We live at a unique point in history. For the first time, we humans know the entire story of our species, at least in broad outline. We know how the universe expanded from the initial Big Bang, how generations of stars manufactured a periodic-table’s worth of elements and then dispersed them throughout space as those stars exploded, and how clouds seeded with those elements gravitationally collapsed to form planets. We understand the evolution of the life that arose on this particular planet, and how an astronomical impact led to the dominance of the hairless apes that eventually became our friends and neighbors.
But what was it like to be alive four centuries ago when almost everything was a mystery? What was it like to discover, for the very first time, that the Moon has mountains, that there is a universe filled with stars we cannot see with the naked eye, and that other moons orbit Jupiter? Fortunately, we know exactly what it was like because the man who made these discoveries has told us: Galileo Galilei.
Sidereus Nuncius (The Starry Messenger) is not filled with the dry dialectics of Galileo’s other tomes. In this book you can sense Galileo’s exuberance, his sense of wonder at what he has seen for the very first time through the crude telescope he made with his own hands. He tells you how he labored over its construction until he could see objects “over sixty times larger.”
Galileo writes that “having dismissed Earthly things, I applied myself to explorations of the heavens.” He grabs your sleeve to pull you toward his eyepiece so you can see these wonders for yourself.
And what wonders they were to his eyes! Galileo sees the tops of mountains on the Moon lit by the Sun, and asks us, “On Earth, before sunrise, aren’t the peaks of the highest mountains illuminated by the Sun’s rays while shadows still cover the plains?” Galileo alone now knows that the Moon is not a perfect sphere. Using shadows, he calculates that one lunar mountain is “higher than 4 Italian miles.”
Galileo swings his telescope toward the constellation of Orion, and breathlessly tells us that “to the three [stars] in Orion’s belt and six in his sword that were discovered long ago, I have added eighty others.”
Then, on January 7, 1610, Galileo trains his telescope on Jupiter to see “three little stars” near Jupiter that are “arranged exactly along a straight line and parallel to the ecliptic.” Night after night Galileo keeps track of these stars, now grown to four, as they stalk Jupiter, passing back and forth across its disk.
Finally, on March 2, Galileo calls them “planets,” and later, the “Medicean planets.” (In the opening passages of Sidereus Nuncius, Galileo, in his never-ending quest for patronage, proposes naming these four moons of Jupiter for Cosimo II de’ Medici, the Fourth Grand Duke of Tuscany.)
Thirty years ago, I attended a meeting of the American Astronomical Society in Ann Arbor. There on display was a draft of a short letter Galileo sent to the Doge of Venice on August 24, 1609 that described his telescope. But at the bottom of the letter are Galileo’s first recordings of the moons of Jupiter, made on this paper he happened to have nearby.
I felt overwhelmed knowing that when Galileo’s hand made these marks upon this sheet of paper, the world changed. Galileo now knew with certainty that Earth was not the center of the all things, because here were four moons orbiting Jupiter. Galileo went on to make more astronomical discoveries. He discovered spots on the Sun and the phases of Venus, but his Sidereus Nuncius announced his first discoveries to the world.
Reading the Sidereus Nuncius, I am struck by encountering a fully modern mind, so different from the mysticism of Johannes Kepler. It marked a revolution. After Sidereus Nuncius, astronomy no longer had to rely on the word of ancient authority for its conclusions. Astronomy became an observational science, and anyone with a telescope could see what Galileo saw. Sidereus Nuncius is a short book, just 62 pages. My version, translated by Albert Van Helden, has useful notes along with an introduction and conclusion. Read it for yourself and be present with Galileo at the beginning of modern astronomy.