Why does Uranus spin on its side?

Uranus spins on its side because something enormous probably hit it when the solar system was young. That crash, or perhaps several crashes, pushed the planet’s rotation almost sideways. So while Uranus travels around the Sun, its poles take turns pointing toward the sunlight for long stretches.

One beautiful hint comes from its moons, which circle in the same tilted plane as the planet. That suggests the impact may have scattered material that later gathered into moons. The result is a world with seasons far stranger than anything a child sees on Earth.