Cassini and the Division in Saturn's Ring

In 1730 a paper written much earlier by Giovanni Domenico Cassini entitled The Discovery of the Division in Saturn's Ring appeared in Volume X of the Mémoires de I'Académie Royale des Sciences. The paper, of course, was written in French but we give below an English translation:-

The Discovery of the Division in Saturn's Ring

After the discoveries which have been made at different times concerning the globe of Saturn, its ring and its satellites, in part by Huygens who discovered one of the satellites which revolves around Saturn in 16 days less 47 minutes, and in part by Cassini who discovered two others of which we will give the history at an early date, it seemed that there was nothing more to discover concerning the planet; however, the latest observations that Cassini has made concerning the body of Saturn and its ring, show that in the Heavens as well as on the Earth, something new to observe always appears.

After the emergence of Saturn from the rays of the Sun as a morning star in the year 1675, the globe of the planet appeared with a dark band, similar to those of Jupiter, extending the length of the ring from East to West, as it is nearly always shown by the 34-foot telescope, and the breadth of the ring was divided by a dark line into two equal parts, of which the interior and nearer one to the globe was very bright, and the exterior part slightly dark. There was about the same difference between the colours of these two parts that there is between dull silver and burnished silver, which had never before been observed but which has since been seen in the same telescope, more clearly at twilight and in moonlight than on a darker night.

This appearance gave an impression of a double ring, of which the inferior ring, being larger and darker, had superposed upon it another that is narrower and brighter, and reminds one that in the year 1671, when the extensions of Saturn were on the verge of disappearing they contracted beforehand, perhaps because the outer part of the ring, which was single and dark, disappeared before the inner part, which was double and brighter.

In the same year, 1671, the shorter diameter of the ring was still less than the diameter of the globe which extended outside the ring on the North and South sides, and this phase lasted until the immersion of Saturn in the rays of the Sun in the year 1676. But after its emersion, which took place last summer, the shorter diameter of the ring exceeded that of the globe. There is an observation by Hevelius in the English Journal, which corresponds to the first of these two phases; but as he has noted neither the band of Saturn, nor the distinction which can he seen in the ring, one has reason to judge that the telescopes which he uses are much inferior to those of the Royal Observatory.

JOC/EFR March 2006

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