History behind discovery of Saturn’s rings


Ah, Saturn! No astronomical sight is more breathtaking, even in a small telescope, than Saturn’s celestial hat brim. Around 9 p.m., look for the ringed planet low in the southern sky as a pale yellow point of light. You can’t miss it if you start by finding much brighter Jupiter off to its right.

A few weeks ago, I outlined a thumbnail history of the discovery of Saturn’s rings. This week, I thought I’d fill in a few of the stranger details, and there are plenty of those.

Most folks imagine that science in general and astronomy in particular are filled with calm, single-minded people who quest unceasingly after truth. The fact is that the history of astronomy is filled with weirdness on a cosmic scale. Even the greatest discoveries are often cloaked in dark portents and strange events.

Such was the case with the discovery of the rings of Saturn.

Saturn is the farthest planet from the sun visible to the unaided eye. At about a billion miles away from our daystar, it takes almost 30 years to make one orbit. As a result, it moves very slowly against the background stars, a motion that our ancient forebears saw as stately and beautiful.

The ancients had no telescopes, of course, so the rings that girdle the planet were unknown to them. They saw the planets as perfect and unchanging, their motions predictable. To say otherwise was heresy, a blasphemy against the gods who created them.

This was true even up to the early 17th century when the inquisition still wielded its velvet-gloved fist over the intellectual life of Europe.

Galileo Galilee was a follower of the Copernican model of the cosmos, which placed the sun at the center and the planets revolving around it. The hierarchy of the Catholic Church still espoused the old Ptolemaic model, which placed Earth at the center. They saw the heavens as unchanging, perfect reflections of the power of a perfect God.

But it was not Galileo’s sun-centered model that got him in trouble with the church. You see, Galileo was a scientist. He believed that direct observation of the universe produces our best understanding of its workings. The church believed that the senses can trick us. Only divine revelation produces true understanding.

When Galileo first pointed his tiny telescope at Saturn, he saw something that shocked him so much that he had to tell the world: “SMAISMRMIMLEPOETALEUMIBUNENUGTTAURIAS,” he wrote.

Ha! Foolish Galileo. That conclusion is, as you can clearly see, totally incorrect.

Confused? You should be. This cryptic message is an anagram — of a Latin sentence, no less. It says, “Altissimum planetam tergeminum obseruani.”

Galileo’s telescope was so poor by today’s standards that he had not seen the rings. Instead, he had observed two circular lumps perched on each side of the planet like planetary Mickey-Mouse ears. As the Latin loosely translates into English, “I have seen the highest planet (Saturn) to be triple in form.”

A few months later, Galileo translated his anagram for the world. This and other telescopic observations of the heavens got him into a world of trouble. He tried showing others what he had seen, but remember, the senses can deceive us. Telescopes are hard to use, especially when you don’t know how to look through them. Galileo’s guinea pigs were among the first humans to look through a telescope, and they cannot be blamed for seeing nothing at all.

Besides, the planets had to be perfect representations of God’s power. What better representation of perfection is there that a perfect, uniform sphere. Mickey Mouse ears didn’t exactly fit into that picture.

By 1665, Dutch astronomer Christian Huygens had developed a better telescope, which he trained on the planet Saturn. Here is his startling discovery in his own words:


Ha! In honor of Galileo, Huygens had published his observations as another Latin anagram, which in English reads, “Saturn is girdled by a thin, flat ring, touching it nowhere, and tilted to the ecliptic.” I’ll leave it to all you anagram fans to figure out what the Latin is.

By 1665, most astronomers had warmed to the ring theory. In 1675, Italian astronomer G.D. Cassini discovered a gap in the ring structure, a dark line all the way around that divided the ring system in two. The number of rings increased slowly as telescopes got better.

In 1787, Pierre-Simon Laplace proposed that the rings were made up of many solid ringlets. They would have to be spinning rapidly around Saturn or Saturn’s gravity would make them collapse into the planet.

