Polar Radius of Uranus

Uranus carries a modest rotational bulge: the poles sit 586 km below the equator, a flattening of 2.3 per cent from the 17.24-hour spin — gentler than Jupiter's or Saturn's because the ice-giant interior is denser and the rotation slower. The measurement, like the equatorial figure, rests on Voyager 2's occultation data and stellar occultations from Earth, and its uncertainty is genuinely larger: a planet seen closely once, pole-on geometry, and a bland atmosphere make the polar figure the softer of the two numbers.

The 98° axial tilt gives the polar radius an unusual job. For decades at a time a pole points nearly at the Sun — and nearly at Earth — so the disc we observe, and the angular size a telescope measures, is often bounded by the polar radius rather than the equatorial one. Any angular-diameter calculation for Uranus should pause to ask which radius the season has turned toward us.