Polar Radius of Saturn

Saturn is visibly out of round: the poles sit 5904 km closer to the centre than the equator, a flattening of 9.8 per cent — the largest of any planet, and detectable in a backyard telescope once you look for it. The distortion is what a ten-and-a-half-hour rotation does to the least dense planet in the solar system: with gravity per unit volume so weak, the centrifugal term wins a bigger share of the balance than anywhere else.

Sitting over the north pole is the strangest structure the flattening frames: a hexagonal jet stream, each side longer than Earth's diameter, discovered by Voyager in 1980 and still turning when Cassini arrived a quarter-century later. Cassini's polar orbits used the planet's precise shape and gravity harmonics to weigh the rings and to show the interior's rotation is layered — measurements that only make sense against an accurately known polar figure.