How Was Galileo Converted?

painting by Cristiano Banti (1857)

Do you believe that the sun is the center of our solar system?  Why?  Very few people have ever carried out the measurements necessary to determine that earth moves around the sun.  Most of us simply accept it by faith, backed by scientific authority.   But what would we believe if authorities told us that earth didn’t move?  That is the situation that Galileo found himself in 400 years ago.  Though Copernicus had already published his theory of a sun-centered universe, he only had some elegant mathematics but no physical laws to support him.  Few scholars took the heliocentric theory seriously.  What convinced Galileo?

Owen Gingerich in Yale's Beinecke Library, examining a first edition of Copernicus' De revolutionibus (1543)

Owen Gingerich, Professor emeritus of Astronomy and History of Science at the Harvard-Smithsonian Center for Astrophysics, recently pinpointed the exact day and astronomical observation that transformed Galileo from a timid believer to a Copernican crusader.  The precision of this date is remarkable given that Galileo’s conversion happened 400 years ago at a turbulent time in European history, in which only fragmentary evidence remains.  Gingerich, in collaboration with Dutch colleague Albert van Helden, presented these findings at the Library of Congress during its celebration of the 400th anniversary of the publication of Galileo’s Sidereus Nuncius (Starry Messenger).

Galileo was not the first public champion of Copernicus.  In 1596, Johannes Kepler published The Sacred Mystery of the Cosmos, which explicitly argued for a sun-centered universe.  Galileo received two copies of the book and wrote a thank-you letter congratulating Kepler on his achievement.  Nevertheless, Galileo continued to teach geocentric theory to his students for the next 12 years.

Galileo's drawing of Earth's moon (source: National Library-- Florence, Italy)

But everything changed when Galileo started using the newly invented telescope in the fall of 1609.  By November he began producing images of the phases of the moon.  With the increased magnification of his instrument, he detected small patches of light beyond the dark edge of the moon.  Though scholars had long believed that the moon was perfectly spherical, Galileo concluded that the moon must have mountains.  Just like on Earth, the tops of mountains catch the first rays of sunlight each morning, briefly illuminating them while the rest of the landscape remains dark.  Through Galileo’s telescope, traditional theories about the heavens began to collapse.

Galileo's original notebook, depicting the relative positions of Jupiter's moons (National Library-- Florence)

But an even greater discovery awaited Galileo when he turned his attention to Jupiter.  With his telescope, he detected small stars next to Jupiter not visible to the naked eye.  But even more surprising, during subsequent nights, it appeared that they had moved!  Clearly these were not fixed stars like other heavenly bodies.  Also peculiar was that these stars remained in a nearly straight line, regardless of where they moved.  What could possibly produce this effect?

On the night of January 13, 1610, Galileo’s worldview changed forever. Using two different Galilean manuscripts from that night, Gingerich and Van Helden deduced that Galileo was at first puzzled, and then later realized that the little stars were actually in orbit around Jupiter.  That night, Galileo became an enthusiastic believer in the Copernican system, and he began aggressively promoting his views, even though it would put him at considerable personal risk.

Modern edition of Galileo's "Sidereus Nuncius" (University of Chicago Press)

Galileo knew that his discovery would become an international sensation, so two days later he started taking notes in Latin instead of Italian.  He quickly published Sidereus Nuncius, named Jupiter’s moons after his future patron Cosimo de Medici, and assured his place in the pantheon of scientific heroes.  Moreover, many historians have claimed that Galileo’s Sidereus Nuncius was the first publication of truly modern scientific research, signaling a new era of human history.

That same year, Galileo marshaled additional evidence for a sun-centered universe.  With his telescope he observed that Venus went through phases much like our moon does.  That would only be possible if Venus orbited the sun, not the earth.  Nevertheless, Galileo’s convictions did not become mainstream until long after his death.  Since telescopes were rare and rudimentary in the early 17th century, critics questioned the quality and reliability of Galileo’s observations.  They also wondered, “If the earth is twirling around at high speed, why don’t people just fly off into space?”  Even now, if you were asked this question, would you be able to provide an adequate answer?

When examining such complicated events, we must recognize that scientific theories are rarely subject to indisputable proof.  Owen Gingerich, a highly accomplished astronomer himself, maintains that “scientific knowledge comes from building a coherent picture of complex phenomena.  In science, we don’t look for proofs; we rely on evidence and reason to create plausible explanations.”  If critics refuse to grant these well-reasoned arguments, there is no dissuading them.

