After developing an improved technique to grind lenses to precise shapes, Huygens constructed an improved 50-power telescope that helped him identify the unusual elongation of Saturn as a ring or disk surrounding the planet. Huygens also discovered Titan, Saturn’s largest moon, and showed that the Orion nebula was composed of many stars.

In 1610, Galileo Galileo;Saturn and was the first to observe Saturn with a telescope. He recorded that Saturn had an odd appearance, with projections that appeared to be “handles” at both sides. Galileo, however, did not understand his observations. He thought the handles could be two large moons, one on each side of the planet, so he described Saturn as a group of three, nearly touching objects that do not move relative to one another. Two years later, in 1612, Galileo became even more puzzled when he observed that Saturn’s “handles” had disappeared. [kw]Huygens Identifies Saturn’s Rings (Feb., 1656)
[kw]Rings, Huygens Identifies Saturn’s (Feb., 1656)
[kw]Saturn’s Rings, Huygens Identifies (Feb., 1656)
Astronomy;Feb., 1656: Huygens Identifies Saturn’s Rings[1900]
Physics;Feb., 1656: Huygens Identifies Saturn’s Rings[1900]
Mathematics;Feb., 1656: Huygens Identifies Saturn’s Rings[1900]
Science and technology;Feb., 1656: Huygens Identifies Saturn’s Rings[1900]
Netherlands;Feb., 1656: Huygens Identifies Saturn’s Rings[1900]
Huygens, Christiaan
Astronomy;Saturn’s rings[Saturns rings]

Although Saturn’s ring system was first observed by Galileo, Dutch physicist and mathematician Christiaan Huygens is credited with their discovery because he was the first person to identify the observed elongation of Saturn as the presence of a disk or ring surrounding the planet.

Huygens had studied law and mathematics at the University of Leiden from 1645 until 1647, and he published a series of papers on mathematics, but actually he had trained to be a diplomat. In 1649, Huygens was a member of a diplomatic team that was sent to Denmark, but he was not offered a permanent position in diplomacy. In 1650, Huygens returned home and lived on an allowance from his father.

Both Huygens as well as his brother Constantine were interested in astronomy, but they found that the telescopes Telescope then available were too short to resolve features on the planets. The brothers gained an interest in lens grinding and telescope construction to improve the quality of their observations, and, around 1654, they developed a new and better way of grinding lenses for telescopes. Their techniques significantly reduced chromatic aberration, an effect that causes simple lenses to focus different colors of light at different points of the telescope lens. They also introduced the use of optical stops, masks along the tube of a telescope that intercept light reflected from the walls of the tube, keeping reflected light from reaching the lens and blurring the image.

Christiaan Huygens.

(Library of Congress)

Using one of his own lenses, Christiaan Huygens built a self-designed 50-power refracting telescope. With this new telescope, in 1655, he discovered Titan, the first and largest moon of Saturn. Later that year, he visited Paris and informed the astronomers there, including Ismaël Boulliau Boulliau, Ismaël (1605-1694), of his discovery. By this time, Boulliau was a well-recognized astronomer who had published his Astronomia philolaica
Astronomia philolaica (Boulliau) (1645), in which he adopted Johannes Kepler’s Kepler, Johannes idea that planets moved in elliptical orbits around the Sun.

Huygens’s discovery of Titan was near the time of the “ring plane crossing” phenomenon, that is, when Saturn’s rings are viewed edge-on from Earth, making them difficult to see. Thus, Huygens was unable to see the rings when he discovered Titan. In February of 1656, the true shape of the Saturn’s rings was apparent to Huygens. He recognized that the bulge, which Galileo thought were two moons, actually was a thin, flat disk or ring, which did not touch the planet and was inclined to the ecliptic plane.

Huygens reported his conclusions in a message to Boulliau, in order to establish the priority of his discovery. However, Huygens did not make a public announcement of his results until 1658, in a letter to the scientific academy in Paris.

Huygens’s description of Saturn’s rings was not immediately accepted. At least three other astronomers offered different explanations for Saturn’s bulge after Huygens’s discovery. Gilles Personne de Roberval Roberval, Gilles Personne de (1602-1675) proposed that Saturn emitted vapors, like a volcano, from its equatorial region. When the concentration of vapors was high enough, they would become visible as a belt around the planet. Johannes Hevelius, Hevelius, Johannes an astronomer from Gdansk, proposed that Saturn was not a sphere, but rather an ellipsoidal, and the bulge was simply part of the planet. Giovanni Battista Odierna Odierna, Giovanni Battista (1597-1660) suggested that Saturn had two large dark areas at its equator, which appeared to observers as “handles.”

Even with the excellent view of Saturn that Huygens had through his improved telescope, it was not until 1659 that he correctly inferred the geometry of Saturn’s rings, because he had to wait until he had observed them over a significant part of their cycle. In his Systema Saturnium, sive de causis mirandorum Saturni phænomenôn, et comite ejus planeta novo
Systema Saturnium (Huygens) (1659; the system of Saturn, or on the matter of Saturn’s remarkable appearance, and its satellite, the new planet; better known as Systema Saturnium), Huygens explained the phases and changes in the shape of the ring based on the expected view of a rigid disk surrounding the planet and inclined relative to Earth’s orbital path around the Sun. Huygens noted that all earlier observations of Saturn suffered from inadequate resolution. He argued against the models proposed by Roberval, Hevelius, and Hodierna, and offered his idea of a disk surrounding Saturn at its equator but tilted at an angle of about 20 degrees to the plane of Saturn’s orbit. He explained that this tilt is what causes the appearance of Saturn’s ring to vary as Saturn moves around the Sun.

