Astro Bob: Watching the seasons change at Saturn

New Hubble photos show the ringed planet transitioning from summer to fall.

Saturn HST 2018 to 2020 seasonal change NASA ESA.jpg
Subtle changes in Saturn's cloud bands and brightness from 2018 to 2020 point to a transition from summer to fall. Look closely in the planet's polar region (top) and you can see how the number and thickness of the bands varies. The yellow equatorial region also brightens from left to right in these Hubble Space Telescope photos. (NASA / ESA)

Any planet with a tipped axis like the Earth has seasons. On Earth, the northern hemisphere angles steeply sunward from May through August, so the sun shines high in the sky. As a consequence, daylight hours are long, heat builds up and summer happens. In winter, when the Earth moves to the other side of its orbit, the northern hemisphere is tipped away from the sun. It shines low in the sky, resulting in shorter hours of daylight and falling temperatures.

Seasons Earth orbit Sonoma U_S.jpg
During northern hemisphere winter, the Earth's axis points away from the sun, causing it to describe a short, low path in the sky. In summer, we "lean into" the sun (left), and it appears high in the sky. At the equinoxes our planet sits sideways to the sun so the entire globe experiences equal amounts of sunlight and darkness. Seasons are reversed in the southern hemisphere. (Courtesy of Sonoma State University)

It's rather incredible to think that the profound differences between the seasons stem from a single cause: the varying amount of solar energy each hemisphere receives across a year. This fact holds for any planet with a significant nod to its axis.


Planet tilts axis axes NASA JPL Caltech Richard Barkus.jpg
Planets that have little to no tilt (or are tilted all the way around like Venus) don't experience seasons like those with tilted axes. (NASA / JPL-Caltech / Richard Barkus)

Other planets with pronounced seasons include Mars (axis tipped 25.2°), Saturn (26.7°), Uranus (97.8°) and Neptune (28.3°). All the others rotate nearly straight up and down with little to no seasonal variation. Like Earth, the four "seasonal" planets have four seasons, but the length of their length depends on how long it takes the planet to go around the sun.

On Earth as on any tilted planet, the start of each new season is defined by the four key spots in its orbit — the winter and summer solstices and spring and fall equinoxes. Because it takes our planet only a year to complete solar revolution, each season lasts 1/4 of that time or about three months. More distant Mars needs 687 days to circle the sun, making each season about twice as long as those on Earth. Saturn orbits the sun every 29 years at a distance of 886 million miles (1.4 billion km). There each season lasts more than 7 years and each sees changes in the planet's climate.

Saturn orbit seasons King through 2032 S.jpg
As Saturn orbits the sun it experiences seasonal changes. Its tipped axis also means we see the rings from different perspectives. At summer solstice in the planet's northern hemisphere, the rings are fully tipped open, and we see their north face. 15 years later the south face is tilted our way. In between, at the equinoxes, the rings appear edgewise. (Bob King with NASA images)

Summer began in Saturn's northern hemisphere in May 2017. Four years later we're starting to see the transition to fall, which starts in May 2025. Astronomers have been busy tracking changes in the planet's equatorial and polar regions as well as its atmosphere with the Hubble Space Telescope Because seasons are long-lasting affairs at the ringed planet, changes occur more slowly than they do on Earth.

The Hubble data show that from 2018 to 2020 the equator got 5 to 10 percent brighter, and the winds shifted. In 2018, winds measured near the equator were about 1,000 miles per hour (1,600 kph), some 200 mph faster than those measured by NASA’s Cassini spacecraft during 2004-2009. Then in 2019 and 2020 they decreased back to the Cassini speeds. Saturn’s winds also vary with altitude, so the change in speeds could possibly mean the clouds in 2018 were around 37 miles (60 km) deeper than those measured during the Cassini mission.

A drop in the intensity of seasonal sunlight appears to be behind these variations. You can read more about the observations in this free, scientific paper published earlier this month in the Planetary Science Journal. There's also a cool animation of the changing tilt and appearance of Saturn from 2018 to 2020 — click here to see it.


Saturn and rings close up NASA Cassini.jpg
NASA's Cassini spacecraft took this close-up of Saturn's clouds and its many rings. (NASA)

Speaking of atmosphere, that's pretty much all Saturn is. — gas. Like Jupiter, Saturn's air is mostly hydrogen and helium with traces of methane and other gases. Both planets are massive enough to hold onto these light gases. On Earth they routinely escape from the atmosphere into space because gravity here is so much less.

When you look at Saturn in a telescope or photograph the first thing you notice are its bright, pale-white rings, which are composed primarily of water ice. In contrast, the planet's globe shines pale yellow from clouds made of ammonia and ammonium hydrosulphide ice crystals. Enormous storms , some almost as large as Earth, occasionally erupt from deep within the atmosphere. They occur about every 30 years and are called Great White Spots.

Saturn Great white spot storm 2010 2011 NASA JPL Caltech.jpg
A gigantic storm bubbled up from deep within Saturn's clouds in late 2010. NASA's Cassini spacecraft captured this image from orbit. (NASA / JPL-Caltech)

The last one started in late 2010 and lasted well into the following year. Since many of the planets discovered around other stars are gas giants as well, astronomers look to Saturn and Jupiter to teach us more about giant-planet atmospheres work.


Jupiter Saturn dawn 6_15 March 19 2021 S.jpg
The gas giants Jupiter and Saturn rise over Lake Superior from Duluth, Minn. on March 19, 2021. (Bob King)

Right now, Saturn shines low in the southeastern sky at the start of dawn. An hour before sunrise is the best time to find it. The ringed planet is joined by Jupiter, which is brighter and easier to see. In case you spot Jupiter first, Saturn is located one horizontal fist held at arm's length to the upper right of the planet. Both Jupiter and Saturn rise higher and get easier to see as we approach the end of March.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at .

"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at
What To Read Next
Our February event calendar highlights Comet ZTF and Mars, Venus and Jupiter at dusk and a stellar reminder of spring.
How to see returning comet 96P/Machholz as it feels the heat.
For observers in the southern U.S. the moon will cover Mars on Monday night, Jan. 30. Elsewhere we'll see a very close conjunction.
Cold weather and clear skies means it's time to see the comet!