I first saw this drawing in Thomas Christensen’s book 1616: The World in Motion, which I recently reviewed. According to a blog post Christensen made before the publication of his book, this sepia-toned drawing of the phases of the moon “was found in a manuscript copy in Galileo’s hand of his Starry Messenger, the book that made him an international celebrity.” The manuscript now resides in Florence’s National Library.
In 1609, Galileo, at that point a professor of geometry and astronomy at the University of Padua, heard about a new invention, the “spyglass,” that made distant objects appear closer. With help from other designers, he refined the instrument, increasing its magnifying power to something like 20x. Turning his new telescope to the sky, Galileo recorded his observations of the phases of the Moon and Venus, the four largest moons of Jupiter (now, in tribute to their discoverer, known as the Galilean satellites – Io, Europa, Ganymede and Callisto), sunspots, and the Milky Way. The drawing of the phases of the moon may represent sketches that Galileo made as he was actually looking in his telescope, or they may be the model for the engravings that eventually appeared in Starry Messenger (Sidereus nuncius), which was published in March 1610.
It’s worth remembering that drawings like this attracted much controversy. The idea that the moon wasn’t an ideal sphere, as most people then thought, but rather, as Galileo described it, “uneven, rough, and crowded with depressions and bulges” and “chains of mountains and depths of valleys,” was thought blasphemous. In later years Galileo was in fact tried by the Inquisition and imprisoned. Ultimately, though, these and other observations, and Galileo’s interpretations thereof, helped finish off the Earth-centered Ptolemaic model of the universe and to confirm the heliocentric model proposed by Copernicus in 1543.
By the way, in case you’re still looking for the perfect holiday present, Galileo’s drawing is available on a t-shirt from Zazzle.