Using a telescope, Galileo discovered several bodies orbiting Jupiter, showing that celestial objects did not all have to revolve around Earth.
His Starry Messenger, published in 1610, circulated these observations. Telescopic evidence became part of debates about the universe, while also provoking religious and scholarly conflict.
Historical context
Sidereus Nuncius (usually Sidereal Messenger, also Starry Messenger or Sidereal Message) is a short astronomical treatise (or pamphlet) published in Neo-Latin by Galileo Galilei on March 13, 1610. It was the first published scientific work based on observations made through a telescope, and it contains the results of Galileo's early observations of the imperfect and mountainous Moon, of hundreds of stars not visible to the naked eye in the Milky Way and in certain constellations, and of the Medicean Stars (Galilean moons) that appeared to be circling Jupiter.
Content
Sidereus Nuncius contains more than seventy drawings and diagrams of the Moon, certain constellations such as Orion, the Pleiades, and Taurus, and the Medicean Stars of Jupiter. Galileo's text also includes descriptions, explanations, and theories of his observations.
Moon
In observing the Moon, Galileo saw that the line separating lunar day from night (the terminator) was smooth where it crossed the darker regions of the Moon but quite irregular where it crossed the brighter areas. From this he deduced that the darker regions are flat, low-lying areas, and the brighter regions rough and mountainous. Basing his estimate on the distance of sunlit mountaintops from the terminator, he judged, quite accurately, that the lunar mountains were at least four miles high. Galileo's engravings of the lunar surface provided a new form of visual representation, besides shaping the field of selenography, the study of physical features on the Moon.
Stars
Galileo reported that he saw at least ten times more stars through the telescope than are visible to the naked eye, and he published star charts of the belt of Orion and the star cluster Pleiades showing some of the newly observed stars. With the naked eye observers could see only six stars in the Pleiades; through his telescope, however, Galileo was capable of seeing thirty-five – almost six times as many. When he turned his telescope on Orion, he was capable of seeing eighty stars, rather than the previously observed nine – almost nine times more. In Sidereus Nuncius, Galileo revised and reproduced these two star groups by distinguishing between the stars seen without the telescope and those seen with it. Also, when he observed some of the "nebulous" stars in the Ptolemaic star catalogue, he saw that rather than being cloudy, they were made of many small stars.
From this he deduced that the nebulae and the Milky Way were "congeries of innumerable stars grouped together in clusters" too small and distant to be resolved into individual stars by the naked eye.
Medicean Stars (Moons of Jupiter)
In his drawings, Galileo used an open circle to represent Jupiter and asterisks to represent the four stars. He made this distinction to show that there was in fact a difference between these two types of celestial bodies. Galileo used the terms planet and star interchangeably, and "both words were correct usage within the prevailing Aristotelian terminology."
At the time of Sidereus Nuncius' publication, Galileo was a mathematician at the University of Padua and had recently received a lifetime contract for his work in building more powerful telescopes. He desired to return to Florence, and in hopes of gaining patronage there, he dedicated Sidereus Nuncius to his former pupil, now the Grand Duke of Tuscany, Cosimo II de' Medici. In addition, he named his discovered four moons of Jupiter the "Medicean Stars," in honor of the four royal Medici brothers. This helped him receive the position of Chief Mathematician and Philosopher to the Medici at the University of Pisa. Ultimately, his effort at naming the moons failed, for they are now referred to as the "Galilean moons".