Astronomical Unit

For two centuries the astronomical unit was the hardest number in astronomy. Kepler's laws gave the whole solar system's shape in units of the Earth–Sun distance, but not one absolute kilometre of it. Cassini and Richer attacked the problem in 1672 by observing Mars simultaneously from Paris and Cayenne; Halley proposed the better method and did not live to use it, urging astronomers to time the transits of Venus of 1761 and 1769 from widely separated stations. Those expeditions — Cook's voyage to Tahiti among them — were the first coordinated global scientific campaigns, and they landed the au within a few per cent. Radar echoes off Venus in the early 1960s finished the job to seven digits.

By then the definition had become an embarrassment. The au was formally tied to the Gaussian gravitational constant and therefore to the Sun's mass parameter GM⊙ — which shrinks measurably as the Sun radiates mass away, so the unit of length was slowly changing. The IAU's 2012 Resolution B2 cut the knot: the au is now simply a defined number of metres, exact, with no physics attached. Earth's actual mean orbital distance is a separate measured quantity that happens to sit within a few parts per million of it.