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| Space on May 18, 2010

What are the seasons like on Uranus?

Seasons on Uranus are very different from Earth’s. Each season on Uranus lasts 21 years.

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.

Imagine the Earth is a large bead on a stick. The bead spins on the stick – which gives us our 24-hour day. The stick travels around the sun. But the stick isn’t straight up and down relative to the sun. Instead, it’s tilted a little bit off the vertical. Scientists call the stick the “axis” of the planet.

But the position of the stick, or axis, stays the same as it goes around the sun. Imagine that at one point, the tilt points the bottom half of Earth more directly at the sun. This is when it’s summer in the southern hemisphere and winter in the northern hemisphere. When the planet has traveled to the other side of the sun, the situation is reversed. Now the northern hemisphere is more directly lit and the southern hemisphere is less directly lit.

Now think about what happens at the poles. You probably know that during each hemisphere’s winter, days are really short near the north and south poles. Sometimes the sun barely comes up at all in midwinter.

The situation at the poles is the situation most similar to days and seasons on Uranus. Basically, if you imagine Uranus on a stick (and by the way, about 64 Earths could fit inside Uranus), the tilt is so large that the stick is almost horizontal. This means for two 21 year seasons out of the 84 year journey, the poles are pointed more or less at the sun. It means that even as the planet rotates in its approximately 17-hour day, the side of the planet facing away from the sun will never see the sun. That hemisphere won’t see the sun until the planet has traveled on in its orbit, to a part of its orbit where the axis of Uranus is no longer pointing directly at the sun.

Uranus has been visited by one spacecraft -the NASA spacecraft Voyager. At the time, Uranus was in its northern hemisphere winter. Since then, Uranus has moved in its 84-year orbit around the sun. Its northern hemisphere spring equinox occurred in 2007. There were 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.