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Sidereus Nuncius
Sidereus Nuncius, Galileo Galilei, 1610, printed book. Observatories of the Carnegie Institution for Science Collection. The Huntington Library, Art Museum, and Botanical Gardens. 487000:0071.
A New Way of Knowing
In 1610, Galileo Galilei began a scientific revolution when he shared a new way of studying the universe in his book Siderius Nuncius. Sidereus Nuncius means “Starry Messenger” in Latin. Galilei viewed the 550 copies of his book as messengers of his technological and scientific discoveries. In Siderius Nuncius, Galilei described the telescope and shared two revolutionary discoveries he had made using his new telescope.
Galileo Galilei
Portrait of Galileo Galilei, 1624 reproduced in an unidentified book. Observatories of the Carnegie Institution for Science Collection. The Huntington Library, Art Museum, and Botanical Gardens. COPC 2868.
Galileo Galilei was born in 1564 in Pisa, Italy. As a university student at the University of Pisa, Galilei decided to study mathematics even though his father wanted him to study medicine. After graduating, he had trouble getting steady work because his mathematical ideas contradicted many of Aristotle’s ideas. In the end, his patrons helped him become the chair of mathematics at the University of Padua (near Venice) from 1592 to 1610.
To support his family, Galilei worked side jobs in addition to his job as a professor. One of his side jobs was designing scientific instruments.
In 1610, with the publication of Sidereus Nucius, Galilei’s life changed. He dedicated the work to Cosimo II, who was the Grand Duke and a member of the very powerful Medici family. Because Galilei earned the favor of the Medici family, Cosimo II appointed Galilei as his personal mathematician and philosopher. This was a big status increase for Galilei.
Galilei’s ideas continued to challenge existing ideas about the universe. Many of the existing ideas about the universe were very important to the Catholic Church’s ideology. Galilei’s life changed again when he was sentenced to house arrest in 1633 by the Catholic Church.
Telescopes: A New Technology
Photograph of Galileo Galilei's object glass and telescopes reproduced in an unidentified book. Observatories of the Carnegie Institution for Science Collection. The Huntington Library, Art Museum, and Botanical Gardens. COPC 1406.
Galilei did not invent the telescope, but he was by far the telescope’s most successful developer and spokesperson.
In 1609, Galilei heard about telescopes being manufactured in Europe and he constructed his first telescope. This telescope magnified objects to the eighth or ninth power, meaning it made the objects appear eight or nine times larger than they appear to the naked eye.
The telescopes being developed by Galilei and others were not originally intended to help humans learn about the night sky. They were intended to help humans wage war. When Galilei gave Cosimo II his first telescope as a gift, he wrote about the possible military uses of the technology.
Galilei continued to improve the telescope’s technology by manufacturing, polishing, and refining his own lenses. His early efforts were held back by the poor quality of the available glass and a scarcity of chemicals to treat the glass. Only about 10 percent of the telescopes Galilei built were useable. Through trial and error, Galilei figured out that magnification depends on the focal lengths of two lenses. Once he figured this out, he knew he needed weaker convex lenses (lenses that curve up in the center, like a hill) or stronger concave lenses (lenses that curve down in the center, like a bowl). By November 1609, Galilei had created a telescope that magnified objects to twenty times their original size.
Engineers define a problem, imagine solutions, make creations, test their creations, and make improvements based on the results. How is Galilei an engineer?
Around this time, Galilei began considering the ways this new technology could be used for scientific discovery. With a new goal of learning more about the night sky, Galilei turned his 20× power telescope to the moon.
The Moon
The Moon, United States Environmental Protection Agency
Before Galilei published Sidereus Nuncius, accepted astronomical knowledge of the time described the heavens as perfect. Aristotle described the heavens as being the “realm of perfection.” Because of this description, the moon presented a conundrum. Even without telescopes, we can see darker and lighter parts of the moon. Aristotle developed an explanation for the moon’s features that worked with his understanding of the heavens, and Medieval Islamic and Christian scholars continued to develop possible explanations that aligned with Aristotle’s ideas. Perhaps the moon had varying densities that made the perfectly spherical moon appear imperfect? Perhaps vapors between the sun and the moon were reflecting light and creating shadows on the moon?
