Galileo

by Mario Livio

  Things started to progress in leaps and bounds at the end of 1609. In December of that year and January 1610 alone, Galileo probably made more earthshaking discoveries than any other person in the history of science. In addition, he managed to improve his telescope to fifteen-power by November 1609 and to twenty-power or more by March 1610. Turning this improved device to the night skies, he started by observing the surface of the Moon, moved on to resolving stars in the Milky Way, and then to making the revolutionary discovery of the satellites of Jupiter. Armed with these truly surprising findings, he decided to promptly publish his results, fearing that another astronomer might scoop him out of what he correctly perceived as momentous revelations. Indeed, The Sidereal Messenger (Figure 4.1) appeared in Venice as early as March 13, 1610. Perhaps not unexpectedly, Galileo’s burst of creativity followed—and was almost certainly helped by—the departure of his mother from Padua. Not only did Giulia Ammannati not support her son in his research, she did no less than attempt to convince Galileo’s servant, Alessandro Piersanti, to spy on his master. Constantly suspicious that Galileo’s lover, Marina Gamba, would somehow persuade Galileo to reduce the financial support he was providing his mother, or that she would steal her linen, Giulia recruited Piersanti to secretly report to her about the couple’s private conversations. If that wasn’t enough, she even asked the servant to steal some of Galileo’s telescope lenses, which she intended to give to her son-in-law, the husband of Galileo’s sister Virginia, in what she perceived to be an act of gratitude for the son-in-law’s alleged generosity. Fortunately, Piersanti immediately turned over Giulia’s conspiratorial letters to Galileo.

  Politically savvy at that point in his life, Galileo dedicated The Sidereal Messenger to Cosimo II de’ Medici, Fourth Grand Duke of Tuscany. He went even further and named the four satellites of Jupiter the Medicean Stars, because, he said, “the Maker of the stars himself admonished me to call these new planets by the illustrious name of Your Highness.” The effect of these “heavenly” gifts was expeditious and gratifying. By June 1610, Galileo had been appointed philosopher and mathematician to the grand duke, and chief mathematician, free of teaching obligations, at the University of Pisa. When applying for the position, Galileo insisted upon having the title of “philosopher” added to his position of a court mathematician. One reason for this request was simple: philosophers enjoyed a higher status than that of mathematicians. This was not, however, merely a status-affirming desire; Galileo confessed at the time that he had “studied more years in philosophy than months in mathematics.”

  Figure 4.1. Title page of The Sidereal Messenger.

  Two of the distinguishing characteristics of many of those who truly made a difference in the history of science were: first, their ability to immediately recognize which discoveries could be genuinely impactful; and second, their effectiveness in disseminating their findings and in making them intelligible to others. Galileo was masterful at both. In 1610 he became unstoppable: in the course of just about one year, he discovered the phases of Venus, the fact that Saturn appeared to have a bizarre shape, and that variable spots move across the surface of the Sun. Over the succeeding couple of years, he also published two more books, Discourse on Bodies in Water in 1612, and History and Demonstrations Concerning Sunspots the following year.

  The Sidereal Messenger became an instant best seller—its initial printing of 550 copies sold out in no time. Consequently, by 1611, Galileo became the most famous natural scientist in Europe. Even the Jesuit scientists in Rome had to take notice, and they rolled out the red carpet for him when he arrived for a visit on March 29. While the distinguished astronomer Christopher Clavius had some reservations about the interpretation of a few of the results, generally the mathematicians of the Collegio Romano expressed their trust in the accuracy of the observations themselves and certified the phenomena revealed by the telescope as real. As a result, Galileo was received in audience by Pope Paul V, and by Cardinal Maffeo Barberini, who years later (as Pope Urban VIII) played a crucial role in what has become known as the “Galileo affair.” In addition, both Cardinal Roberto Bellarmino (often Anglicized as Robert Bellarmine), former rector of the Collegio, and Clavius himself, met with Galileo during his Roman visit, and Bellarmino even discussed some aspects of Copernican astronomy with him. The only sign of a potential cloud on the horizon came in the form of a somewhat ominous comment Bellarmino made to the Tuscan ambassador at the end of Galileo’s stay in Rome: “If he [Galileo] had stayed here too much longer, they [church officials] could not have failed to come to some judgment upon his affairs.”

  Another honor conferred upon Galileo during the same trip was his election as a sixth member of Federico Cesi’s Accademia dei Lincei (literally, the “Academy of the Lynx-Eyed”). This prestigious science academy had been founded in 1603 by Cesi, a Roman aristocrat (later Prince of Acquasparta), and three of his friends, and its idealistic goals had been stated as “not only to acquire knowledge of things and wisdom, and living together, justly and piously, but also peacefully to display them to men, orally and in writing, without any harm.” It was named after both the sharp-eyed lynx and Lynceus, “the most keen-eyed of the Argonauts” in Greek mythology. The academy, its membership soon to grow even beyond the borders of Italy, published Galileo’s book on sunspots in 1613, and later his book The Assayer in 1623. Galileo always felt very honored to be an academician, and he often signed his name as: “Galileo Galilei, Linceo.” He and Cesi became bonded not only by their personal affinity, but also by their shared conviction that many beliefs held since antiquity about the natural world had to be abolished.