However, in 1859, James Clerk Maxwell proved mathematically that solid rings in rapid rotation would simply tear themselves apart. Instead, he suggested that the rings had to be made up of an enormous number of small particles, tiny “moons,” all of which were independently orbiting Saturn.

His mathematical calculations run 60 pages, so I won’t describe them here (not that I understand them anyway). However, his conclusions, based on the math alone were positively prescient:

“… (T)he only system of rings which can exist is one composed of an indefinite number of unconnected particles, revolving round the planet with different velocities according to their respective distances. These particles may be arranged in series of narrow rings, or they may move through each other irregularly. In the first case, the destruction of the system will be very slow, in the second case, it will be more rapid, but there may be a tendency towards an arrangement in narrow rings, which may retard the process.”

Maxwell had predicted the large number of rings we see in spacecraft today! More significantly, he had done so without direct observation. Instead, he used mathematics, the language that the universe speaks.

By 1895, spectroscopic analysis of the rings proved that Maxwell was right, at least as far as the “indefinite number of unconnected particles” was concerned.

We now know that the rings are about 235,000 miles wide around the planet. Saturn and its ring system would just fit between the Earth and its moon.

Yet at their widest, they are no more than half a mile a thick, and in some places, they are no thicker than 10 yards or so. They are far thinner than a sheet of paper if you scaled it up to ring size.

They are made of mostly tiny particles of mostly water ice with a trace of rocky material mixed in. In other words, they are like the dirty hunks of ice you might find frozen to your car’s wheel wells in winter.

The individual ring particles, trillions of them, are all rotating rapidly around the planet. Because they are 800 million miles away, we see them as a solid ring. Distance alone creates the illusion of a solid ring.

What looks like a single ring is really a host of thin concentric circles. A medium-sized scope at higher magnification will show Cassini’s Division, the widest division in the rings, about halfway from the planet.

Even the Cassini spacecraft, which was in orbit around Saturn from 2004 to 2017, could not resolve the rings into particles. It could, however, resolve the seven or eight main ring sections into thousands of ringlets with strange swirls, eddies, and gaps that perturbed the perfect coherence of the rings. It could also see within the rings some of the larger hunks — moons and moonlets of Saturn — that had never been seen before.

Cassini provided compelling evidence that the ringlets and their eddies and gaps were created by the moons and moonlets, which exerted their gravitational influence on the rings.

Those “shepherd moons” divided the rings into their many divisions but not by their individual gravitational force. Working in consort, their gravitational interactions with the rings and each other created “gravitational resonances” that accounted for both the large number of rings and the perturbations within them.

Cassini’s insights into the rings produced at least one more important discovery. Because Saturn does not have a solid surface, that surface is constantly changing. Consequently, it is difficult to determine the length of Saturn’s rotation, i.e., just how long a Saturnian day is.

As it turns out, not all of the perturbations in the rings are caused by its shepherd moons. Some of them are caused by Saturn itself. By studying those Saturn-caused perturbations, astronomers were able to calculate that Saturn rotated every 10 hours, 33 minutes, and 33 seconds.

Sends shivers up your spine, doesn’t it? It should. Such calculations surpass the mathematical dexterity of Maxwell over 100 years earlier, except that he had to do his 60 pages of mathematical calculations by hand. Now far-more-complex calculations have become computerized and commonplace.

If such mathematical legerdemain still sounds a bit nerdy and esoteric, then try this:

Cassini determined once and for all that Saturn’s enormous gravitational and magnetic fields were sucking the substance of its rings out of orbit. Soon, in cosmic time at least, Saturn will lose its staggeringly beautiful, easily visible rings entirely. All that may be left are faint, nearly invisible dust rings caused by micrometeorite impacts on some of Saturn’s more solid moons.

Better get to a telescope soon, my stargazing friends. In 100 million years, it will be too late.


By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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