Galileo Galilei

Sidereus Nuncius

“In this short treatise I propose great things for inspection and contemplation by every explorer of Nature.”

Galileo Galilei’s (15 February 1564 – 8 January 1642) Sidereus Nuncius (The Sidereal Messenger or The Starry Messenger) was a landmark publication in 1610 that changed humankind’s understanding of the moon, stars, sun, universe, and, by extension, our position among them.1

In this short treatise I propose great things for inspection and contemplation by every explorer of Nature. Great, I say, because of the excellence of the things themselves, because of their newness, unheard of through the ages, and also because of the instrument with the benefit of which they make themselves manifest to our sight. Certainly it is a great thing to add to the countless multitude of fixed stars visible hitherto by natural means and expose to our eyes innumerable others never seen before, which exceed tenfold the number of old and known ones.

Sidereus Nuncius also introduced the telescope as a legitimate means of technical observation. The unseen details of the sky were suddenly visible. Wrote Galileo of this remarkable device:

… A new contrivance of glasses [occhaile], drawn from the most recondite speculations of perspective, which renders the visible objects so close to the eye and represents them so distinctly that those that are distant, for example, 9 miles appear as they were only 1 mile distant.

Galileo’s telescope, fashioned with his own specially cut lenses and his advanced knowledge of optics, allowed him to view the moons of Jupiter (previously considered stars) as well as the mountains and valleys of the earth’s moon.

Galileo Galilei's Phases of the Moon showing earth-like scapes demonstrating the imperfectibility of these heavenly bodies. Published in Sidereus Nuncius
Galileo Galilei’s Phases of the moon showing earth-like scapes demonstrating the imperfection of these heavenly bodies. Source: Sidereus Nuncius

When Galileo looked through his telescope in 1609, there were essentially two prevailing schools of thought about non-earth structures: The Ptolemaic system considered the earth the center of the universe and everything above part of the unchanging divine, and the Copernican model, conceived by the eponymous Polish mathematician a century before Galileo, considered both the sun and earth as fixed centers of the universe.

Both theories had weaknesses, and neither had been proven using strict observational measurement.

Galileo did not prove the Copernican system, but he introduced significant doubt in the logic of the geocentric theory. (His subsequent study of the phases of Venus—only observable with a telescope—proved the Ptolemaic system impossible.) He also disproved the Aristotelian logic that bodies in heavens were unchangeable.

As Albert Van Helden states in the introduction:2

What intrigued Galileo … about the Moon was the irregularity of its surface as revealed by the new instrument. According to the then prevailing geocentric cosmology of Aristotle, the heavens were perfect and unchanging, and heavenly bodies were perfectly smooth and spherical.

What Galileo saw and noted were moving and rising shadows on the moon, what he described simply as “lighter and darker shades” and “shadows of rising prominences.” But his conclusion left no doubt that the moon was, in fact, earth-like.

It is thus known for certain and beyond doubt that they appear this way because of inequalities in the shapes of their parts and shadows moving diversely because of the varying illumination by the Sun.

Sidereus Nuncius
“I felt about for my journal and laid there holding it, waiting for the moon to reappear” a night wakefulness captured by Patti Smith. Did Galileo feel similarly? Photograph by Ellen Vrana.

The great leap in consciousness that accompanies advances in science (like Rachel Carson’s 1962 study of pesticides) can both expand our sense of being and render us quite helpless.

In 2019 we learned how a black hole appears—how does that change things? What else do we not know that could change all we know?

Galileo's drawing of the moons of Jupiter featured in "Sidereus Nuncius" in The Examined Life Library.
Galileo’s drawing of the moons of Jupiter. Source: Sidereus Nuncius

My trusted guide to certainty in our uncertain time is Alan Lightman, a professor of humanities and physics who organizes knowledge and shows its gaps: “Nothing lasts. Nothing is indivisible. . . . Nothing is whole. Nothing is indestructible. Nothing is still.” Read more in The Accidental Universe and Searching for Stars on an Island in Maine.

Alan Lightman - Searching for Stars on an Island in Maine
Vincent van Gogh’s “Starry-Night” 1889, painted by one of our most compassionate and thoughtful creatives the year before he died.

Galileo’s understanding of our moon, stars, and sky handed humans life-changing knowledge. Yet, the sky and stars remain an illustrative metaphor that extends beyond the boundaries of science. A particular genius and generosity of humankind to imagine beyond perception.