  What precisely, then, were these wondrous observations of Galileo that for the first time showed humankind what the heavens were really like?

  LIKE THE FACE OF THE EARTH ITSELF

  In 1606 someone named Alimberto Mauri published a satirical book in which he speculated (based on reasoning inspired by naked-eye observations) that the features seen on the surface of the Moon indicated that the lunar surface was covered with mountains surrounded by flat plains. Many science historians suspect that Alimberto Mauri was really Galileo, writing under a pseudonym. Be that as it may, with the telescope in hand, Galileo finally had the opportunity to test this conjecture. Indeed, the Moon was the first celestial object to which he turned his spyglass. What he saw was a surface covered with blemishes and small circular areas that looked like craters. This was, however, where his artistic education came in handy. Observing in particular the terminator—the boundary separating the illuminated part from the dark one—and using his imaginative understanding of light and shadow and his grasp of perspective, Galileo was able to argue convincingly that the lunar terrain was very rugged. He described it as “uneven, rough, and crowded with depressions and bulges. And it is like the face of the Earth itself.” Galileo’s spectacular wash drawings and etchings (Figures 4.2 and 4.3) show points of light in the dark portion, which gradually increase in size toward the boundary.

  Figure 4.2. Galileo’s wash drawings of the Moon, as seen through his telescope.

  Figure 4.3. Galileo’s etchings of the Moon.

  This is precisely what one would expect when, at sunrise, only the tops of mountains are lit up, with the light later creeping down the mountains till it reaches the dark plains. Estimating the distance of one such point of light from the terminator to be about one-tenth of the Moon’s radius, Galileo determined the height of that mountain to be more than four miles. The numerical value itself was later challenged in October 1610 by the German scientist Johann Georg Brengger, who suggested that ranges of mountains on the Moon probably overlap, or the Moon’s rim would have appeared jagged rather than smooth. The precise height notwithstanding, Galileo demonstrated that he could not just see but also—in principle, at least—estimate fairly accurately the size of features in the lunar landscape. Today we know that the tallest mountain on the Moon is Mons Huygens, which is about 3.3 miles high. When we compare Galileo’s drawings
of the Moon’s surface to images of the Moon taken with modern telescopes, it becomes immediately apparent that he deliberately exaggerated the dimensions of a few elements (such as the one known today as the Albategnius crater, shown in the bottom half of the lower etching in Figure 4.3), probably in order to didactically highlight the different levels of illumination and shadowing that he observed in the crater.

  Galileo’s drawings of the Moon provide us with yet another wonderful example of the overlap and interconnections between science and art in the late Renaissance. Somewhat surprisingly, in a famous painting entitled The Flight to Egypt, a German artist who worked in Rome at the time and died in December 1610, Adam Elsheimer, depicted the Moon in a fashion that is strikingly similar to Galileo’s drawings. So much so, in fact, that a few art historians have even speculated that Elsheimer might have observed the Moon through one of the early telescopes, which could have been provided to him by his friend Federico Cesi.

  An intriguing story related to the Sidereus Nuncius and art emerged in 2005, when an Italian art dealer named Marino Massimo De Caro offered to sell to the New York antiquarian Richard Lan a remarkable copy of the Sidereus Nuncius. Instead of the usual etchings, this copy contained five wonderful watercolor drawings of the Moon, presumed to have been painted by Galileo himself. A battery of experts in the United States and Berlin confirmed the authenticity of the copy, which Lan bought for a half a million dollars. One of those experts, Horst Bredekamp, was so fascinated by the beauty of this specimen that he wrote a book about the exciting find. Then things took an unexpected turn. While writing a review of the English version of Bredekamp’s book in 2011, Nick Wilding, a Renaissance historian at Georgia State University, started to suspect that something was not quite right with the new copy of the Sidereus Nuncius. To make a long story short, further examination and inquiry revealed that the copy was indeed a masterful forgery by the Italian seller De Caro.

  Galileo used his lunar observations to discuss another puzzling topic that had generated many false interpretations over the years: the Moon’s secondary light. Observers had been baffled by the fact that even the portions of the Moon that are dark when the Moon is at its crescent phase, are not pitch black—they appear to be dimly illuminated. In Galileo’s words: “If we examine the matter more closely, we will see not only the extreme edge of the dark part shining with a faint brightness, but the entire surface… made white by some not inconsiderable light.”

