Siderius Nuncius – Galileo Galilei

Sidereus Nuncius is a short astronomical treatise (or pamphlet) published in New Latin by Galileo Galilei in March 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, the hundreds of stars that were unable to be seen in either the Milky Way or certain constellations with the naked eye, and the Medicean Stars that appeared to be circling Jupiter. The implications of the text left Galileo and the Roman Catholic Church in a series of debates throughout the latter part of Galileo’s life, ending with his condemnation by the Church in 1633.

The Latin word nuncius was typically used during this time period to denote messenger; however, albeit less frequently, it was also interpreted as message. While the title Sidereus Nuncius is usually translated into English as Sidereal Messenger, many of Galileo’s early drafts of the book and later related writings indicate that the intended purpose of the book was “simply to report the news about recent developments in astronomy, not to pass himself off solemnly as an ambassador from heaven.” Therefore, the correct English translation of the title is Sidereal Message (or often, Starry Message).

Sidereus Nuncius contains more than seventy drawings and diagrams of the Moon, certain constellations such as Orion, Pleiades, and Taurus, and the Medicean Stars of Jupiter. Galileo’s text also includes descriptions, explanations, and theories of his observations.


In his observations of the Moon, Galileo observed 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 observation, he deduced that the darker regions are flat, low-lying areas, while the brighter regions are rough and covered with mountains. Based on the distance of sunlit mountaintops from the terminator, he estimated, quite accurately, that the lunar mountains were at least four miles in height. Galileo’s engravings of the lunar surface provided a new form of visual representation, as well as shaped the field of selenography, the study of physical features on the Moon.


In observing the stars, Galileo reported that he saw at least ten times as many stars through the telescope as with 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 only see six stars in the Taurus constellation; however, with his telescope, Galileo was capable of seeing thirty-five – almost six times as many stars. When he pointed his telescope at Orion, he was capable of seeing eighty stars, rather than the previously observed nine – almost nine times as many stars. 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.[6] 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.

Galileo’s drawings of Jupiter and its Medicean Stars from Sidereus Nuncius. Image courtesy of the History of Science Collections, University of Oklahoma Libraries.

Medicean Stars (Moons of Jupiter)

In the last portion of Sidereus Nuncius, Galileo reported his discovery of four objects that appeared to form a straight line of stars near Jupiter. The first night, he witnessed two little stars on ether side of Jupiter; the following nights brought two additional stars into his views, totaling four stars around Jupiter. Throughout the text, Galileo gave illustrations of the relative positions of Jupiter and its apparent companion stars as they appeared nightly from late January through early March 1610. The fact that they changed their positions relative to Jupiter from night to night, but always appeared in the same straight line near Jupiter, brought Galileo to deduce that they were four bodies in orbit around 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. It is important to note that 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 who later became 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.”

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