A flat ring nowhere connected

narrow arms projecting to either side of the planetary disk. In early 1656, using a refractor with a 2.5-inch lens, a 23-foot focal length and a magnification of 100, he noted that the ansae had disappeared. However, the narrow arms reappeared in October, and using an 'arial' refractor with a focal length of 136 feet, Huygens later noticed that a shadow had been cast on the planet - an observation which showed that the ansae were not merely crescent-forms 'alongside' the globe, but formed a structure that continued in front of it.

In 1650 an Italian Jesuit, F.M. Grimaldi, noted that Saturn's globe was not round, but was flattened at the poles, implying that the planet was in a state of

A depiction of Christiaan Huygens observing with his 136-foot-focal-length 'arial' refractor.

In his Systema Saturnium in 1659, Christiaan Huygens showed how an inclined ring explained how Saturn's appearance varies in a cyclical manner, including how it can 'disappear' as the Earth passes through the ring plane.

rotation. Even though the period of its rotation had yet to be determined, it was unlikely to be outlandishly long, so Huygens rationalised that if the ansae structure was rotating with the planet the only physical form that could maintain its appearance was an annular disk. After compiling observations for several more years, Huygens published Systema Saturnium in 1659 in which, after summarising earlier observations, he announced that Saturn's globe was ''surrounded by a flat ring, nowhere connected to the body of the planet, and is inclined to the ecliptic''. In fact, to assure his claim of precedence in this discovery, on 5 March 1656 he had appended a note - in cipher, naturally - to a short paper on his discovery of the satellite the previous year.

With fifty years of observations to consider, Huygens realised that the ring had been viewed at a variety of angles as Saturn had pursued its 29-year orbit around the Sun. The plane of the ring is significantly inclined from the ecliptic: for a part of Saturn's orbit the ring is tilted Earthward and is viewed obliquely from 'above'; during the second part of the orbit it is viewed from 'below'. Huygens realised that the ring becomes 'invisible' when the Earth passes through its plane - a perspective that occurs at 14.5-year intervals. The ring had been absent for Galileo in 1612, absent in 1641, and absent again in early 1656. In 1655, when Huygens had first inspected the planet, the virtually edge-on ring had projected its shadow onto the northern hemisphere. After the ring-plane crossing, the shadow had been on the southern hemisphere. In the first case, the southern face of the ring had been illuminated and, later, the northern face had been illuminated. William Ball of Exeter in England had noted the shadow as a thin band on the disk between February and July 1656, when the ansae had been absent. This sequence of observations during this very dynamic time were crucial to Huygens's insight.

Although Huygens's analysis accounted for all the observations, the suggestion of a ring was so ridiculous that it was rejected by some of his contemporaries. In 1658 Christopher Wren had suspected that Saturn had an elliptical corona that was in

Christiaan Huygens's insight into the ring-form of Saturn's ansae derived from his observation that the shadow cast by the rings appeared in opposite hemispheres in 1655 and 1656, in between which the ansae were edge-on and hence invisible.

contact with the globe in two places and rotated with it, but before he could publish his idea he found Huygens's analysis to be more compelling. G.B. Hodierna, a Sicilian priest and mathematician suggested that Saturn was a distended spheroid on which there were two dark patches; however, the dynamics of such an arrangement were untenable. In 1660 Honore Fabri, a Jesuit philosopher, pontificated rather strangely that Saturn had a pair of very large but unreflective satellites close in and a pair of somewhat smaller but reflective ones farther out, but the dynamics of such a system were again untenable. G.P. de Roberval, a mathematician and a founder member of the French Academy of Sciences, argued that Saturn was surrounded by ''a torrid zone given off by vapours'' which was transparent if present in small quantity, reflected sunlight at the edges if present in medium quantity, and appeared as an ellipse if present in large quantity.

Huygens predicted that after the viewing angle had opened and closed again, the rings would disappear in July 1671 (he was slightly out, because the ring-plane crossing actually occurred in May of that year). By that time, however, there was no doubt of the truth of his interpretation because improved telescopic observations in 1665 had given conclusive proof. Indeed, in 1662 Guiseppe Campani in Italy and Adrien Auzout in France had independently observed the shadow of the planet cast upon the ring, clipping a narrow section out of one of the ansae.

Later in life, Huygens founded the Academie Royale des Sciences, and became its first director. His many contributions to mathematics, astronomy, time measurement and the theory of light are regarded as fundamental.

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