space.

How long does it take the sunlight to reach us on earth? It's 8 minutes.The light travelling from the nearest star is 4.3 light years! If we worked it out in metres it would be an impossibly large number. That's why we use the simpler light year unit.

We are part of the Milky Way and we can see lots of stars. The distance from here to the edge of our galaxy is 100,000 light years!