When Galilei looked at the moon through his telescope, he saw the familiar dark spots, but he also saw smaller spots that no one had ever seen before. These spots gradually changed as the angle of illumination changed. He focused his attention on the dividing line between lightness and darkness (the terminator). He noticed that this line was not a smoothly curved line (which would be expected from a smooth surface). Instead, it was a jagged line with light and dark parts that varied depending on the time of night. Based on his observations of the changes in light and shadow on the moon’s surface, Galilei reached a conclusion. He concluded that the moon is not a perfectly smooth sphere, but instead it has mountains and valleys.
Galilei created a series of illustrations and sketches of the Moon. These illustrations and sketches documented what he observed while looking through his telescope. Use the questions and prompts below to explore Galilei’s moon illustrations and sketches.
Questions & Prompts
Describe what you see.
Look closely at one drawing for a full minute. What do you notice? Compare this drawing with the other five. In what ways is it unique? What does it have in common with other drawings?
Compare one of the illustrations to the photograph in the above section. How do Galilei’s illustrations capture what you see when you look through Galilei’s telescope? Is there anything Galilei’s illustrations missing?
What artistic process do you think Galilei used to create these illustrations?
What do you think Galilei was most proud of with these drawings?
What do you think Galilei hoped people would learn by looking at his work?
The text on this page is Latin, not English. Because English borrows many words from Latin, some of the words might look familiar. Do you recognize any words? What do these words mean?
Jupiter
Before Galilei published Sidereus Nuncius, accepted astronomical knowledge of the time described the Earth as the center of the universe. Only seven celestial bodies had ever been observed to move (Earth, sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn), and only the Earth had ever been observed to have a moon circling it. If the Earth wasn’t the center of the universe, why was it the only planet with a moon circling it?
After making his observations of the moon, Galilei made adjustments to his telescope to improve the image quality of small, bright objects. At the beginning of January 1610, he was ready to observe the planets. Planets and their moons look to us like stars because they reflect light from the sun. Venus was only visible in the mornings during this time, and Saturn and Mars were close to the sun, so Jupiter became Galilei’s focus.
The first time Galilei looked at Jupiter through his telescope, he observed three little stars in a row. He assumed that Jupiter happened to be passing through these stars and that the next night Jupiter would have moved to the West (continuing its orbital journey) and the stars would have moved to the east. However, the next night, Galilei saw that his prediction had been wrong. The stars had moved to the west on the same straight line.
Almost every night from January 7th to March 6th 1610, Galilei made observations of Jupiter. Over time, his observations showed that there were four little stars moved in straight lines with Jupiter and with respect to each other. The stars and Jupiter didn’t drift apart. They stayed together and moved in predictable patterns. Based on his observations of these bodies’ movements, Galilei came to the conclusion that Jupiter has four moons. This conclusion showed that the Earth is not the only planet with moons circling it.
Use the image slider below to compare the first page of Galilei’s research notes to the first few pages of the Jupiter section of Sidereus Nuncius. Each diagram in these pages of Sidereus Nuncius directly corresponds to one of the diagrams in the page of research notes. Match each published diagram to Galilei’s notes. Do you notice any patterns?
References and Resources
Brashear, Ronald, Daniel Lewis, and Owen Gingerich. 2001. Star Struck: One Thousand Years of the Art and Science of Astronomy. San Marino, CA: The Huntington Library in association with the University of Washington Press, Seattle.
Department of History and Philosophy of Science of the University of Cambridge. n.d. “Sidereus Nuncius: The Starry Messenger.” Accessed May 4, 2022. http://www.sites.hps.cam.ac.uk/starry/galsidnun.html.
Galilei, Galileo. 1989. Sidereus Nuncius or The Sidereal Messenger. Translated by Albert Van Helden. Chicago: University of Chicago Press.
Micro Observatory. n.d. “The Moon – Then & Now.” Accessed May 4, 2022. http://mo-www.harvard.edu/microobs/guestobserverportal/Galileo/ThenNow/Moon/mObsMoonWeb.htm.
PBS. 2019. “Galileo’s Moon: About the Film.” May 8, 2019. https://www.pbs.org/wnet/secrets/galileos-moon-galileos-moon-about-the-film/4309/.
Van Helden, Albert. 1995. “The Galileo Project.” Accessed May 4, 2022. http://galileo.rice.edu/sci/observations/moon.html.
