Saturnus Triformis

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In the same year that Kepler announced the laws that governed the movements of the planets, a paradigm shift revolutionised the study of astronomy.

Galileo Galilei, the son of a musician, was born in Pisa, Italy, in 1564. Although he attended the University of Pisa as a medical student his passion was mechanics, and in pursuing this interest he became the first real experimental physicist since Archimedes of Syracuse, almost two thousand years earlier. He is reputed to have dropped differently sized masses from the top balcony of the Leaning Tower to demonstrate that they would all fall at the same speed - a prediction that was at odds with the conventional view that the heavier ones would fall more quickly. This simple observation was counter to the 'world view' of the Church, which derived from Aristotle, a student of the philosopher Plato. Galileo then accepted the post of professor of mathematics in Padua, which fell under the authority of Venice where the administration was rather more open-minded.

In 1608 Hans Lippershey, a spectacle-maker in Middleburg in the Netherlands, invented a device, utilising two lenses, by which it was possible to make distant objects appear closer. In May of the following year, Galileo heard about this invention and, after building one, set out to improve the design to enable ships on the horizon to be identified a few hours earlier than was previously possible, which gave a significant advantage to Venetian merchants. In November, Galileo indulged his own long-standing interest in astronomy. He began with the Moon, and was the first person to realise that its face was disfigured by vast holes and rugged mountains, which was contrary to the accepted view that all objects in the sky were 'perfect'. His discovery of dark spots on the Sun further undermined this classical viewpoint.

On 7 January 1610, Galileo turned his attention to Jupiter, and, in addition to resolving it as a disk, he noted three small nearby star-like points arranged in a line. Over successive nights, not only did he notice that these three subsidiary objects changed their relative positions, but he also spotted a fourth. As he accumulated more observations, he realised that these objects were circling around the planet, but this was contrary to the accepted view, as taught by Ptolemy, that things moved around the Earth on 'celestial spheres'. Galileo favoured the theory by Copernicus that only the Moon circles the Earth, and that everything else moves around the Sun, and in March 1610 he recorded his observations in a small pamphlet entitled Sidereus Nuncius, which he chose to dedicate to Grand Duke Cosimo de Medici, a leading member of one the merchant families that had ruled Florence and Tuscany for some two centuries. As there were four de Medici brothers, Galileo suggested that the Jovian attendants be called the 'Medicean Stars'. While the existence of Jovian satellites did not actually prove Copernicus's theory, it certainly contradicted Aristotle, and Galileo once again found himself in serious strife with the Church. Undeterred, he aimed his telescope at the Milky Way and discovered that it comprises a multitude of faint stars, as indeed do some of the 'nebulosities' listed in the catalogue of stars in Ptolemy's Almagest, which extended Hipparchus's catalogue.

In September, having won de Medici's patronage, Galileo was invited to become the mathematician to the Tuscan court in Florence. Soon after arriving, he observed that Venus showed lunar-like phases, which was incontrovertible proof that it was orbiting the Sun, and hammered the final nail into the coffin of the Ptolemaic system.

Meanwhile, in July Galileo had unveiled a real mystery: Saturn appeared to have a substantial attendant on each side of its disk. He wrote to Kepler announcing his discovery of Saturnus triformis, saying: ''I have observed the most distant planet to have a triple form.'' However, in the style of the time, he used an anagram cipher, which Kepler misinterpreted as saying that Mars had two moons. Kepler believed that there should be a progression in which the Earth had one moon, Mars had two, and Jupiter - as Galileo had himself discovered - had four, so he was predisposed to this error. ''The planet Saturn is not alone,'' Galileo wrote later to de Medici, ''but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the ecliptic, and the middle one is about three times the size of the lateral ones.''

First Picture Saturn
Upon first turning his telescope to Saturn in July 1610, Galileo Galilei was astonished to find that the planet appeared to be three objects in line, as his first drawing shows.

Galileo continued his observation through the rest of the year and into 1611. The appendages or ansae (meaning handles) became progressively less noticeable, and in 1612 they disappeared, leaving only the large central object, which, if he had never suspected the presence of the ansae, would have appeared thoroughly unremarkable. He was, however, not only baffled but also gravely concerned because his discovery of satellites around Jupiter had attracted criticism from more traditionally minded non-telescopic astronomers and philosophers. Having announced that Saturn had strange ansae, he would be ridiculed if he now reported that they had disappeared.

Recalling from Greek mythology that after siring Zeus (Jupiter), Kronos (Saturn) had devoured his subsequent offspring at birth in an attempt to prevent them from supplanting him, Galileo wrote of the disappearance of the ansae:

What is to be said concerning so strange a metamorphosis? Are the two lesser stars consumed in the manner of sunspots? Have they vanished, or suddenly fled? Has Saturn perhaps devoured his own children? Or were the appearances indeed illusions or fraud with which the glasses have so long deceived me, as well as many others to whom I have shown them? Now, perhaps, is the time come to revise the well-nigh-withered hopes of those who, guided by more profound contemplations, have demonstrated the utter impossibility of their existence. I do not know what to say in a case so surprising, so unlooked-for, and so novel. The shortness of the time, the unexpected nature of the event, the weakness of my understanding and fear of being mistaken have greatly confounded me.

Reluctant to report conclusions that would subsequently be shown to be wrong, and suspecting that his early telescopes had in any case been flawed, Galileo made a succession of better instruments and, upon discovering that Saturn looked even more baffling in 1616, he ceased to observe the planet.

Others continued to study Saturn, however. In 1614 C. Schreiner noticed that the ansae had reappeared, indicating that Galileo's early observations had been valid. A

Saturn Saturne 1616
In 1616, Galileo found Saturn to be even more puzzling. Although with the benefit of hindsight we readily interpret his drawing as depicting a ring around the planet, he did not recognise the 'appendages' (ansae) as such.

progression in the changing appearance of the ansae was thence revealed. Firstly, they were thin arms which projected to either side of the seemingly unchanging central disk. A dark gap then appeared within each arm. Over the next seven years this progressively opened up, then the trend reversed and the arms closed again.

When G.B. Riccioli, a Jesuit philosopher in Bologna, examined Saturn in 1640 he

Early sketches of Saturn's rings: I: Galileo Galilei, 1610; II: Christopher Schreiner, 1614; III: G.B. Riccioli, 1641 or 1643; IV to VII: Johannes Hevelius circa 1645 (these are theoretical forms); VIII and IX: G.B. Riccioli 1648 to 1650; X: Eustachio Divini 1646 to 1648; XI: Francesco Fontana, 1636; XII: Giuseppe Bianchini, 1616 and Pierre Gassendi, 1638 to 1639; XIII: Francesco Fontana, 1644 to 1645. (From Systema Saturnium by Christiaan Huygens, 1659.)

saw the ansae as thin arms. After vanishing in 1641, they reappeared and started a new cycle. From 1647 to 1650 he watched them evolve, then published a book in 1651 summarising the state of knowledge, but this served only to confound the mystery.

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