0In the sixteen century a new way of conceiving science arised and spreaded through Europe: the beginning of this new period is identified with the publication of Copernicus’s book De Revolutionibus in 1543.
One of the main protagonists of this important stage of modern science is Galileo Galilei (1564-1642) as he put great effort to promote new theories based on experimentation and observation instead of accepting the old Aristotelian ideas. Before 1609 he focused prevalently on mechanics where he made major accomplishments. A turning point in Galileo’s life came when he received news about a Dutch instrument capable of making objects look bigger and nearer. He did not have the possibility to examine it directly but probably had some hints about the fact that it used a convex and a concave lens: using what his knowledge on dioptric he set to work and succeeded in reproducing the instrument with two lenses that were both flat on one side but one concave and the other convex on the other side. Then he put a lot of effort to try to improve it and, in 1610, obtained an instrument that would bring objects 30 times nearer. He showed it to the Signoria in Venezia obtaining grate praise. He reported that many nobles wanted to climb several steps to get to the top of the highest tower to see ships appearing in his “glass” (telescope) two hours before they were visible to the naked eye.
Galileo used his “glass” to look at the sky and made an incredible discovery: the Moon was not a perfect sphere, but had mountains and craters, just like the Earth. That was in contrast with the accepted model according to which the Moon and the skies were made of perfect crystal, in contrast with the Earth, which was corruptible and imperfect. Again he looked through the telescope and found four moons orbiting around Jupiter, a fact which was in strong contrast with Aristotle’s system, in which everything in the sky should rotate around the Earth. Observing Saturn he found it had a slightly different shape, though he could not distinguish it was a flat ring. He was among the first to observe solar spots and seeing their motion understood that the Sun rotates. The telescope also showes in all parts of the sky minute stars that were not visible with the naked eye. Basically all the discoveries that could be done with an instrument of such a resolution were made around 1610 by Galileo and a few others, while, for further achievements, such as the clear observation of Saturn’s rings, a more advanced kind of telescope was needed, with two or more convex lenses.
Actually, Galileo was not the first to look at the Moon through a telescope: the English mathematician Thomas Harriot had done it (probably using a less refined instrument) one year before, but did not publish his results, thus not being recognized the credit.
In 1610 Galileo published a book with the result of his observation, the Sidereus Nuncius; in the following years he dedicated himself to defending his discoveries and the Copernican system and promoting a new conception of science based on experimentation and rational reasoning.
In 1632 Galileo publishes the Dialog Concerning the Two Chief World Systems, a work of fundamental importance for popularizing the Copernican system from a scientific but also a philosophical point of view: structured as a dialog between one supporter of the Aristotle’s system and one supporter of the Copernicus’s system, the book shows how the latter could fit a conception of the world that was rational and based on mathematics. To do this he exposes his theories in mechanics and applies them to solve those inconsistencies that many scholars, learned in Aristotle’s mechanics, would use as arguments against the heliocentric system. Galileo had no doubt that the laws of motion that proved themselves correct for bodies on Earth were the same that ruled celestial mechanics. He introduced the concept of indistinguishability between real and perceived motion: as well as a mariner in the cabin of a ship sailing on a perfectly calm sea cannot perceive the motion on water and the result of any action (like throwing a ball) will be the same as if he were on firm land, the same happens to us on the Earth, not perceiving its motion. One failure of Galileo was that, while discussing this, he did not analyze the importance of the trajectories (circular or linear) and did not recognize that circular motion requires accelerations and therefore forces.
Galileo did not go any further in the analysis Copernicus had made and did not bother to find an explanation on how exactly the planets move in the sky and why they move, and admitted that the movement of celestial bodies is perfectly circular, because only this type of motion could be preserved without any alteration; this theory was incompatible with observations but, as he says in many parts of the book, Galileo distrusted the accuracy of astronomical measurements.
Galileo’s purpose was not to explain the irregular movement of planets that troubled astronomers for ages: it was instead to promote the idea that Aristotle was not right using the power of rational arguments.
In 1633 he was summoned to Rome where he was put under trial for the content of the Dialogo: he was forced to repent and abjure and was condemned to live in isolation in his house near Florence, where he spent the latter years of his life. The Dialogo was during many years inscribed in the Index (list of forbidden books).