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Rings of Saturn

Historical Background of Saturn's Rings

(Courtesy of NASA/JPL/Ron Baalke)

1600 - 1699

1610 - Galileo Galilei becomes the first to observe Saturn's rings with his 20-power telescope. He thought the rings were "handles" or large moons on either side of the planet. He said "I have observed the highest planet [Saturn] to be tripled-bodied. This is to say that to my very great amazement Saturn was seen to me to be not a single star, but three together, which almost touch each other".

sketch of Saturn by Galileo in 1610

Sketch of Saturn by Galileo in 1610

1612 - Galileo was astounded when he found that the rings he first observed a couple of years earlier had now disappeared. He wrote "I do not know what to say in a case so surprising, so unlooked for and so novel". The rings were, in fact, edge-on from Earth's perspective. Galileo inadvertently became the first person to observe a Saturn ring plane crossing.

1616 - Galileo now observes the rings as two half ellipses. He wrote "The two companions are no longer two small perfectly round globes ... but are present much larger and no longer round ... that is, two half ellipses with two little dark triangles in the middle of the figure and contiguous to the middle globe of Saturn, which is seen, as always, perfectly round".

Sketch of Saturn by Galileo in 1616

Sketch of Saturn by Galileo in 1616

1626 - The ring plane crossing for this year went virtually unnoticed.

1655 - Christaan Huygens proposes that Saturn was surrounded by a solid ring, "a thin, flat ring, nowhere touching, and inclined to the ecliptic." Using a 50 power refractor that he designed himself, Huygens also discovers the first Saturn moon, Titan, just 7 months before the ring plane crossing for that year.

Huygens' Sketch of Saturn in 1655

Huygens' Sketch of Saturn in 1655

1656 - Johannes Hevelius postulates that Saturn's rings were two crescents attached to an ellipsoidal central body.

1658 - Christopher Wren argues that an elliptical corona was attached to Saturn, and the planet and corona rotated about the major axis of the corona. He speculated that this corona was so thin that it was invisible when it was edge-on from Earth's perspective.

1659 - Christaan Huygens publishes his book, Systema Saturnium, in which he explains that every 14 to 15 years the Earth passes through the plane of Saturn's ring.

1660 - Jean Chapelain makes an insightful suggestion that Saturn's rings are made up of a large number of very small satellites. Since most astronomers of the time strongly believed that Saturn's ring was solid (with one notable exception: Cassini), Chapelain's suggestion went mostly unnoticed. It would be another 200 years until Maxwell in 1856 makes a similar deduction.

1664 - Giuseppe Campani observes that the outer half of Saturn's ring is less bright than the inner half, but fails to recognize this as being two separate rings.

1671-1672 - Giovanni Cassini discovers two new Saturn moons, Iapetus and Rhea, during the ring plane crossings of 1671-1672. Cassini first observed Iapetus in 1671 on the west side of Saturn, but failed to observe it on the east side in 1672 even though he knew it was there. He correctly postulated that Iapetus had light and dark sides, and it always kept the same face turned to Saturn.

1676 - Giovanni Cassini discovers a gap in the rings which would later be named the Cassini Division. The outer ring would be called the A Ring and the brighter inner ring would be called the B Ring.

Cassini's Sketch of Saturn in 1676 Showing the Gap in the Rings

Cassini's Sketch of Saturn in 1676 Showing the Gap in the Rings

1684 - Giovanni Cassini discovers two more Saturn moons, Tethys and Dione, just prior to the ring plane crossing of 1685. It would be over 100 years until the next Saturn moon would be discovered.

1700 - 1799

1780 - William Herschel reports seeing a "black list", or linear markings on one side of the B Ring near its inner edge. He had probably observed what would later be called the Encke Division.

1787 - Pierre de Laplace suggests that Saturn has a large number of solid rings. William Herschel suspects he has observed a new moon of Saturn (which would later turn out to be Enceladus), but waits until the next ring plane crossing in 1789 to pursue it further.

1789 - William Herschel suggests that Saturn is surrounded by two solid rings. Herschel also discovers two new Saturn moons, Enceladus and Mimas, during the ring plane crossing of 1789-1790. Herschel also finds that Saturn is flattened at its poles, something he suspected since 1776. He also makes one of the earliest estimates of the thickness of the rings at 300 miles, and reports observations of eclipses of Saturn's moons by the planet's shadow.

