Astronomical unit
An astronomical unit is the average distance from Earth to the Sun, about 149.6 million kilometres.
The astronomical unit, written au, is a unit of distance used for the Solar System. It is defined as exactly 149,597,870,700 metres, roughly the average distance between Earth and the Sun.
Using astronomical units keeps numbers manageable. Mars orbits at about 1.5 au, the main asteroid belt lies mostly between about 2.1 and 3.3 au, Jupiter is at 5.2 au and Neptune at 30 au. The Kuiper Belt stretches from about 30 to 50 au, and the Oort Cloud may extend tens of thousands of au.
Measuring the astronomical unit accurately was a great challenge in the history of astronomy. Observers in the eighteenth century timed transits of Venus across the Sun from places around the world to work it out. Later measurements of the near-Earth asteroid Eros helped refine it. Today, radar and spacecraft tracking give its value with great precision.
Related terms
Level: Explorer Ages 10 and up