Uranus, like Earth, has four seasons. But that’s where the similarity between our seasons ends.
For starters, the length of Uranus’ seasons are different from ours. It takes Earth 365 days to orbit around the sun, but it takes Uranus 84 years, more or less. So, each season on Uranus lasts 21 (Earth) years.
Uranus’ seasons are also different from Earth’s because the tilts of our planets are different.
Uranus, like Earth, has a nearly circular orbit, so it remains at the same distance from the sun throughout its long year. It’s the planet’s tilt that gives Uranus its seasons, just as Earth’s seasons are caused by our world’s tilt on its axis. But Uranus tilts much more than Earth. Imagine Uranus as a large bead on a stick. The bead spins on the stick about every 17 hours. The stick also travels around the sun. But the stick isn’t straight up and down relative to the sun. Instead, it’s tilted off the vertical by 82 degrees. In other words, Uranus is lying down nearly sideways with respect to its orbit around the sun.
Uranus’ axis of rotation – that is, its angle of tilt – stays the same as the planet moves around the sun. So imagine the sun in the image above, just outside the frame, either to the left or right. It would be shining on Uranus’ poles, making it summer on half the planet, and winter on the other half. That is the case for half of Uranus’ orbit. For two 21-year seasons out of its 84 year orbit around the sun, the poles are pointed more or less at the sun.
During Uranus’ winter or summer, even as the planet rotates in its approximately 17-hour day, the winter side of the planet never sees the sun. It doesn’t see the sun for 21 long years.
Now think about the other two seasons – spring and fall on Uranus. Now you’re imagining the sun at the top or bottom of the image above. At those seasons, Uranus is oriented in its orbit so that sunlight strikes its equatorial region. During the planet’s spring and fall, there’s a day and night cycle in the equatorial regions that’s not dissimilar to that on Earth, except, on Uranus, a day lasts about 17 hours.
Uranus has been visited by one spacecraft – the NASA spacecraft Voyager 2 in 1986. At the time, Uranus was in its northern hemisphere winter, and Voyager saw Uranus as blue and featureless.
Around the year 2000, and as Uranus moved in its 84-year orbit around the sun, the planet was just coming out of the grip of its decades-long northern winter.
Its northern hemisphere spring equinox occurred in 2007; that’s when the sun was shining above Uranus’ equator. Sunlight was reaching some latitudes for the first time in years. The light and warmth in the atmosphere triggered gigantic springtime storms comparable in size to North America (but with temperatures of 300° below zero). Observers on Earth saw more clouds in the atmosphere of Uranus – and bands encircling the planet that had changed in size and brightness – as sunlight struck parts of the planet for the first time in over two decades.
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