Historical Background of Saturn's Rings
Written by Ron Baalke
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
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
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
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
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.
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.
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. Charles 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
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.
