Every 1151 years, seven planets of the Solar System, from Mercury to Neptune, find themselves at almost the same points in their orbits. Their positions scatter by less than a degree, as if the celestial mechanism is wound to the same tune. This rhythm was discovered by comparing planetary positions over 2600 years using precise tables of celestial motion.
But one member falls out of step. Uranus lags by a third of an orbit, while its neighbor Neptune is off by only five degrees. Such a gap between the two ice giants looks like a scar from an ancient catastrophe: perhaps, in the Solar System's youth, a large body slammed into Uranus and knocked it off rhythm.
Had that impact never happened, all eight planets might be marching in perfect formation. Instead, in the planetary choir, seven voices sing in harmony, while one is off-key, a reminder of our system's turbulent youth.
The 1151-year period was known to Babylonian astronomers as the time for Venus to return to the same spot in the sky — long before Uranus and Neptune were discovered.
Arthur C. Clarke's story "Collision at Dawn" describes a catastrophe that changed a planet's axial tilt, a similar scenario might have happened to Uranus.