Beyond Galileo

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 a line of three little stars close to Jupiter parallel to the ecliptic; the following nights brought different arrangements and another star into his view, totaling four stars around Jupiter. (Galilei 1618) 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. On January 11 after 4 nights of observation he wrote:

“I therefore concluded and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury round the Sun; which at length was established as clear as daylight by numerous subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions round Jupiter...the revolutions are so swift that an observer may generally get differences of position every hour.” (Galilei 1610)

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.” (Mendillo 2010)

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 dedicatedSidereus 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. (Byard 1988) Ultimately, his effort at naming the moons failed, for they are now referred to as the “Galilean moons.”


  1. G. Galilei. The Assayer, as Translated by Stillman Drake (1957). (1618). Link

  2. G. Galilei. Sidereus nuncius. (1610).

  3. M. Mendillo. The Appearance of the Medicean Moons in 17\(^{th}\) Century Charts and BooksHow Long Did It Take?. 269, 33-41 (2010). Link

  4. M. M. Byard. A New Heaven: Galileo and the Artists. History Today (1988).