Ring Division

G.D. Cassini made the momentous discovery in 1675 of a thin dark line on Saturn's ring. It was presumed that this was a mark on the ring which divided the broad bright inner section from the narrower outer section. A year later, he noticed a 'belt' on the planet's disk, just south of the equator. Such latitudinal banding had been discovered on Jupiter in 1630 by Francesco Fontana of Naples, but Saturn's version was more subdued in character.

In 1705 J.J. Cassini (G.D. Cassini's son, and successor as director of the Paris Observatory) ventured that the rings comprised a myriad of meteoroids, but he had no evidence to support this view. The elder Cassini had hired a nephew, G.F. Miraldi, as an assistant. Miraldi's observations of irregularities moving to and fro in the ring plane when it was almost edge-on in 1714 was the first evidence of rotation.

Cassini Drawing Saturn Rings Division
Although G.D. Cassini's discovery of a dark division in Saturn's ring was made in 1675, he produced this drawing the following year. Notice also the depiction of the band on the planet.

After reviewing all the evidence, he concluded that the dark line was a gap separating two concentric rings, each of which was a rigid structure turning synchronously with the planet. Proof that Cassini's Division, as the line became known, was indeed a gap came in October 1852, when William Lassell in England noted that a slice of the planet's disk was visible through it. Considering the obliqueness of the line of sight where the ring system crosses in front of the globe, it was a remarkable observation. In fact, such a line of sight is feasible only when the ring system is open wide and when the Sun is at the same angle to the ring plane as the viewing angle, allowing the slice of the globe visible through the gap to be illuminated; at other times, the part of the globe observed through the gap is in the ring system's shadow.

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