The Polygons of Saturn

Yesterday, NASA reported that the Hubble Space Telescope had captured a vast, ten-sided wave circling Saturn’s south pole, roughly 167,820 kilometres across. That’s wide enough to lay more than a dozen Earths side by side inside it, with each of its ten sides running nearly 16,800 kilometres. Scientists are calling it a decagon.¹

»The decagon is something Saturn is doing; the hexagon is something Saturn has«

It isn’t a fixed marking on the planet’s surface. Saturn doesn’t have one. It’s weather: an enormous atmospheric wave embedded in the planet’s jet stream. And it appears to be new. Astronomers have been checking Saturn’s south pole for a matching pattern since 1990. NASA’s Cassini spacecraft orbited Saturn from 2004 to 2017 and saw no trace of a long-lived formation. Hubble observations show the decagon was already present in 2023, and that it has become more pronounced since.² Something is happening at Saturn’s south pole that, as far as anyone can currently tell, wasn’t happening before.

Except that Saturn already has a polygon.

The famous hexagon at its north pole is also weather. It’s also an atmospheric wave in a jet stream. But we don’t usually describe it that way. We say that Saturn has a hexagon. The decagon is something Saturn is doing; the hexagon is something Saturn has.

How much weather does it take to make a climate? Almost a century ago, we agreed on an answer: thirty years. Today, the World Meteorological Organization uses thirty consecutive years to calculate what it calls a climatological standard normal. The convention dates back to 1935, when its predecessor, the International Meteorological Organization, instructed member nations to calculate climate normals over thirty-year periods.³

Weather becomes climate after thirty years. Not because anything happens on the thirtieth anniversary: the atmosphere doesn’t cross an invisible boundary and emerge on the other side as climate. Thirty years is a convention, a period over which we have decided that enough individual events have accumulated to describe an actual pattern.

The number itself was never discovered in nature. It was chosen because, in 1935, thirty years was a useful period for which enough reliable records existed.⁴ A hot day is weather. A hot summer is weather. Thirty summers, averaged together, become climate. The thing that changes isn’t the weather, but our description of it.

When I was a child, Saturn had rings. It’s difficult to imagine the sentence being otherwise. But the rings were once a mystery to astronomers and physicists. Galileo Galilei noticed something odd about Saturn through his telescope. It took Christiaan Huygens, working with better instruments, to work out that the planet was surrounded by a thin, flat ring. Later, James Clerk Maxwell showed that the ring couldn’t be solid, but must consist of countless particles orbiting the planet.⁵

Now we teach children the resulting fact: Saturn has rings. Galileo’s strange observation, Huygens’s explanation, Maxwell’s mathematics, and centuries of subsequent observation have been compressed into three words. The sentence no longer tells us that anyone discovered anything. It simply tells us what Saturn is like.

The same thing has happened to the hexagon. Voyager saw it in 1980, Cassini watched it for years, and Hubble has continued to see it. After more than four decades, it has accumulated enough duration to become part of our idea of Saturn. We don’t have to say that there’s a persistent atmospheric wave circling its north pole. We can just say that Saturn has a hexagon.

The decagon hasn’t earned that sentence yet. It’s still too young. We don’t know whether it will persist, exactly how it formed, or whether it will settle into a stable configuration like the hexagon or continue changing and eventually disappear. The scientists observing it intend to keep watching.

So we still have to describe what it’s doing rather than what it is. It’s forming, strengthening, changing. It’s happening. Perhaps that’s what duration does: it turns things we have to describe as events into things we can describe as properties. It allows the process to recede and the noun to appear.

Weather becomes climate when we’ve watched it long enough. A mystery becomes a ring when we’ve watched it long enough. Weather becomes a hexagon when we’ve watched it long enough.

The decagon isn’t there yet. It’s still just weather.

Ask again in thirty years.

References