Sidereal Orbital Period of Uranus

An 84-year orbit crossed with a 98° tilt produces the strangest calendar in the solar system: each pole spends 42 years in continuous sunlight, then 42 in darkness. Voyager 2 arrived in 1986 at southern-summer solstice and saw a featureless pole-on disc; by the 2007 equinox, with sunlight raking the equator, storms and bands erupted across a planet once dismissed as bland. Whatever a Uranus orbiter finds, it will see a single season of it.

The period is also a human yardstick: one Uranian year spans a long lifetime. The planet has completed barely three orbits since Herschel found it in 1781 — and one of those orbits carried the accumulating errors that betrayed Neptune's existence. In 2033 it returns to the point of its discovery for only the third time.