Although Boulliau generally accepted Huygens’s idea of a ring, he believed the ring should still be seen from Earth even when edge-on. Many other astronomers were not convinced. In 1660, Eustachio Divini Divini, Eustachio (1610-1685), an Italian instrument (and telescope) maker, published his “Brevis annotatio in Systema Saturnium Christiani Eugenii” (brief comment on Christian Huygens’s Systema Saturnium), which attacked not only Huygens’s ring theory but also the validity of his observations. This book suggested Saturn had four moons, two dark ones near the planet and two bright ones farther out. The handles appeared when the bright moons were in front of the dark ones, partially blocking them from Earth.

Huygens quickly replied with his “Brevis assertio Systematis Saturnii sui” (1660; brief defense of Systema Saturnium), pointing out that the work of other astronomers contained incorrect observations, which could only be explained by their use of inferior telescopes. Hevelius accepted the ring theory after reading “Brevis assertio Systematis Saturnii sui.” By 1665, the matter was finally settled, when telescope quality had improved to the point that most astronomers were able to replicate Huygens’s observations.

The question that faced the astronomers next was how such a disk could be stable. Huygens thought the ring was a solid structure, but Gian Domenico Cassini Cassini, Gian Domenico proposed that the ring consisted of a large number of small particles, all orbiting around Saturn. Cassini, who conducted extensive observations of Saturn using telescopes at the new Paris Observatory, noted that there was a dark gap separating the ring into two separate rings. This showed that Saturn’s rings could not be a single, rigid disk, as proposed by Huygens.

In was not until 1858 that James Clerk Maxwell (1831-1879), a Scottish physicist, was able to perform a detailed mathematical analysis that showed how a ring composed of many tiny particles could be stable. By the end of the nineteenth century, astronomers were able to measure the speed of the particles at the inner and outer edges of the ring. This measurement was inconsistent with a solid rotating disk, and it agreed with the orbital speeds calculated from Kepler’s laws of motion.

Huygens also used his improved telescope to view other astronomical objects. In 1656, he observed the Orion nebula and was able to show that it consisted of a large group of individual stars. He also discovered several other nebulae and double stars.

Huygens’s greatest contribution to astronomy was the improvement of techniques for making telescopes. This work significantly improved their resolution, making it possible for him and future astronomers to resolve details that had been hidden from earlier astronomers. The improved telescope allowed Huygens to discover Titan, Saturn’s largest moon, and Saturn’s ring system. The rings around Saturn remained a unique planetary feature until 1977, when fainter rings were discovered around Uranus, and shortly after, around the two other gas giant planets, Jupiter and Neptune.

Even more important than these observations, however, was Huygens’s insight that the Saturnian system was a miniature solar system, with Titan orbiting Saturn the way Earth orbits the Sun, as Nicolaus Copernicus and Kepler had proposed. Thus, Huygens’s observations supported the Copernican idea of a Sun-centered (heliocentric) rather than an Earth-centered (geocentric) solar system. His work was done at a time when a great debate on the issue of a heliocentric versus geocentric system was raging among the best minds in astronomy in Europe.

Because of his great contribution to the understanding of Saturn, the National Aeronautics and Space Administration (NASA) named its Titan space probe the Huygens probe.

• Alexander, Arthur F. O’Donel. The Planet Saturn: A History of Observation, Theory, and Discovery. 1962. Reprint. New York: Dover, 1980. A classic work on the scientific history of Saturn. Includes an index.
• Andriesse, Cornelis D. Christian Huygens. Paris: Albin Michel, 2000. An excellent account of the life and achievements of Huygens.
• Bell, A. E. Christian Huygens and the Development of Science in the Seventeenth Century. London: Edward Arnold, 1947. An excellent account of Huygens’s contributions to astronomy and mathematics.
• Brashear, Ronald. “Christiaan Huygens and His Systema Saturnium.” http://www.sil.si.edu/DigitalCollections/HST/Huygens/huygens.htm. Accessed February, 2005. Brashear, the curator of Science and Technology Rare Books in the Special Collections Department of the Smithsonian Institution Libraries, provides an introduction to Huygens’s major work on Saturn. The site also includes a digital version of the work in its original Latin.
• Moore, Patrick. Eyes on the University: The Story of the Telescope. New York: Springer-Verlag, 1997. Written to commemorate the fortieth anniversary of the British television series “The Sky at Night,” this illustrated account of the telescope’s history is intended for general readers and amateur astronomers.
• North, John. The Norton History of Astronomy and Cosmology. New York: Norton, 1995. North, a professor in the history of the exact sciences, discusses the telescope as part of the evolution of astronomy. Includes an index and a thirty-four-page bibliographical essay.
• Struik, Dirk J. The Land of Stevin and Huygens: A Sketch of Science and Technology in the Dutch Republic During the Golden Century. Boston: Kluwer, 1981. A short, illustrated work that centers on Huygens as the major claim to fame of the Netherlands for the seventeenth century scientific revolution.
• Yoder, Joella G. Unrolling Time: Huygens and the Mathematization of Nature. New York: Cambridge University Press, 2004. A 252-page account of the interrelationship between mathematics and physics in the work of the Dutch mathematician, physicist, and astronomer.

Gian Domenico Cassini; Galileo; Johannes and Elisabetha Hevelius; Christiaan Huygens; Johannes Kepler. Huygens, Christiaan
Astronomy;Saturn’s rings[Saturns rings]