[9][3] 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 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.

[9] In the last part of Sidereus Nuncius, Galileo reported his discovery of four objects that appeared to form a straight line of stars near Jupiter.

[11][3] 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.

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.

"[13] 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.

Three works of art were even created in response to Galileo's book: Adam Elsheimer's The Flight into Egypt (1610; contested by Keith Andrews[14]), Lodovico Cigoli's Assumption of the Virgin (1612), and Andrea Sacchi's Divine Wisdom (1631).

A common response to the Medicean Stars was simply to say that the telescope had a lens defect and was producing illusory points of light and images; those saying this completely denied the existence of the moons.

[13] That only a few could initially see and verify what Galileo had observed supported the supposition that the optical theory during this period "could not clearly demonstrate that the instrument was not deceiving the senses.

"[15] By naming the four moons after the Medici brothers and convincing the Grand Duke Cosimo II of his discoveries, the defence of Galileo's reports became a matter of State.

The secretary to Giovanni Antonio Magini, a Bohemian astronomer named Martin Horký, published an incendiary pamphlet criticizing the Sidereus Nuncius, alleging in it that Galileo's observations were the result of poor lenses and influenced by personal ambitions.