Plate from ‘Systema Saturnium’ by Christiaan Huygens, 1659
Diagram showing how Saturn’s appearance to us changes due the changing positions of the Earth (E) and Saturn as they orbit the Sun (G).
Introduction by Ronald Brashear (Curator of Science and Technology Rare Books Special Collections Department Smithsonian Institution Libraries)
Christiaan Huygens (pronounced How’-kenz) was born in The Hague, Netherlands, on April 14, 1629. […] He published three mathematical books, produced a manuscript on hydrostatics, wrote a work on collision of elastic bodies, did research on centrifugal force, and invented the pendulum clock. […] Huygens was particularly intrigued by Saturn, mainly because of its puzzling appearance. To an observer using a telescope from the early 17th century, Saturn did not look like the other planets but at times appeared to have unexplainable protrusions extending out from either side. These protrusions were commonly referred to as ansae or “handles.”
When Huygens initially observed Saturn with his telescope (which provided a magnification of fifty), he […] realized that in order to learn more about the cause of the planet’s odd appearance he would have to construct a telescope with greater magnification and resolution. […]
Huygens was able, in February 1656, to resolve Saturn’s handles into a ring around the planet. Huygens wanted to make more observations of Saturn and develop a coherent theory to explain the ring before he published an official announcement. This would take time […] In the meantime, he penned a very brief treatise, De Saturni luna observatio nova [New observation of a moon of Saturn] (The Hague, 1656).
So as not to give away his explanation, Huygens disguised it in the form of an anagram:
a a a a a a a c c c c c d e e e e e h i i i i i i i l l l l m m n n n n n n n n n o o o o p p q r r s t t t t t u u u u u.
If anyone came forward with the same theory as Huygens, the latter would then reveal his anagram to be:
Annulo cingitur, tenui, plano, nusquam cohaerente, ad eclipticam inclinato [It is surrounded by a thin flat ring, nowhere touching, and inclined to the ecliptic].
[…] Huygens confided his Saturnian secret to few people. An important confidant was the well-respected Parisian astronomer Ismael Boulliau (1605-1694). Should anyone else discover Saturn’s ring, Boulliau could act as an independent authority, vouching for Huygens’s claim to priority. […]
In his letter of March 28, 1658, Huygens announced his discovery of the ring to Chapelain and gave him authority to present the announcement to the academy, whose members greeted it with much enthusiasm and praise. Huygens was satisfied with the extent of his research on Saturn by 1659, and his book, Systema Saturnium, was printed and ready for distribution by July of that year.
[…] On page 34, Huygens begins the discussion of the changing and unusual nature of Saturn’s appearance. He discusses earlier observations of the planet going back to Galileo, notes how these observations suffered from the use of inadequate telescopes, and goes into some detail on the hypotheses of Hevelius, Roberval, and Hodierna. After arguing against these explanations, Huygens offers his theory of a thick solid ring circling Saturn at its equator and in equilibrium under Saturn’s gravitational force. He then goes into detail about how the plane of the ring is tilted 20 degrees to the plane of Saturn’s orbit and that the ring maintains a constant orientation as the planet orbits the Sun. This means that the ring’s angle changes with respect to us and thus explained the varying appearance of Saturn. When the ring was edgewise to the Earth it would seem to practically disappear and then slowly the angle would change and the rings would open themselves back up to us. The book ends with Huygens’s observations of all the planets and his calculations of their sizes in relation to the Sun. […]
The concept of a ring around Saturn was generally accepted by 1670. What remained a mystery was the exact nature of the ring. Was it a solid thick ring as Huygens proposed? Always a point of contention, Huygens’s theory was weakened by the discovery of a gap in the supposedly solid ring by Giovanni Domenico Cassini, the director of the Paris Observatory, in 1675. Cassini also believed that the ring was actually composed of a large number of small satellites orbiting Saturn. In 1785, Pierre Simon, Marquis de Laplace demonstrated the mathematical instablility of solid rings orbiting around Saturn. James Clerk Maxwell wrote a mathematical essay in 1857 which destroyed the notion of a solid ring. His proof noted that the only possible explanation for the ring was that it was composed of small particles orbiting the planet and dense enough to give the appearance of a ring.
Finally, in 1895, Maxwell’s theory was proved by James E. Keeler at the Allegheny Observatory in Pittsburgh. Keeler used a spectroscope to show that the rings were actually rotating around Saturn and that the velocity of rotation could only be explained by a ring like the one described by Maxwell. Today, thanks to investigations made possible by unmanned spacecraft, we now know that the ring system is 270,000 km in diameter, but only a few hundred meters thick. There are four main ring groups and three more faint, narrow ring groups separated by gaps called divisions (the largest one being the Cassini Division). The rings are composed of particles which range from centimeters to tens of meters in size and are mainly made of ice (though there are traces of silicate and carbon minerals).