1790 - William Herschel is able to determine the rotation period of Saturn's ring to be 10 hours 32 minutes.

1800 - 1899

1825 - Henry Kater reports seeing three gaps in the A Ring, but no one else could verify Kater's claim for several years.

1835 - Friedrich Bessel is able to determine the orientation of Saturn's pole with unprecedented accuracy and was able to determine the precession period of the pole at 340,000 years. (The modern estimate is 1.7 millions years).

1837 - Johann Encke observes a dark band in the middle of the A Ring that matches one of the gaps that Kater observed in 1825. This dark band in the A Ring would be later known as Encke's Division, even though Encke never really observed it as a gap in the rings.

1848 - William Bond, George Bond and William Lassell discover a new Saturn moon, Hyperion, during the ring plane crossing of 1848-1849. William Bond and George Bond observe that the unlit side of the rings was barely visible, and infer a ring thickness of 40 miles.

1849 - Edouard Roche suggests that Saturn's ring system was formed when a fluid satellite had approached Saturn so closely that it had been torn apart by tidal forces.

1850 - William Bond and George Bond observe a dark band across Saturn immediately adjacent to the interior edge of the B Ring. Charels Tuttle suggests that this might be caused by a dusky ring inside the B Ring. This ring was intially known as the crepe ring, and later officially became the C Ring. George Bond concludes that a system of narrow solid rings could not be stable and that Saturn's rings had to be fluid.

1852 - Several observers notice that the limbs of Saturn are visible through the C Ring. This observation made it difficult to defend the theory that the rings were solid.

1856 - James Maxwell deduces that the Saturn rings cannot be solid and must be made of "an indefinite number of unconnected particles".

1861-1862 - The dark unlit side of Saturn's rings was observed by James Carpenter, William Wray and Otto Struve.

1866 - Daniel Kirkwood notices that a particle in the Cassini Division would be in 3:1 resonance with the peroid of Enceladus.

1872 - Daniel Kirkwood is able to associate the Cassini Division and Encke's Division with resonances of the four then known interior moons: Mimas, Enceladus, Tethys and Dione.

1876 - Spectacular white spots are observed on Saturn by Asaph Hall.

1883 - The first photograph of Saturn's rings is taken by Commons.

The First Photograph of Saturn by Commons, 1883

The First Photograph of Saturn by Commons, 1883

1888 - James Keeler becomes the first person to clearly observe the Encke Division (Encke only saw it as a dark band in 1837).

1889 - Edward Barnard observes an eclipse of Iapetus by Saturn's rings. Watching Iapetus become dimmer going through the shadow of the C Ring and disappear completely in the shadow of the B Ring, Barnard concludes that the C Ring must be semitransparent and that the B Ring is opaque.

1895 - James Keeler and William Campbell observe that the inner part of the rings orbit more rapidly than the outer part of the rings, confirming Maxwell's deduction in 1856 that Saturn's rings were made up of small particles.

1898 - William Pickering discovers a new Saturn moon, Phoebe. This is the first and only Saturn moon discovered from ground based observations that was *not* around the time of a ring plane crossing.

1900 - 1999

1903 - Edward Barnard observes a white spot on Saturn.

1907-1908 - Observations during the 1907-1908 ring plane crossing led to an estimate for the ring's thickness of no more than 15 km.

1911 - Edward Barnard takes a photograph of Saturn from Mount Wilson Observatory which shows that Saturn is visible through the A Ring.

1921 - The first observation of a mutual event, an eclipse by Rhea by Titan, is observed by L Comrie and A. Levin.

1923 - B. Lyot was able to detect the polarization of light scattered by Saturn's rings.

1933 - Will Hay observes a brilliant white spot on Saturn.

1936 - Leslie Comrie and Bertrand Peek predict that the ring inclination would only be 0.0001 degrees, and that the Earth would not cross the ring plane. Observations seem to bear out this prediction.

1940's - Harold Jefferys provides definitive work showing that Saturn's rings must be composed of separate solid bodies.

