## What Galileo Saw!
Through the next year Galileo observed things that nobody had previously imagined. In November and December of 1609 he carefully studied the Moon and with his excellent artistic abilities, drew detailed images showing the mountains and craters. This was revolutionary because the Aristotelian model required the celestial bodies to be perfectly unblemished spheres. By carefully mapping the shadows of the Moon, Galileo estimated the height of crater edges and found them to be Earth-like in size. Then a month later he studied Jupiter and found what looked like three bright stars, all in a line. He continued looking in successive nights and saw a fourth “star” peek out from behind the planet and found all four of them to be moving together! Subsequent observations convinced him that they were bound to Jupiter, and not stars at all: Jupiter has moons which today we call the Galilean moons. Opportunistically he named them the Medicean moons and realized that this Jupiter-moons system was Copernican-like. Finally, when he looked into deep space the stars multiplied. He found hundreds of stars that nobody had ever seen before.
In 1611 he published *Sidereus Nuncius*, or *Starry Messenger*, reporting these and other revolutionary observations and interpretations. This figure shows the elaborately constructed title page
and here are a few pages from the text.
The Moon has a rough surface with mountains and valleys. Other planets in our solar system have moons that move according to Kepler’s laws.
When he was at Pisa, he had become friendly with the Medici family, especially the the Grand Duchess Christina. A few summers while he was in Pisa he was brought back by her to Florence in order to tutor her young son, the young Cosimo d’Medici. His bold naming of the moons after his former student -- by then the reigning Duke of Tuscany -- and his dedication of *Sidereus Nuncius* to him was an obvious ploy to again improve his circumstances and to get a new job without teaching responsibilities. He was not above groveling before royalty when he needed to: > "...scarcely have the immortal graces of your soul begun to shine forth on Earth than bright stars offer themselves in the heavens, which, like tongues [longer lived than poets] will speak of and celebrate your most excellent virtues for all time.” A tad syrupy perhaps? The figure above shows his sketches of multiple nights’ viewing of the Medician moons. What he had found was a miniature Copernican system within the bounds of our own solar system. In fact, Kepler's Third law would hold, but for Jupiter there would be a different proportionality constant, say $k_J$. Further, *Siderius* described his discovery that Venus had phases like the Moon which explained why it appeared to change brightness periodically, just as Copernicus had predicted. And finally, the number of stars visible with the telescope dwarfed what everyone believed was the full compliment of stars that had been carefully tallied by the Babylonians, Greeks, and Tycho. The universe appeared to be a much more interesting place than anyone had imagined. ```{aside} It’s important to recognize that Galileo did not invent the telescope (one of those persistent myths) and he was not the first to use it to discover things in the sky. A British natural philosopher, Thomas Harriot, was first to observe many of the things that are credited to Galileo. Harriot was not as self-promoting nor did he publish as quickly as Galileo, so he lost his historical moment. ``` The effect of all of this news electrified Europe and overnight, Galileo became famous, and remained so for the rest of his life. The good news? He got the job back in Florence. And the bad news: Florence was within the sphere of influence of Rome and the Pope. In fact, there had been a number of Medici popes in the family. Venice was much more liberal and was often at odds with the Vatican over one or another issue. (Pope Clement V excommunicated the entire population of Venice in 1309! Interdicts – forbidding any ecclesiastical functions were instituted against Venice in 1202, 1284, 1480, 1509, and again in 1609.) Galileo’s…unusual views…were safe in Venice, but dangerous in Florence. He negotiated the position that not only paid well, but also importantly, raised his stature: he was *The Chief Mathematician of the University of Pisa and Philosopher and Mathematician to the Grand Duke*. The last title was important, for it was Philosophers who ruled the academic roost and mathematicians were the least respected. Galileo insisted on this dual, contradictory title. He took multiple victory laps in Rome where he was celebrated by the College of Jesuits and where the then Cardinal Barberini took great pleasure in Galileo’s friendship. The 47 year old was riding high.Other planets in our solar system have moons that according to Kepler’s model.
In years to come, Galileo studied many things and wrote books on Sunspots (He learned to train his telescope on the Sun, but a student taught him to project the image onto a piece of paper so that he would not damage his eyes. The result was another kind of blemish in a heretofore perfect celestial sphere: sunspots.) and buoyancy. Here he began to get himself in trouble as a respected Jesuit competitor disagreed with him on the origin of sunspots (were they just another set of planets?) and buoyancy…what caused things to float. In both cases, Galileo was over the top and *ad hominem* in his nasty criticisms of his scientific adversaries. This cost him support among some of his Jesuit colleagues.The Sun has blemishes on its surface that change in time.
```{margin}Only measurements can challenge observations about the physical world. Not a religious authority or text.
Galileo had become a Copernican and there’s some evidence that this evolution in his belief happened early, but it was first enunciated in a letter to Kepler in 1597 ("....Like you, I accepted the Copernican position several years ago”) but the letter to Catherine was his coming-out. > “All our Fathers of the devout Convent of St. Mark feel that the letter contains many statements which seem presumptuous or suspect, as when it states that the words of Holy Scripture do not mean what they say; that in discussions about natural phenomena the authority of Scripture should rank last... [the followers of Galileo] were taking it upon themselves to expound the Holy Scripture according to their private lights and in a manner different from the common interpretation of the Fathers of the Church...” Letter to a member of the Inquisition. A council of advisors was established by Pope Paul V to review the theological aspects of Copernicanism. They reached conclusion on two issues: Does the Sun sit immobile? Does the Earth move? - On the first, they concluded that to hold that the Sun was immobile and at the center of the solar system: “...foolish and absurd in philosophy, and formally heretical since it explicitly contradicts in many places the sense of Holy Scripture.” - On the second, that the Earth moves: “...receives the same judgment in philosophy and... in regard to theological truth it is at least erroneous in faith.” This led to a banning of Copernicus’ book until corrections were made. The figure below is a page of _Revolutiononibus_ showing some of the Inquisitor’s corrections.