*Question*:What is a light-year?

A light-year is a unit of distance.

One light-year is equal to the distance travelled by light in one year.

The value of a light-year can be calculated as follows. Using the equation from P3, Distance travelled = Speed x Time of travel.

One light-year = 300,000,000 meters per second x (365 days x 24 hours x 60 minutes x 60 seconds (time in seconds)) = 9.461 x 10^{15} meters.

The most common misconception is to think that a light-year is a unit of time, or more rarely a unit of speed.

Why do we use light-years instead of kilometres to describe the distance to objects in space?

The sizes of distances through space that astronomers must work with are so large that using meters or kilometres is impractical - the numbers are too big.

For instance: the nearest star to the Earth, after the Sun, is Proxima Centauri, which is only 4 light-years away, but 36,000,000,000,000 km.

So the light year is just a more useful unit when it comes to measuring the very large distances between object in space.