Medician stars

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The Medician Stars are four moons of Jupiter, which were discovered by Galileo Galilei, mostly likely in December of 1609 or January of 1610. These moons were dedicated to the four brothers of the prominent Medici family in Galileo’s Sidereus Nuncius, or The Sidereal (Starry) Messenger, published in Italy in March of 1610.

As a result of improvements he made to the telescope, Galileo was able to see celestial bodies more distinctly than ever before; Galileo was the first to observe the moons of Jupiter.¹

On January 7, 1610, Galileo wrote a letter containing the first mention of Jupiter’s moons. At the time, he only saw three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these “stars” from January 8 through March 2 and quickly discovered that there were four bodies instead of only three and that they were not fixed. Jupiter was not moving past them, but they were orbiting Jupiter.²

This was a very important discovery, for these moons were never mentioned by ancient philosophers, suggesting that they had no knowledge of Jupiter’s satellites. Galileo’s discovery showed that there were new things to be discovered and objects in space that could not be seen without the help of scientific instruments. The discovery also provided criticism for the Ptolemaic world system, which held that the Earth was in the center of the universe with everything else evolving around it. If Jupiter was also a center of motion, with moons orbiting it, then the Earth could not be the center of all motion in the universe. Also, the fact that Jupiter has more moons than Earth (with larger lunar area than Earth’s moon) goes against the idea that Earth is the center of the universe. While Galileo does not mention the Copernican Universe specifically, he believed in this system.³

In 1605, Galileo was employed as a math tutor for Cosimo de’ Medici, and in 1609, Cosimo became Grand Duke Cosimo II of Tuscany. Galileo, seeking patronage from his now-wealthy past student, decided to use the discovery of Jupiter’s moons to his advantage. On February 13, 1610, Galileo wrote a letter to the Grand Duke’s secretary,

…God graced me with being able, through such a singular sign, to reveal to my Lord my devotion and the desire I have that his glorious name live as equal among the stars, and since it is up to me, the first discoverer, to name these new planets, I wish, in imitation of the great sages who placed the most excellent heroes of that age among the stars, to inscribe these with the name of the Most Serene Grand Duke.⁴

He goes onto present his dilemma: he did not know whether to name the moons “Cosmian Stars,” after Cosimo alone or “Medician Stars,” which would honor all four brothers in the Medici family. The secretary wrote back, informing Galileo that the latter name would be best.⁵

On March 12, 1610, Galileo wrote his dedicatory letter to the Duke of Tuscany, and he sent a copy of this letter to the Grand Duke the very next day, hoping to ensure the Grand Duke’s support as quickly as possibly. On March 19, he sent the telescope he used to first view Jupiter’s moons to the Grand Duke, along with an official copy of Sidereus Nuncius.⁶ In his dedicatory introduction, Galileo stated,

…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, will speak of and celebrate your most excellent virtues for all time. Behold, therefore, four stars reserved for your illustrious name…which…make their journeys and orbits with a marvelous speed around the star of Jupiter…like children of the same family…Indeed, it appears the Maker of the Stars himself, by clear arguments, admonished me to call these new planets by the illustrious name of Your Highness before all others.⁷

He went on to officially name them the Medician Stars. By doing this, he hoped to honor the Medici family and to gain the financial and social support necessary to continue his work and better his position in life.⁹