astronomical unit

Non-SI unit accepted for use with SI

Name	Symbol	Quantity		SI units
astronomical unit	au	length	149 597 870 700 m
The astronomical unit, symbol au, is a non-SI unit of length accepted for use with the SI. One astronomical unit is equal to 149 597 870 700 m, or approximately 150 gigametres.
Definition				c Δν_Cs^-1
$1 \mspace{4mu} \text{au} \mspace{6mu} = \dfrac{(149 \mspace{4mu} 597 \mspace{4mu} 870 \mspace{4mu} 700)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)}{299 \mspace{4mu} 792 \mspace{4mu} 458} \mspace{6mu} \dfrac{c}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{au} \mspace{6mu} \approx 4.587 \mspace{4mu} 167 \mspace{4mu} 229 \mspace{4mu} 274 \mspace{4mu} 233 \mspace{4mu} 907 \times10^{12} \mspace{6mu} c \mspace{4mu} {\Delta \nu _{Cs}}^{-1}$

Astronomy

The astronomical unit is approximately equal to the average distance from the Earth to the Sun. It is used primarily in the field of astronomy for measuring distances within the Solar System, or interplanetary distances around other stars.

Examples of distances of planets from the Sun

Name	Distance in au	Distance in SI units
Mercury	0.387	58 Gm
Venus	0.723	108 Gm
Earth	1	150 Gm
Mars	1.52	228 Gm
Jupiter	5.20	778 Gm
Saturn	9.54	1427 Gm
Uranus	19.2	2871 Gm
Neptune	30.1	4495 Gm

Parsec

The astronomical unit and the arc second, both of which are non-SI units accepted for use with the SI, together form the definition of the parsec – another non-SI unit of length used in the field of astronomy.