Galileo Galilei

Sidereus Nuncius

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

Galileo Galilei’s (1564–1642) Sidereus Nuncius (The Sidereal Messenger or The Starry Messenger) was a landmark publication that changed humankind’s understanding of the moon, stars, sun, universe, and, by extension, our position among these things.

It 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 illustrations of the phases of the moon, Published in 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 observed with a telescope—proved the Ptolemaic system wasn’t possible.) He also disproved the Aristotelian logic that bodies in heavens were unchangeable.

As Albert Van Helden states in the introduction:1

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
Earth’s moon as it appears to the naked eye. Galileo’s telescope revealed much more detail. 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. We now know how a black hole appears—how does that change things? What else do we not know that could change all we know?

My preferred 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 of Lightman’s work in The Accidental Universe and Searching for Stars on an Island in Maine.

Galileo’s understanding of our moon, stars, and sky changed the way humans viewed themselves. Yet, stars remain an illustrative metaphor that extends beyond the boundaries of science. Read more in “The Meaning and Metaphor of Stars.”