The telescope and astronomical discoveries

Galileo "space explorer"
Galileo was the first to bring closer heaven and earth. Physically, showing that they consisted of one and same matter, and not two distinct substances, perfect and imperfect, as believed until then. Visually, pointing up the telescope, which had been perfected and developed in comparison to the specimens that could be found in the markets, so that it became an instrument of science. Tenaciously and methodically, Galileo began to observe the aspect and movements of the heavenly bodies, hitherto seen only by the naked eye, with unimaginable results that were collected in the Sidereus Nuncius (Starry Messenger). The work, published very hastily in a modest-style edition in the Spring of 1610, announced to the world the series of extraordinary celestial discoveries made by Galileo in just a few weeks. Nothing was as it previously seemed to the human deceptive senses, and the Aristotelian-Ptolemaic cosmology, universally accepted until then, proved to be false.

Original telescope of Galileo Galilei preserved in the Museo Galileo in Florence

In the Starry Messenger Galileo Galilei announces the celestial discoveries made thanks to the telescope

The "terrestrial" Moon
Galileo referred to the face, full of mountains and valleys, of the Moon (that was, therefore, not a perfect body, as the followers of Aristotle claimed) The idea of a Moon similar to the Earth refuted the Aristotelian theory of the different nature of the celestial bodies. Furthermore, conceiving of the Moon as a satellite orbiting around a centre, which was the Earth, suggested that the latter, being made of the same substance, might behave in the same way, orbiting in turn around a centre of its own. A famous series of six water-colours of the Moon in different phases were realized “live” by Galileo, engaged in telescopic observation of the Earth’s satellite in the autumn of 1609. The first realistic representations of the Moon, they were destined to produce a veritable earthquake not only in natural philosophy but also in art.

Galileo manuscript with drawings of the Moon

Donato Creti, Observation of the Moon

Jupiter and the Medicean Stars
It soon became obvious that four satellites revolved around Jupiter and, contemporaneously with the planet, around the Sun, a phenomenon whose impossibility had always been viewed by the Ptolemaics (Aristotelians) as proof of an earth-centred system. Once it had been demonstrated that this was by no means impossible, the same thing could be true of the Earth, which might very well revolve around the Sun, accompanied by its satellite the Moon. Among all these discoveries, that of Jupiter's satellites was thus the one with the greatest impact. And it was not by chance that Galileo, who had long been seeking the protection of a prince in order to continue his studies without having to teach, named them the Medicean Stars, dedicating them to the House of Medici. In the following years, on the basis of observations and calculations aimed at the accurate calculation of the periods of the four satellites, Galileo created the jovilabe, an instrument that would have favored the use of the system of Jupiter as a perfect clock to solve the long-time problem of determining the longitude at sea.

The Jovilabe is an extraordinary calculating instrument for predicting the apparent configurations of Jupiter's system at any given moment

Donato Creti, Astronomical observations: Jupiter and the Medicean Planets

Galilean manuscript with the observations of Jupiter and the Medicean Planets

Diary of the observations of the satellites of Jupiter

Verso of medal with the personification of the River Arno observing Jupiter and the Medicean Planets

Fixed stars, nebulae and the Milky Way
The observation of an enormous quantity of stars never before seen cast doubt on the small size of the Ptolemaic universe and substituted for the restricted covering of the sky a great sidereal space. Swept away were also all the sterile disputes of philosophers around the matter of the Milky Way, which was nothing more than a congeries of innumerable stars, thousands of which could be seen wherever the telescope was aiming. Even that milky matter that seemed to form nebulae consisted of groups of small stars.

The Pleiades and Aldebaran in the head of Taurus

The constellation of the Pleiades as seen by Galileo with the telescope

The spots on the Sun
In observing the Sun through the telescope, Galileo had seen numerous spots on its surface. Elsewhere, these same spots had been noted by Christoph Scheiner, a Jesuit mathematician residing at the time in Bavaria. Their conflicting ideas on the nature of sunspots reflected the basic split between the inflexibly philo-Aristotelian position of Scheiner, unable to forego the concept of an incorruptible heaven, and Galileo’s increasing acceptance of the new Copernican cosmology. The Istoria e dimostrazioni intorno alle macchie solari e loro accidenti [History and demonstrations concerning sunspots and their properties] did not stop at close examination of the phenomenon of sunspots, or at the even revolutionary hypothesis that the Sun revolved on its own axis, but took it as a springboard for attacking the non-critical conformism of his adversaries, and ended by proclaiming the natural truth of the heliocentric position and the Earth’s motion.

