Blowing & Drifting

Tonight, the moon is waxing toward a full moon on New Year’s Eve, which makes this as fitting a moment as any to note that almost exactly four hundred years ago, between November 30 and December 18, 1609, the Italian scientist Galileo Galilei used a hand-made telescope (and other tools, as shown in this excellent Smithsonian article) to look at the moon, examining it for the first time in human history like a scientist – indeed, using his looking to help invent modern science. Using the telescope, Galileo scrutinized the discolored areas that are visible with the naked eye. As put on an excellent webpage by an undergrad at Rice,

Galileo’s observations led him to the startling conclusion that the moon is anything but perfect. With his telescope, he noticed small dark spots that had never been seen before on the illuminated part of the moon’s surface, along with similar light spots in the dark area. He observed that as time passed, these and other spots changed, either getting lighter and eventually disappearing or getting darker and more distinct. The interface between the light and dark sides of the moon was rough and uneven, rather than smooth as would be expected on a perfectly smooth sphere. Galileo also observed that the spots all “have a dark part on the side toward the Sun while on the side opposite the Sun they are crowned with brighter borders like shining ridges.” (Sidereus Nuncius, p 41)

From these and many other observations, Galileo concluded that the moon’s surface consists of valleys, plains and mountains much like the surface of the Earth (Sidereus Nuncius, p 48-49). The dark spots are shadows cast by these mountains and valleys as the sun falls on them. As the moon’s position relative to the sun changes, the shapes and intensities of these shadows change. Galileo’s conclusion was a shocking one– how can the moon, a heavenly body, not be perfect and spherical? If the moon is imperfect, could there be other imperfect heavenly bodies as well? If heavenly bodies can be imperfect, why can the Earth not be a heavenly body? Galileo’s conclusions about the moon did not give solid proof of Copernicus’ theory, but his observations paved the way for the theory’s eventual acceptance.

Not only did Galileo help advance the earthshattering ideas that Earth is not the center of the universe, and that perhaps God had not made all of the universe in a perfect form, but he drew some awfully good pictures of the moon as he saw it through his telescope. Indeed, the art historian Samuel Edgerton argues, in his recent book, The Mirror, the Window, and the Telescope: How Renaissance Linear Perspective Changed Our Vision of the Universe (thoroughly reviewed here), that Galileo’s skill as a draftsman, and especially his knowledge of perspective (an artistic and scientific innovation which was not even a century old when Galileo learned it), enabled him to understand that the discolorations visible on the moon were actually the evidence of not just of imperfections like spots on cowhide, but of irregularities such as mountains and craters. In other words, drawing led directly to scientific discovery – and spun off great art such as these sketches by Galileo.

Six Phases of the Moon, by Galileo

Four More Phases of the Moon, by Galileo