Imagine that you have a type of star that is easy to recognise and is always exactly the same. In other words, if you were to put two of these stars side-by-side, it would be very hard to tell them apart.

Measuring distances in astronomy is very important for many reasons. If we know the distance to an object we can learn more about:

The light-year is a way of measuring distance - and not time as you might think from the name.

It is the distance that light travels in one year.

The Astronomical Unit or AU is the average distance between the Earth and the Sun. It works out as 149,600,000 km.