# Astronomical distances

in space are vast. If we measure them in miles or the numbers would very large, eg the distance from Earth to the Sun is 150 million kilometres ($$150,000,000 km$$).

Outside of the Solar System distances are even greater so we use a measurement of distance called the .

## The light year

Distances in space are so enormous so it is more appropriate to use the light year as the unit of distance. We know light travels a distance of 300 million metres in one second ( of light in a is $$3 \times m\,s^{-1}$$) so in one year it will cover a distance of $$9.47 \times 1015$$ metres.

One light year is the equivalent distance light travels in one year. Other than the Sun the nearest star to us is Proxima Centauri which is 4.2 light years from Earth. Light from our Sun takes only 8 minutes to reach us.

Calculating the speed of light