2.3 The astronomical unit: a convenient way to measure distances in the Solar System
Distances in the Solar System are easiest to describe in terms of the distance between the Sun and the Earth. This distance is called 1 astronomical unit, or 1 AU for convenience. 1 AU is roughly equivalent to 150 million kilometres (150 000 000 km) (1.5 × 108 km). This is about 93 million miles – a number which may be more familiar to some.
Using this unit you can rewrite what was explained in Section 1: Mars’s distance from the Sun is about 1.5 AU, while Jupiter’s is about 5 AU. This unit helps to appreciate the vast scale of the Solar System: Neptune and Pluto are approximately 30 AU and 40 AU from the Sun, respectively. Eris, which until 2018 was the most distant known natural object in the Solar System, has a maximum distance from the Sun of 98 AU. Eris’s record was recently displaced by an object nicknamed ‘Farout’, observed in 2018 at a distance of around 120 AU (18 billion kilometres just sounds like another very big number which is difficult to mentally compare with other distances in the Solar System!).
In all sciences it is extremely important to state the units of any quantity. This is particularly true in astronomy, where the numbers can be very different if expressed in everyday units.