Uranus rolls around the Sun on its side

Uranus is tilted by about 98 degrees. Its poles take turns pointing at the Sun, and a season there lasts about twenty-one years.

Earth’s axis leans by 23 degrees, which is enough for summer and winter. Uranus leans by roughly 98, past the horizontal, so the rotation is retrograde and one pole points toward the Sun at one solstice while the other pole points into the dark. Half an orbit later they exchange. The orbit takes 84 years, so a season — the long day at one pole, the long night at the other — runs about 21 years. The equator, which on Earth gets the most sun, spends the solstice in twilight.

The tilt is probably the scar of a giant collision early in the planet’s life, though the models of that collision are still being argued. What is not argued is the geometry. Voyager 2, in 1986, is the only spacecraft to have been there and seen the faint rings and the bland face. The blandness was the season: the probe arrived near southern summer, looking down on a pole. A planet on its side does not show you the same planet twice.