Discoveries and Opinions of Galileo

Nonfiction | Book | Adult | Published in 1957

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Part 1

Part 2

Part 3

Part 4

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Part 1Chapter Summaries & Analyses

Part 1 Summary: “The Starry Messenger”

Galileo addresses an elaborate introduction to his patron, Cosimo II de’ Medici, on the nature of human fame. In Galileo’s view, we build monuments to great men to preserve their achievements, but we ought instead to look beyond mere earthly realms to the heavens. Galileo even decides to name Jupiter’s suns—which he recently discovered—the Medicean stars after Cosimo. He hopes that these stars will immortalize Cosimo’s strengths and virtues.

Via a “new spyglass” (the newly invented telescope), Galileo was able to view the surface of the moon magnified to 900 times larger than it appears to the naked eye. In doing so, Galileo discovered that the moon’s surface is not smooth and polished, as previously thought, but rough and uneven and “covered everywhere, just like the earth’s surface, with huge prominences, deep valleys, and chasms” (28). In addition, the telescope enabled Galileo to view the Milky Way more accurately than ever before, and—a discovery which “surpasses all wonders by far” (28)—to discover four “wandering stars” attached to Jupiter. Galileo hopes that the telescope will aid in the discovery of “still more remarkable” (28) things in the future.

Galileo next explains how he came by the telescope and describes its nature and construction. Galileo heard that a lens grinder in Holland had constructed a spyglass composed of two lenses, which allowed the viewer to see distant objects as if they were nearby. Through information provided by one of his former students in Paris, Galileo constructed his own version of the spyglass, which operated on the principle of refraction. Although realizing that the telescope would have important uses at sea and on land, Galileo instead used it to observe the heavens.

After explaining how the telescope is constructed and how it magnifies objects and measures distances, on Page 31 Galileo begins to review the discoveries he made with it. Galileo sees the moon’s surface as divided into a lighter and a darker portion. Both portions are covered with spots: Larger spots predominate on the darker portion and smaller spots predominate on the lighter portion—the latter of which Galileo was the first in history to notice. Observing these spots at different times of night and throughout the lunar cycle, Galileo notes that they change in shape and brightness, indicating that the sun shines on them differently according to the moon’s distance from the sun during its cycle. From this observation, Galileo deduces that the moon’s surface is uneven, covered with chasms and mountains much like the surface of the earth.

Galileo observes further that one section of the moon seems to shine with its own light, regardless of whether the sun is shining on it or not. Galileo proposes that this “secondary light” of the moon is the result of secondary reflection of sunlight from the earth. Galileo thus hypothesizes an exact relation between the moon and the earth: “[W]henever the earth is most brightly lighted by the moon, the moon is least lighted by the earth, and vice versa” (45).

On Page 45, Galileo proceeds to discuss his discoveries concerning Jupiter’s stars. Looking at stars, one perceives them not as round but as having fringes or rays. Using the telescope, one can see stars in their natural shape and size. Through the telescope, Galileo can see that the Milky Way consists of innumerable stars clustered in various formations. He presents drawings of several constellations, including the Pleiades (the six stars of Taurus) and the Head of Orion.

On Page 50, Galileo begins to detail Jupiter’s stars. One night in January, Galileo discovers two stars next to Jupiter. On preceding nights, and continuing into March, these stars change position in relation to Jupiter, and eventually a fourth star appears. Galileo concludes that four stars revolve around Jupiter, much as the moon revolves around the earth and as it in turn revolves around the sun. Galileo considers this a momentous discovery, since it gives credence to the Copernican theory of the earth and sun.

Finally, Galileo questions why the sun and moon appear large to us, while stars appear extremely small and seem to change shape. He hypothesizes that the sun, moon, and other planets are covered with “terrestrial vapors” that make them appear larger and that, as the vapors move, the stars appear larger or smaller to us.

Part 1 Analysis

“The Starry Messenger” was originally written in Latin as “Sidereus Nuncius” (“the starry message” or “the starry messenger”). Galileo chose Latin to reach a wider scholarly audience than would have been possible through Italian. In the early 17th century, Latin was still the international language of the learned in Western Europe, although vernacular languages were quickly gaining traction and prestige. The other works in this volume were originally written in Italian. Galileo’s choice of Latin for this one shows that he considered the discoveries he wanted to communicate in this work to be particularly important.

Like the other three works in this volume, “The Starry Messenger” is written in the form of a letter. The addressee is Galileo’s patron and student, the Grand Duke Cosimo II de’ Medici. A member of the powerful Medici family of Florence, Cosimo appointed Galileo as his teacher of philosophy and mathematics in 1610, the same year that “The Starry Messenger” was published.

Following literary conventions of the day, Galileo writes an elaborate and flattering dedication to Cosimo, declaring his intention to name the newly discovered stars of Jupiter after him. The dedication reflects an era when scientists and artists relied on the favor of powerful patrons to support their work. In flowery language, Galileo likens Cosimo’s moral qualities to the stars that orbit Jupiter: “[W]e are able, most serene Prince, to read Your Highness in the heavens” (24).

Additionally, Galileo gives his discoveries a religious significance, attributing them to his being “illuminated by divine grace” (28). This language reflects how, in Galileo’s era, science and religion were still regarded as intricately connected.

For Galileo, the significance of his discoveries about Jupiter is that they confirmed the truth of the Copernican system, since they showed that planets other than Earth orbited the sun. Galileo’s advocacy of heliocentrism (see Index of Terms) became the focus of the controversy surrounding his work in the years after “The Starry Messenger” was published. Putting the Copernican theories in context can help shed light on the opposition against Galileo.

Before Galileo’s time and going back to ancient times, the sun, moon, and planets were assumed to embody an ideal perfection. They were believed to be perfectly spherical in shape and to move in symmetrically circular patterns. They were not regarded as physical objects made of ordinary matter but rather as objects consisting of “some kind of superior substance free from all change” (Introduction: First Part, Page 11).

The astronomers Nicolaus Copernicus, Tycho Brahe, Johann Kepler, and later, Galileo used close observation of the planets to show that these bodies did not always conform to human ideas of “perfection.” The sun had blemishes, or spots, on its surface. The moon was not perfectly smooth but filled with craters and valleys. What’s more, the Earth did not enjoy a privileged place at the center of the universe; instead, it orbited the sun along with the other planets.

These discoveries proved disturbing to many since they contradicted long-held notions of what the cosmos was like—ideas that touched on science, philosophy, and theology. Judeo-Christian theology held that God had a special predilection and care for humankind; this was shown in His establishing the heavens with the earth at the center and the sun revolving around it. Heliocentrism seemed to unsettle this conception of the heavens. Moreover, literal readings of certain biblical passages seemed to clash with a heliocentric model.

Additionally, many opposed heliocentrism on philosophical and scientific grounds. The theory seemed at first to have weak evidence behind it and to contradict our everyday sense experience. After all, the sun appears to rise and set, and we are not physically aware of the earth turning. Thus, Copernicus’s ideas seemed strange and absurd to many when first proposed. Only a generation later did a series of new discoveries lend increasing credence to Copernican theories. Among these groundbreaking discoveries were Galileo’s observations about Jupiter’s stars, documented in “The Starry Messenger.”

Galileo alternately uses the words “planets” and “stars” to describe the bodies orbiting Jupiter. The distinction between the two was not airtight in Galileo’s day, and scientists frequently speculated on what material—whether solid or fiery or vaporous—various planetary bodies were made of.