Astronomical Unit (AU)

Demonstrate an understanding that one astronomical unit (AU) is the mean distance between the Earth and Sun.

The Earth rotates around the Sun in an elliptical orbit. This means it orbits in a slight oval shape.

At some point in the year it is nearer the Sun, At another point it is further away. See the Perihelion & Aphelion page for more information. We therefore make an average.

The mean average distance from Earth to the Sun is 149,600,000 km.

This is called 1 Astronomical Unit, or AU. This figure is a standard number in astronomy. Many distances to planets and stars are calculated using this number.

It is normally rounded to 150,000,000 km.


  • What is the AU?
  • How many km is the AU?
  • Why is this number important to astronomers?

The shape of Earth's orbit does not contribute to our seasons. Seasons are caused by the tilt of Earth's axis.


Imagine at Nasa Calculate the AU