1564 - 1642

Galileo's Sidereus Nuncius, published in Venice in 1610. The book instantly made Galileo a European celebrity, and earned him, in July 1610, the position of chief mathematician and philosopher mathematician to the Grand Duke of Tucsany, Cosimo de Medici II, in Florence. Reproduced from the introductory essay in A. van Helden's 1989 translation. The book described Galileo's groundbreaking telescopic discoveries, including his lunar observations, observations of faint stars invisible to the naked eye, and discovery of Jupiter's four larger Moons. Originally greeted with a good measure of scepticism, Galileo's telescopic discoveries benefited from an enthusiastic endorsement by Kepler, and shortly thereafter by the Christoph Clavius and other Jesuit astronomers at the Roman College.

The title page of Sidereus Nuncius served, to a certain extent, as an advertisement and was directed to Grand Duke Cosimo. The text of the title page demonstrates Galileos skills as a self-publicist:


unfolding great and very wonderful sights
and displaying to the gaze of everyone,
but especially philosophers and astronomers,
the things that were observed by
Florentine patrician
and public mathematician of the University of Padua,
with the help of a spyglass lately devised by him,
about the face of the Moon, countless fixed stars,
the Milky Way, nebulous stars,
but especially about
four planets
flying around the star of Jupiter at unequal intervals
and periods with wonderful swiftness;
which, unknown by anyone until this day,
the first author detected recently
and decided to name


Manuscript pages: you see Galilei's handwriting and the position of the moons with respect to Jupiter.
Probably the most significent contribution that Galileo Galilei made to science was the discovery of the four satellites around Jupiter that are now named in his honor. Galileo first observed the moons of Jupiter on January 7, 1610 through a homemade telescope. He originally thought he saw three stars near Jupiter, strung out in a line through the planet. The next evening, these stars seemed to have moved the wrong way, which caught his attention. Galileo continued to observe the stars and Jupiter for the next week. On January 11, a fourth star (which would later turn out to be Ganymede) appeared. After a week, Galileo had observed that the four stars never left the vicinity of Jupiter and appeared to be carried along with the planet, and that they changed their position with respect to each other and Jupiter. Finally, Galileo determined that what he was observing were not stars, but planetary bodies that were in orbit around Jupiter. This discovery provided evidence in support of the Copernican system and showed that everything did not revolve around the Earth.
Sketches of the four moons of Jupiter, as seen by Galileo through his telescope. What he saw are the four larger moons of Jupiter, now known as Io, Europa, Ganymede and Callisto. The drawing depicts observations from the time period January 7 to 24, 1610. Galileo had considerable difficulty in recognizing the true meaning of what he was seeing; Callisto often lay outside the (restricted) field of view of his telescope, Io often lost in Jupiter's glare, and some moon occasionally disappeared in Jupiter's shadow or behind or in front of the planet itself.
Galileo named the moons Medicean Stars, after the ruling Florentine family Medici. This was a move calculated to improve his chances of moving back to Florence, and it succeeded. The names used today were coined by Simon Mayr (1573-1624).
It is nice to compare Galileo's observations and notes with the positions of Jupiter's moons as shown in a planetarium program like Skymap or Cartes du Ciel and to find out that Galileo did a good job !!!

Much more about Galileio Galilei.