Polar Radius of Neptune

Neptune's poles sit 423 km below its equator — a flattening of 1.7 per cent, the smallest of any giant planet, the combined result of the highest density among the four and a moderate 16.11-hour rotation. The value derives from Voyager 2's occultation profiles joined to stellar occultations observed from Earth, and its quoted uncertainty is the largest of the giant-planet radii: one flyby, one geometry, 1989.

Even a modest oblateness works hard at 30 au. The J₂ harmonic it generates precesses the orbits of the inner moons and the narrow, clumpy rings — the "arcs" of the Adams ring, whose stubborn persistence against spreading is still being explained, are confined partly by resonances that depend on exactly this figure. When the arcs were first detected by occultation in 1984, before Voyager arrived, the flattening was among the unknowns that made the data so hard to read.