Uranus Orbit Semi-Major Axis

When Herschel's new object proved to be a planet at 19.2 au, the solar system doubled in radius overnight — Saturn's 9.6 au had bounded it since prehistory. The distance also fit the Titius–Bode rule's prediction of 19.6 au almost eerily, giving that numerical curiosity four decades of credibility until Neptune finally broke it. Sunlight at Uranus is 0.27 per cent of Earth's; high noon resembles Earthly twilight.

The orbit's residual misbehaviour became more important than the orbit itself. By the 1840s Uranus was straying tens of arcseconds from prediction, and the discrepancies — tabulated against this semi-major axis — were the raw data from which Le Verrier and Adams independently computed the position of an unseen outer planet. Galle's first-night discovery of Neptune in 1846 turned a table of errors into the greatest triumph of celestial mechanics.