Astronomical Images : Riccioli explains Galileo's Copernican argument from the motion of sunspots

Giovanni Battista Riccioli

Astronomical Images

<p style='text-align: justify;'>This image and excerpt are found in the section of Giovanni Battista Riccioli's <i>Almagestum novum</i> (1651) that addressed the question of the world systems. Educated at Jesuit schools in Ferrara, Piacenza, and Parma, Riccioli began teaching at the Jesuit College in Parma in 1632. His career also took him to Mantua and Bologna, where, like most Jesuit professors of his day, he taught a wide variety of subjects, including logic, natural philosophy, mathematics, and theology. Riccioli's publications were equally diverse and included not only astronomical texts, such as his <i>Almagestum novum</i> and <i>Astronomia reformata</i> (1665), but also his <i>Geographia et hydrographia </i>(1661) and various theological treatises. The Jesuit community at Parma and Bologna, where Riccioli studied and taught, was known for its commitment to experimental and observational methods and its interest in the newest developments in astronomy and natural philosophy. Riccioli quickly became immersed in this culture. As a student, Riccioli first learned from and assisted the work of more established professors, including the well-known Jesuit mathematician Giuseppe Biancani (1566-1624). Riccioli almost certainly assisted Biancani and Christoph Scheiner in the latter's observation of sunspots and in the debates over Aristotelian cosmology that followed. Later, as an established professor and scholar, Riccioli directed his students and colleagues in a programme of experimental work. Partly in collaboration with Niccolo Cabeo (1586-1650), a Jesuit from Ferrara who carried out experiments to challenge Galileo's findings on local motion, Riccioli devised an ambitious experimental programme to investigate free fall, the trajectories of projectiles, and the behaviour of pendulums; all topics treated at length in Galileo's <i>Discorsi</i>. This experimental work, along with astronomical observations made at the newly-constructed Jesuit observatory at Santa Lucia College in Bologna, comprised much of the preparatory work for Riccioli's <i>Almagestum novum</i>, which he began in 1640. Though Riccioli intended his <i>Almagestum novum </i>to comprise three volumes, only the first was printed. Riccioli devoted an entire section of the second tome to the Copernican doctrine. He ultimately concluded that arguments drawn from physical and observational evidence were insufficient to determine which world system was correct. After analysing forty-nine proofs for and seventy-seven proofs against the Copernican doctrine, Riccioli sided against the Copernican hypothesis on the basis of the Church's 1616 and 1633 decisions on this matter. This excerpt is from book 9 of the first (and only published) volume of Riccioli's <i>Almagestum novum</i>. Book 9 is titled 'On the World Systems', and treats, in separate sections: (I) the creation and nature of celestial bodies, (II) their motion and movers, (III) various systems of the World in which the Earth is at rest, (IV) the world system in which the Earth is in motion, and (V) the harmonic system of the World. Section IV of book 9, devoted to the Copernican system, is further subdivided into chapters, and this excerpt is found in chapter 12, which is titled 'An argument for the annual motion of the Earth taken from the motion of sunspots is proposed and dispensed with; on the same occasion the doctrine of these spots recounted by Galileo and Scheiner is rendered in abridged form, by which this argument and its solution are shown more clearly' [<i>Proponitur et dissoluitur argumentum pro motu annuo Terrae a motu macularum Solis desumptum; eaque occasione doctrina de his maculis a Galilaeo et Scheinero tradita, in epitomem redigitur; quo luculentius argumentum hoc eiusque solutio declarentur</i>, Vol. I, Part 1, p. 351]. Riccioli begins this section by summarizing the 1612'13 controversy over sunspots between Galileo and Scheiner. He then moves on to consider Galileo's claim in Day Three of his <i>Dialogo</i> (1632) that the motion of sunspots serves as evidence for the Earth's annual motion (<i>Editione nazionale</i>, Vol. VII, pp. 375-9). In an earlier section of his <i>Almagestum novum</i>, Riccioli remarks, in fact, that this reasoning was one of the most powerful arguments for the Earth's annual motion [<i>unum ex potissimis pro motu annuo Terrae</i>, Vol. I, Part. 1, p. 97]. After discussing Galileo's views, Riccioli moves on to explain and endorse Scheiner's geocentric account of the motion of the spots. Riccioli reproduces diagrams found in Galileo's original text, though they differ in quality and typeset. He also reverses the geometry of the physical system in the text accompanying the second image (but not the first or third). In both the 1632 Italian and 1635 Latin editions of Galileo's <i>Dialogo</i>, the description accompanying the second image describes a figure with the pole E towards the viewer and the spots moving along the arc BFD. In contrast, Riccioli describes a figure whose pole I is towards the viewer and whose upper arc BGD is the path of the spots.</p>