The Astronomical Unit
What units we use to measure a distance depends on the distance involved.
To measure really BIG distances we need really BIG units.
We have seen that the distance between the Sun and the Earth actual varies between about 147 and 152 million km. The average value is about 150 million km (actually 149597870.691 kilometres).
We call this distance 1 astronomical unit or AU. It is a very useful measure of distance within the Solar System.
Consider this table.
| Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto | |
| Orbit (AU) | 0.39 | 0.72 | 1 | 1.52 | 5.20 | 9.54 | 19.2 | 30.1 | 39.5 |
How many times further away is Jupiter from the Sun than Earth?
How many times is Neptune further away than Jupiter?
When we need to describe distances beyond our solar system, e.g. to stars and galaxies, then we use larger units of distance called the parsec and the light year. More about them later.