Donato Creti, Astronomical observations: the Sun

Drawings of sunspots observed by Galileo

The "lunar" phases of Venus and Mercury
In the geocentric system, Ptolemy had been forced to assume the existence of an epicycle, which is a minor rotation along the orbit of the planets around the Earth, to justify their motions as they appeared to the eye of the observer on Earth. If Venus and Mercury, the two planets placed between Earth and the Sun in the Ptolemaic system, moved on an orbit and an epicycle between the Sun and Earth, they would have displayed phases different from those of the Moon appearing, at best, as a slender illuminated sickle. When began observing Venus and Mercury with his telescope in 1610, he noticed that the planets showed phases similar to those of the Moon, coming up to the full brightness of the disk. Galileo saw the phenomenon as proof that Venus and Mercury did not move on an epicycle located between the Sun and the Earth; rather, they followed a circle centered on the Sun, as advocated by Copernicus.

Donato Creti, Astronomical observations: Mercury

Maria Clara Einmart, The phases of Venus

The "triple-bodied" Saturn
Starting in 1610, Galileo observed Saturn with his telescope and found it to be "triple-bodied," i.e., composed of a central body flanked by two smaller lumps, motionless and always identical to themselves, that he mistakenly assumed were satellites. The unique aspect of the planet depended on an insufficient power of the telescope used. Faced with (In response to, Given the) the triple Saturn there was a spread of artistic suggestions, ranging between mythology and science. It was only in 1655 that Christiaan Huygens, thanks to a more powerful telescope, observed for the first time the rings of Saturn. The definition of their nature also engaged the members of the Accademia del Cimento. Between 1671 and 1684 Giovanni Domenico Cassini studied the divisions of the rings and discovered four satellites that were added to the ones previously identified by Huygens.

Letter from Galileo Galilei to Belisario Vinta announcing the tricorporal nature of Saturn

Maria Clara Einmart, Appearance of Saturn

The comets and the method
The three comets that appeared between 1618 and 1619 gave rise to another heated argument, this time between Galileo and the Jesuit priest Orazio Grassi. By now, however, Galileo had been silenced by a pronouncement of the Holy Office, which had suspended Copernicus’s De revolutionibus in 1616, admitting the theory of the Earth’s mobility only as a mathematical hypothesis, not to be proclaimed as a natural truth, because contrary to Holy Scripture. Consequently, Galileo could make no effective response to Grassi, who had adopted Tycho Brahe’s geo-heliocentric theory, after Galileo’s astronomical discoveries had demonstrated the erroneous nature of the Ptolemaic system. Il saggiatore [The Assayer] - the most salient moment of the dispute on comets, the nature of which had not been fully understood by Galileo who believed that they were vapors from the Earth condensed in the cosmic space - is a milestone in methodology. Moreover, it proposes a new scale of values for natural philosophy, where authority and tradition, literature and books, are deemed infinitely less important that studying the world and the mathematical laws that govern it.

Frontispiece of "The Assayer" of Galileo Galilei

Donato Creti, Astronomical observations: Comet

The "unfortunate" dialogue
Although his diplomatic attempts to have the Church recognize the heliocentric system and the motion of the Earth had failed, Galileo held firm to his long-time intention of writing a cosmological work. The Dialogo sopra i due massimi sistemi del mondo [Dialogue on the two chief world systems] covers all of the salient reasons for Galileo’s beliefs: the motion of the Earth, the substance of the Moon, Jupiter’s satellites, sunspots, the relativity of motion. It refutes all of the arguments brought against these concepts by the advocates of geocentric and geostatic cosmological systems, by giving better reasons for their greater plausibility, accompanied by unequivocal lessons in methodology. Half-concealed among the other topics the subject of the tides that the pope had demanded (requested) not to be the central thesis of the book, fearing it could reveal itself as a physical proof of the motion of the Earth, not confined to the field of mathematics. But in spite of its myriad precautions, the Dialogo was prohibited and Galileo condemned to abjuration.

Representation of the Copernican system from Harmonia Macrocosmica of Andreas Cellarius

Vincenzo Malinverno, Galileo Galilei before the Inquisition in Rome

The telescope as an emblem
The Galilean revolution, which, thanks to a rigorous method of experiments conducted in a framework of mathematical demonstrations, unhinged centuries-old false knowledge solely based on unreliable human senses and gave rise to modern science, had as its emblem the telescope. The instrument that had allowed him to explore the heavens from a distance became the symbol of a new concept of knowledge which aimed to understand in depth the nature and its phenomena, well beyond the sensible appearances. Over the centuries, art has embraced that symbol, making recognizable by his telescope also the most idealized representation of Galileo, in a kind of allegory of science.

Nicola Ulacacci, Galileo Galilei offers his telescope to the Venetian Senate

Telescopes and lens used by Galileo in his observations of the sky

StreetView of the room devoted to Galileo in the Museo Galileo, Florence

Museo Galileo - Istituto e Museo di Storia della Scienza
Credits: Story

Exhibition content - Sara Bonechi, Museo Galileo

Digital production - Marco Berni, Museo Galileo

Credits: All media
The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.
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