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Uranus is one of the strangest planets in the Solar System. Something huge smashed into the planet billions of years ago and knocked it over on its side. While the other planets look like spinning tops as they make their journey around the Sun, Uranus is flipped on its side, and appears to be rolling around the Sun. And this has a dramatic effect on the seasons on Uranus.
The Earth’s tilt gives us our seasons. When the northern hemisphere is tilted towards the Sun, that’s summer. And when it’s tilted away from the Sun, that’s winter for the northern hemisphere. But on Uranus, one hemisphere is pointed towards the Sun, and the other is pointed away. The position of the poles slowly reverse until, half a Uranian year later, it’s the opposite situation. In other words, summer for the northern hemisphere lasts 42 years long, followed by 42 years of winter.
If you could stand at the north pole of Uranus (you can’t, you’d sink right in), you would see the Sun appear on the horizon, circle higher and higher for 21 years and then circle back down to the horizon over the course of another 21 years. Once the Sun went below the horizon, you would experience another 42 years of darkness before the Sun appeared again.
You would expect this bizarre configuration to give Uranus wild seasons; the day side faces the Sun and the atmosphere never rotates to the night side to cool down. The night side is in darkness, and the atmosphere never gets a chance to warm up. As the Sun first shines on a region that was cold and dark for years, it heats it up, generating powerful storms in the atmosphere of Uranus. Early observers reported seeing bands of cloud on Uranus through their telescopes, but when NASA’s Voyager 2 spacecraft arrived, it was blue and featureless. It might be that the changing seasons will bring the storms back to Uranus.
Here’s an article from the BBC about the changing seasons on Uranus.
We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.