Orwell Astronomical Society (Ipswich)
Saturn - A View From The Past
During the months of Autumn 1993, Saturn has been the only planet of note in the evening sky and, although it is still low in the southerly part of its orbit, observers obtained some fine views of its rings. Every planetary observer nowadays is familiar with the magnificent views of the rings returned by the spacecraft Pioneer 11 in August/September 1979, Voyager 1 in October/November 1980 and Voyager 2 in August/September 1981. However, there is no substitute for observing the rings directly through a telescope and gazing at one of the most spectacular sights in the heavens!
When the spacecraft showed that the rings were composed of hundreds of tiny ringlets, it seemed to come as a surprise to the scientific community. However, in the 1960s, I read the book The Planet Saturn by A F O'D Alexander. The book is a record of observations of Saturn up to about 1960; it makes fascinating reading and I remembered that it contains many references to reports by some of the great observers of the past of the rings appearing to be composed of a great number of narrow ringlets. (The OASI library holds a copy of the book, donated by Mike Harlow in 1988).
Prior to 1785, astronomers assumed that the rings of Saturn were solid. Laplace was concerned about the stability of a single, solid ring and proposed instead the existence of many thin, narrow, solid ringlets; his considerations prompted suspicion among astronomers of the existence of subdivisions within the rings. However, William Herschel, observing at around this time, was convinced of the solidity of the ring and did not generally observe any subdivisions except for Cassini's division. Despite this, his observing records on four occasions refer to additional markings on Ring B.
Observations of ring subdivisions seem to have started in the early l9th century. In 1825, Captain Henry Kater, vice president and treasurer of the Royal Society, was testing a new 16 cm reflector and observed several divisions in Ring A: I fancied that I saw the outer ring separated by numerous dark divisions, extremely close, one stronger than the rest, dividing the ring about equally. In 1851, a remarkable observation by Charles W Tuttle records that under some exceptionally clear conditions he found Ring B to be Minutely subdivided into a great number of narrow rings. Two other astronomers observing with him also witnessed the phenomenon. Tuttle's description later, from memory, was that the divisions were not unlike a series of waves; the depressions corresponding to the spaces between the rings, while the summits represented the narrow bright rings themselves. The rings and the spaces between them were of equal breadth... Dawes and Lassell also recorded subdivisions at this time.
An even more remarkable observation from the space probes was the radial spokes visible on Ring B; these are features that form and disappear in a matter of hours. In 1896, the famous planetary observer E M Antoniadi observed unusual markings on both bright rings - subdivisions in Ring B and white spots separated by dusky radial streaks on Ring A. At the time, these were thought to be illusory, but were they? The sketch made by Antoniadi is copied below and shows the spoke-like markings seen on Ring A, very reminiscent of the spokes seen on the Voyager images on Ring B. Perhaps they were real?
Saturn is a beautiful object for observation. Using high powers on good, clear nights, you may be surprised at the detail that can be seen under rare moments of exceptional seeing!