The Earth’s axis is tilted about 23.5 degrees. This is why we have seasons on Earth. But the axis of Uranus is tilted so far it’s hard to imagine how it might have even happened. The axis of Uranus is tilted at an angle of 98-degrees compared to the Sun’s orbital plane.
While the rest of the planets in the Solar System can be thought of like spinning tops, Uranus is more like a rolling ball going around the Sun. During the point of the Uranian solstices, one pole faces the Sun continuously, while the other pole faces away. Only a thin strip of the surface of Uranus experiences any kind of night/day cycle. Uranus’ poles experience 42 years of continuous sunlight, and then 42 years of continuous darkness. During the time of the equinox on Uranus, the planet’s equator is facing the Sun, and so it experiences day/night cycles like we have here on Earth.
What could have caused Uranus to be tilted over on its side like this? Astronomers think that a large protoplanet smashed into Uranus billions of years ago. This collision set the planet tumbling. Eventually it settles into its current axial tilt.
Here’s the same question posed to “ask a scientist”, and here’s an article from the Planetary Society Blog.
We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.