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, 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 400^th 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.

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.

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.

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 17^th 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.