1960 - J. Botham observes a white spot on Saturn, and a 30 year cycle for the white spot appearances has been established.

1966 - Allegheny Observatory photographs what would be called the E Ring. During the 1966 Saturn ring crossing event, two new moons were discovered around Saturn: Epimetheus by Stephen Larson, John Fountain and R. Walker, and Janus by Audouin Dollfus. Using the Pic du Midi Observatory, Dollfus is able to establish the actual edge-on brightness of the rings, and estimates the thickness of the rings at only 2.4 km.

1967 - Walter Feibelman discovers the E Ring from the Allegheny Observatory images taken the year before.

1969 - Pierre Guerin finds evidence of a possible D Ring.

1970 - Measurements of the spectra of the rings in the near infrared strongly indicates the presence of water ice, which also indicates the surface of the ring particles is predominatey water ice.

1973 - Pioneer 11 is launched to encounter Saturn in 1979.

1977 - Voyager 1 and Voyager 2 are launched to encounter Saturn in 1980 and 1981. Astronomers discover rings around Uranus, making Saturn no longer the only planet with rings. Stephen O'Meara observes dark radial features (which would later be called "spokes") on Saturn's rings and records them in a sketch. (No one pays much attention to this until Voyager 1 sees these spokes up close in 1980).

1978 - H. Reitsema establishes the existence of the Encke Division with his observations of Iapetus as it was eclipsed by the rings. Peter Goldreich and Scott Tremaine propose that density waves are created in Saturn's rings due to resonance of ring particles with moons.

1979 - Pioneer 11 flies by Saturn and discovers the F Ring, and confirms the E Ring. Peter Goldreich and Scott Tremaine propose that shepherd moons could contain a narrow ring. (The discovery of the narrow F Ring and associated shepherd moons by Voyager 1 in 1980 would bear out this theory).

1980 - Three new Saturn moons are discovered during the 1979-1980 ring plane crossing. Teletso is discovered by Brad Smith, S. Larsen and R. Walker (University of Arizona). Calypso is discovered by D. Pascu, P. Seidelmann, W. Baum and D. Currie. Helene is discovered by P. Laques, J. Lecacheux (Pic du Midi Observatory). Bruno Sicardy and Andre Brahic are able to measure the thickness of the B Ring at about 1.1 km. Stephen Larson and William Baum observes that the E Ring extends from the orbit of Mimas to 8 Saturn radii near the orbit of Rhea. The peak brightness of the E Ring corresponds with the orbit of Enceladus, indicating that this moon may possibly be the source of the particles for the E Ring.

1980-1981 - Voyager 1 and Voyager 2 flyby Saturn, and gather a treasure trove of new information on the rings. The G Ring is discovered, "spokes" are observed on the B Ring, and braiding is seen in the F Ring. Additionaly, three new moons are discovered by Voyager: Rich Terrile finds Atlas in the Voyager images, and Prometheus and Pandora are the first shepherding moons ever discovered. Some of the moons are found sharing the same orbit, and these are called co-orbitals. The rings are found to be made up of thousands of ringlets. Ringlets are even found in gaps in the rings. Three new gaps discovered by Voyager are named the Maxwell Gap, Huygens Gap and the Keeler Gap.

1981 - J. Lissauer and M. Henon propose moonlets are embedded in the ring system. (Mark Showalter would discover Pan in the Encke Division in 1990).

1990 - While analyzing Saturn images taken by Voyager 2, Mark Showalter discovers a new moon, Pan, orbiting in the Encke Division of Saturn's rings. Stuart Wilbert observes the expected white spot on Saturn, last seen in 1960.

1995 - Mitchell Gordon, Carl Murray and Kevin Beurle report on the possible existence of additional new moons of Saturn based on their analysis of the Voyager 2 images.

1995-1996 - A triple plane crossing event will occur. This will be the last triple plane crossing until the year 2038. During the first ring plane crossing on May 22, 1995, the Hubble Space Telescope discovers two, and possibly four, new moons of Saturn.

1997 - The Cassini spacecraft will be launched and arrive at Saturn in 2004. It will also drop off an atmospheric probe, Huygens, which will land on the surface of Titan.

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