astronomical unit
Lengthexact by definition
The astronomical unit is exactly 149,597,870,700 metres, fixed by the International Astronomical Union in 2012. It is roughly the mean distance from the Earth to the Sun, and it is the natural ruler for the Solar System. Since 2012 the factor is exact by definition rather than a measured quantity.
| 1 AU | 149,597,870,000 m |
For most of its history the astronomical unit was a measured value, not a defined one. Astronomers could determine the relative sizes of planetary orbits far more accurately than any absolute distance, so the au served as the scale factor tying that relative map to metres, refined over centuries by transits of Venus, then by parallax, then by radar ranging off Venus and by spacecraft telemetry.
About the astronomical unit
Kepler's third law gives orbital distances in relative terms with great precision: knowing a planet's period tells you its distance in units of the Earth's distance without ever measuring a kilometre. What it does not give is the size of the whole thing. Fixing that scale was one of the great observational problems of the seventeenth to nineteenth centuries, and it is why expeditions sailed across the world to observe the transits of Venus in 1761, 1769, 1874 and 1882: seen from widely separated points on Earth, Venus crosses the Sun's disc along slightly different paths, and the difference gives the Sun's distance by triangulation.
Radar changed the answer's precision entirely. Bouncing a signal off Venus in the early 1960s and timing the echo measured the Solar System directly, and spacecraft ranging refined it further, until the uncertainty in the au was smaller than the effects that make the "mean Earth-Sun distance" ambiguous in the first place. The IAU responded in 2012 by cutting the tie to orbital dynamics altogether and declaring the au to be exactly 149,597,870,700 m. Light crosses it in 499 seconds, which is 8 minutes 19 seconds, so the au is also the reason that a command sent to a Mars rover arrives somewhere between 4 and 24 minutes late.