What are the seasons like on Uranus?

Ball with bands, bright spot, and thick atmosphere encompassed by narrow greenish rings.
Composite thermal image released in 2019, showing both Uranus’ atmosphere and rings, via ALMA and the VLT. The rings look look surprisingly bright in this image, but are really very faint and very cold. Read more about this image.

Uranus, like Earth, has four seasons. But the seasons on Earth and Uranus are very different.

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 Earth-years, more or less. So Uranus’ year is 84 Earth-years along, and each season on Uranus lasts 21 earthly years.

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 the tilts of our planets are different. While Earth orbits nearly upright, Uranus is lying down nearly sideways with respect to its orbit around the sun.

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Diagram of orbit of planet on its side, axis pointing in plane of orbit.
Seasons on Uranus. Image via Nanjing University, Nanjing, China.

Earth’s axis is tilted 23.5 degrees from the plane of its orbit around the sun. The axis of Uranus is tilted at an angle of 98 degrees, with respect to its orbit.

Earth has a midnight sun in summer at its poles, and a long polar night in winter. But those dark and bright times at Earth’s poles affect a smaller part of our planet, and don’t last nearly as long as they do on Uranus. For two 21-year seasons on Uranus – the winter-summer seasons – the poles of Uranus are pointed (more or less) either toward the sun, or away from it.

During Uranus’ winter-summer season, the winter side of the planet never sees the sun. It doesn’t see the sun for 21 long years. Meanwhile, the summer side of the planet has continuous daylight.

That’s a long polar night, and a long midnight sun!

five views of planet with vertical rings showing them at different oblique-to-straight on angles.
From Earth, we don’t always see the same orientation of the Uranian rings. These are ground-based images, acquired from 2000 to 2004, showing the change. Image via Keck Observatory.

Now think about the other two seasons – spring and fall – on Uranus. At those seasons, Uranus is oriented in its orbit so that sunlight strikes its equatorial region. At those seasons, the length of a day on Uranus comes into play.

Uranus spins on its axis about every 17 hours, 14 minutes. So its day-night cycle lasts that long.

During the planet’s spring and fall, a large percentage of the planet has day and night – a shift between daylight and darkness – again and again about every 17 hours. So, for much of the planet, where there had been continuous day or continuous night lasting decades on an earthly scale, now there’s a fast change between day and night.

And that change causes dramatic shifts in the cloud pattern on Uranus, as earthly astronomers have now seen. Uranus has been visited by one spacecraft – the NASA spacecraft Voyager 2 in 1986. When Voyager 2 passed Uranus, this world was in its northern hemisphere summer, and Voyager saw Uranus as blue and featureless.

As the years have passed since Voyager 2’s flyby of Uranus – as technologies for observing Uranus from Earth (or Earth-orbit) have become more powerful – and as Uranus moved in its 84-year orbit around the sun – we’ve seen the seasons on Uranus change.

Featureless pale blue sphere.
Uranus, as seen by Voyager 2 in 1986. Image via NASA PhotoJournal.

In the years after the Voyager 2 flyby, the planet came out of the grip of its decades-long winter/summer season. Its northern hemisphere autumn equinox occurred in 2007; that’s when the sun was shining above Uranus’ equator. Sunlight had begun reaching some latitudes for the first time in years. The light and warmth in the atmosphere triggered gigantic storms comparable in size to North America (but with temperatures of -300 Fahrenheit/-184 Celsius), visible as bright spots in the planet’s atmosphere.

Around that time, 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 decades. Plus, they saw a dark spot and more bright spots, which earthly observers were able to follow for years.

What more will we see, in the decades and years ahead, as Uranus moves toward the long winter/summer portion of its orbit once more?

Blue striped planet with inset showing enlarged spot.
The dark spot on Uranus, which appeared as the planet’s northern hemisphere approached its autumn equinox, was large enough to engulf two-thirds of the United States. Image via NASA/ESA/L. Sromovsky and P. Fry.

Banded blue planet, its axis near horizontal, with thin red vertical ring around it.
Composite image of Uranus and its faint ring system, at near infrared wavelengths, showing its sideways tilt. The rings orbit above Uranus’ equator. Image data acquired July 11 and 12, 2004 by Keck Observatory/Lawrence Sromovsky.
Bright blue planet on its side with vertical rings.
False-color image of Uranus – showing bright clouds along its equator – using Hubble Space Telescope data taken on August 8, 1998. Via NASA JPL/Erich Karkoschka.

Bottom line: Seasons on Uranus are determined by the planet’s hugely sideways tilt on its axis.

January 24, 2017

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