page_title: Rings of Saturn
section_title: Ring theory, observations and exploration
hierarchical_section_title: Rings of Saturn / History / Ring theory, observations and exploration
caption_attribution_description: English: Old drawing of Saturn by Robert Hooke. R indicates Saturn's globe, the two letters a indicate the expected overlap of the globe on the rings. Hooke's shading on the rings to the right of the right-hand a only (i.e. not at the left-hand a), indicates the shadow of the globe on the rings. The two letters b indicate the expected overlap of the rings on the globe. Hooke's shading on the globe between the right-hand and left-hand bs could indicate the shadow of the rings on the globe, or the faint C-ring faintly blocking part of the globe. The abbreviations marked around Saturn indicate directions: Sep. for Septentrional or northern; Mer. for Meridional; Occ. for Occidental or western; and Ori. for Oriental or eastern.
context_page_description: The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit about Saturn. The ring particles are made almost entirely of water ice, with a trace component of rocky material. There is still no consensus as to their mechanism of formation. Although theoretical models indicated that the rings were likely to have formed early in the Solar System's history, new data from Cassini suggest they formed relatively late.
  Although reflection from the rings increases Saturn's brightness, they are not visible from Earth with unaided vision. In 1610, the year after Galileo Galilei turned a telescope to the sky, he became the first person to observe Saturn's rings, though he could not see them well enough to discern their true nature. In 1655, Christiaan Huygens was the first person to describe them as a disk surrounding Saturn.
context_section_description: In 1657 Christopher Wren became Professor of Astronomy at Gresham College, London. He had been making observations of the planet Saturn from around 1652 with the aim of explaining its appearance. His hypothesis was written up in De corpore saturni, in which he came close to suggesting the planet had a ring. However Wren was unsure whether the ring was independent of the planet, or physically attached to it. Before Wren's theory was published Christiaan Huygens presented his theory of the rings of Saturn. Immediately Wren recognised this as a better hypothesis than his own and De corpore saturni was never published.
  Huygens was the first to suggest that Saturn was surrounded by a ring detached from the planet. Using a 50× power refracting telescope that he designed himself, far superior to those available to Galileo, Huygens observed Saturn and in 1656, like Galileo, published the anagram "aaaaaaacccccdeeeeeghiiiiiiillllmmnnnnnnnnnooooppqrrstttttuuuuu". Upon confirming his observations, three years later he revealed it to mean "Annuto cingitur, tenui, plano, nusquam coherente, ad eclipticam inclinato"; that is, "It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic". Robert Hooke was another early observer of the rings of Saturn, and noted the casting of shadows on the rings.
  In 1675, Giovanni Domenico Cassini determined that Saturn's ring was composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division. This division is a 4,800-km-wide region between the A ring and B Ring.
  In 1787, Pierre-Simon Laplace proved that a uniform solid ring would be unstable and suggested that the rings were composed of a large number of solid ringlets.
  In 1859, James Clerk Maxwell demonstrated that a nonuniform solid ring, solid ringlets or a continuous fluid ring would also not be stable, indicating that the ring must be composed of numerous small particles, all independently orbiting Saturn. Later, Sofia Kovalevskaya also found that Saturn's rings cannot be liquid ring-shaped bodies. Spectroscopic studies of the rings carried out in 1895 by James Keeler of Allegheny Observatory and Aristarkh Belopolsky of Pulkovo Observatory showed Maxwell's analysis was correct.
  Four robotic spacecraft have observed Saturn's rings from the vicinity of the planet. Pioneer 11's closest approach to Saturn occurred in September 1979 at a distance of 20,900 km. Pioneer 11 was responsible for the discovery of the F ring. Voyager 1's closest approach occurred in November 1980 at a distance of 64,200 km. A failed photopolarimeter prevented Voyager 1 from observing Saturn's rings at the planned resolution; nevertheless, images from the spacecraft provided unprecedented detail of the ring system and revealed the existence of the G ring. Voyager 2's closest approach occurred in August 1981 at a distance of 41,000 km. Voyager 2's working photopolarimeter allowed it to observe the ring system at higher resolution than Voyager 1, and to thereby discover many previously unseen ringlets. Cassini spacecraft entered into orbit around Saturn in July 2004. Cassini's images of the rings are the most detailed to-date, and are responsible for the discovery of yet more ringlets.
  The rings are named alphabetically in the order they were discovered  (A and B in 1675 by Giovanni Domenico Cassini, C in 1850 by William Cranch Bond and his son George Phillips Bond, D in 1933 by Nikolai P. Barabachov and B. Semejkin, E in 1967 by Walter A. Feibelman, F in 1979 by Pioneer 11, and G in 1980 by Voyager 1). The main rings are, working outward from the planet, C, B and A, with the Cassini Division, the largest gap, separating Rings B and A. Several fainter rings were discovered more recently. The D Ring is exceedingly faint and closest to the planet. The narrow F Ring is just outside the A Ring. Beyond that are two far fainter rings named G and E. The rings show a tremendous amount of structure on all scales, some related to perturbations by Saturn's moons, but much unexplained.
caption: Robert Hooke noted the shadows (a and b) cast by both the globe and the rings on each other in this 1666 drawing of Saturn.
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