Title page of Galileo's 'Sidereus Nuncius' (1610, Starry Messenger). This work by Italian physicist, mathematician and astronomer Galileo Galilei (1564-1642) published his pioneering observations of the Moon and the Galilean satellites of Jupiter. The text is in Latin, with Galileo honouring his Medici patrons by naming Jupiter's moons as the 'Medicea Sidera' (Medician stars). These observations were first made in 1610, using the telescopes that Galileo had built in 1608. Galileo saw that the terminator (the line between the Moon's night and day sides) was sometimes irregular and sometimes smooth. He deduced that the irregularities were due to mountains on the Moon. This challenged the existing worldview that said the heavens were perfect and unchanging. This page is from the 1610 edition of 'Sidereus Nuncius', published in Venice.