What is an astronomical unit (AU)?
149,597,871km — one astronomical unit
An astronomical unit (AU) is the average distance from the Earth to the Sun — 149,597,870.7 km (about 93 million miles). Since 2012 it has been fixed at exactly that value by definition. Light crosses one AU in 8 minutes 19 seconds.
▶ See the Earth–Sun gap in 3D
The ruler for the solar system
Kilometres get clumsy fast once you leave Earth, so the AU is the natural yardstick inside the solar system: Mercury orbits at 0.39 AU, Earth at 1 AU by definition, Jupiter at 5.2, and Neptune, the last planet, at about 30 AU. Originally it was the messy, hard-to-pin-down real Earth–Sun distance; in 2012 the IAU simply defined it as an exact number of metres to end the ambiguity.
One light-year is 63,241 AU. So even though the AU already spans 150 million km, you would need to lay more than sixty thousand of them end to end just to reach the nearest star.
The solar system in AU
Exact definition
149,597,870,700 m
In miles
≈ 92,955,807 miles
Light-time across 1 AU
8 min 19 s
Mercury from the Sun
0.39 AU
Neptune from the Sun
30 AU
Edge of the Oort Cloud
≈ 100,000 AU
In the interactive view, True scale and 1:1 scale lay these AU out honestly — and you feel immediately why nobody draws the solar system to scale on paper.
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✓ Facts verified July 2026
Every figure on this page is a real, rounded value checked against primary sources. Found something out of date? See how we keep it accurate.