Galileo’s conversion experience, bolstered by meticulous observations and clear logic, transformed our understanding of the cosmos.  He could not prove his findings to stubborn opponents, but he was vindicated 300 years later when astronomers with high-powered telescopes observed stellar parallax and confirmed the earth’s motion.  Those who lacked Galileo’s faith in the Copernican system, waiting for definitive proof, were left in the dustbin of history.

Modern images of Jupiter's moons, taken by NASA's satellite "Galileo"

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  1. Thanks for this wonderful report on Galileo. It remains quite important in our day to explain the difference between proof and evidence. Think only of global warming (anthropogenic or otherwise), vaccines, genomics, medical diagnostics, radioactive dating, cosmic expansion — each have their detractors who routinely point to lack of proof, but who thus demonstrate their misunderstanding and/or misrepresentation of the character of science.

  2. Philip Bays says:

    This is wonderful. Now we (ASA members) need to promote the availability of this resource to the wider community!

  3. Ron Carlson says:

    Hi, Tom. I at first didn’t notice this “Leave a Reply” and instead posted my comment on Facebook by first clicking on the little Facebook icon. It seems to have posted it to my own profile, though, not to an ASA Facebook page. So, I guess the purpose of the Facebook icon is to tell others about this site and copy one of the images from the site to link back to it. I’m catching on…

    Anyway, VERY NICE JOB ON THE FIRST ISSUE, TOM! Nice layout and GREAT writing! I like, too, that I can zoom in on the Galileo notebook page by clicking on it, then using CTRL+ to get as much magnification as I want with surprising resolution. Thanks for providing that special treat with the high def image.

    You mentioned in your article that when Galileo realized he was onto something of great significance, he switched from writing in Italian to Latin, the universal language of science. I thought that was a really cool, chills-up-the-spine moment for Galileo and for us, 401 years later, in vicariously witnessing what excitement he must have felt. If I recall correctly, Prof. Gingerich (in his lecture to the Wheaton-Naperville ASA chapter at Wheaton College in November) said that he just noticed that switch to Latin recently. I don’t know if the Galileo notebook page in your article is for January and shows that particular switch to Latin (my Latin is more than a little rusty), but if not, I wonder if you or Prof. Gingerich could post that page in high def and tell us where the switch took place. I’d like to print it out, frame it, and put it up on my den wall. It’s not often that you see written historical evidence of an “AHA!” moment, let alone one of such great significance in the history of science! It was nice of Galileo to share it with us.

  4. Dan Claire says:

    Great article! I just talked with my kids about this. They have also enjoyed hearing the story of Galileo’s paradigm shift as told by Jim Weiss. (

  5. Eric Bergerud says:

    Mr. Sikemma’s comment has me scratching my head a bit. I understand the distinction between evidence and proof. However, I’m not sure that because Galileo got it right that the “detractors” in the other fields have it wrong. When I was an undergraduate in the mid-60s much of what we consider foundational science was done: nobody quibbled with Einstein or Newton. (At least not in generalities.) But consider the range of beliefs held by the top people in their respective fields at the time. 1. Biology: Excitement was very high that work inspired by Miller-Urey would lead soon to the creation of life in the libratory: we’re still waiting on that one. 2. Geology: we freshmen were told about the wonderful theory of plate tectonics and continental drift – but warned that it was only a theory and that much had to be done. (This was 1966: we can see that when I was at my desk the work confirming this theory was just being published.) 3. Astronomy: Here again, nippers were taught that although the “Big Bang” theory looked good that many wise heads, like Fred Hoyle, disagreed with it. So the universe might be expanding, but perhaps steady state was possible. 4. Psychology: I was at the University of Minnesota which was a stronghold of behavioralism and most there thought Skinner walked on water. The ideas of Sigmund Freud were still taken extremely seriously as useful tools for therapy and analytical inquiry. I don’t think you’d find quite the same take in our world where busy fans of the intelligent gene consider people like Freud and Skinner quaint (do psychiatrists today give therapy at all or simply monitor the latest brain dart?) while they pursue the music gene, the war gene, the peace gene and the god gene.

    I’ve always envied people in the hard sciences. I don’t see how one could grow bored trying to make sense out of the natural world. But it must be very humbling. I wonder if humility fits the psychological profile of a good research scientist.

  6. Will Duke says:

    I fondly remember my excitement when I had the privilege of looking through a telescope at Jupiter for the first time and realized that the “stars” I saw nearby were the Jovian satellites! I think I felt a little of the thrill Galileo must have known! Awesome! That experience fueled my love of science and learning about the universe God created. Thanks for this article.