Astronomical Units or AU

The distance from Earth to Sun is 1 AU
Credit: NSO

The Astronomical Unit (AU) is the average distance between the Earth and the Sun. It is 149,600,000 km.

The AU is a useful unit for measuring distances in our Solar System. For example, it is much easier to say that Neptune is 30 AU from the Sun, rather than 4,504,300,000 km!

If you start measuring distances between stars, the AU becomes less useful. This is because our galaxy is so much bigger than our Solar System. The nearest star to the Sun is Proxima Centauri. It is roughly 268,000 AU from the Sun. To give you an idea of the size of our galaxy, the centre of Milky Way is around 1,700,000,000 AU from the Sun. When we measure distances across galaxies, we need bigger units like light years.


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