  Previous explanations for this phenomenon ranged from the inconceivable suggestion that the Moon is partly transparent to sunlight, to the almost equally dubious proposition that the Moon doesn’t just reflect sunlight but also shines with its own intrinsic light. Galileo readily dismissed all of these theories, calling some of them “so childish as to be unworthy of an answer.” Then, even though he made it clear that “we will treat this matter at greater length in a book on the System of the World,” he offered a brief explanation that was remarkable in its simplicity: just as the Moon provides some light to Earth at night, he argued, the Earth brightens the lunar night. This phenomenon is known today as earthshine. Probably sensing that this proposal might raise some objections among the Aristotelian faithful, Galileo quickly added a clarifying touch:

  What is so surprising about that? In an equal and grateful exchange, the Earth pays back the Moon with light equal to that which she received from the Moon almost all the time in the deepest darkness of the night.… In this sequence [of lunar phases], then, in alternate succession, the lunar light bestows upon us her monthly illuminations, now brighter, now weaker. But the favor is repaid by the Earth in like manner.

  A beautiful photograph of the lit Earth rising above the lunar horizon was taken from lunar orbit by Apollo 8 astronaut Bill Anders on December 24, 1968 (Figure 6 in the color insert). Because of the Moon’s synchronous rotation with its orbital motion (the same side of the Moon always faces Earth), such an Earthrise can be seen only by an observer in motion relative to the lunar surface.

  Galileo finished the discussion of his sweeping discoveries about the Moon with a powerful proclamation:

  We will say more in our System of the World, where with very many arguments and experiments a very strong reflection of solar light from the Earth is demonstrated to those who claim that the Earth is to be excluded from the dance of the stars [planets], especially because she is devoid of motion and light. For we shall demonstrate that she is movable and surpasses the Moon in brightness [emphasis added], and that she is not the dump heap of the filth and dregs of the universe.

  Even though Galileo did not analyze the full implications of his lunar findings in The Sidereal Messenger—this was left for his Dialogo—what could be inferred from them was fairly transparent. First, according to the Aristotelian cosmology (which over the centuries had become intertwined intimately with Christian orthodoxy), there was a clear distinction between things terrestrial and things celestial. Whereas everything on Earth was corruptible, mutable, could be eroded, decay, or even die, the heavens were supposed to be perfect, pure, enduring, and immutable. Unlike the four classical elements that were supposed to be the constituents of everything earthly—earth, water, air, and fire—heavenly bodies were believed to be made of a fifth, different, immaculate substance dubbed “quintessence,” or literally the fifth essence. Yet Galileo’s observations showed that there were mountains and craters on the Moon and that by reflecting the Sun’s light, the Earth behaved very much like any other planetary object. No proof was given at this stage for the suggestion that the Earth was really moving, but Galileo’s declaration that “she is movable” spoke volumes toward Copernicanism. If the Moon was, in fact, solid and very much like Earth, and it moved in an orbit around the Earth, why couldn’t the Earth, which was Moonlike, move around the Sun?

  Understandably, this new picture of the lunar surface and the Moon’s place in the world provoked vehement objections. After all, it stood in stark contrast to the surreal description in the book of Revelation: “A great and wondrous sign appeared in the heaven: a woman clothed with the sun, with the moon under her feet and a crown of twelve stars on her head.” Traditionally, in artistic portrayals of this biblical description, the Moon had been represented by a perfectly smooth, blemish-free, translucent object, symbolizing the Virgin’s perfection and purity and continuing the Greek and Roman mythology of the personification of the Moon as a goddess. But Galileo’s lunar deviation from prevalent convictions was only the beginning. His other discoveries with the telescope were about to deliver the coup de grace to the old cosmology.

  STARRY, STARRY NIGHT

  After the Moon, Galileo directed his telescope to those other points of light that shine brightly in the night’s sky—the stars—and there, too, a few surprises awaited. First, unlike the Moon (and later the planets), the stars did not appear any larger through the telescope than they did to the naked eye, although they seemed brighter. From this fact alone, Galileo concluded correctly that the apparent sizes of the stars when observed with the unaided eye were not real but, rather, just artifacts. He did not know, however, that the apparent sizes were actually caused by the stellar light being scattered and refracted in the Earth’s atmosphere rather than by anything related to the stars themselves. Consequently, he thought that the telescope removed the stars’ misleading “adventitious irradiation.” Nonetheless, since he couldn’t make out the images of stars with the telescope, Galileo deduced that the stars were much farther from us than the planets.

  Second, Galileo discovered scores of faint stars that couldn’t be seen at all without the telescope. For example, in close proximity to the Orion constellation, he counted no fewer than five hundred stars, and he found tens more close to the six most brilliant Pleiades stars. Even more consequential for the future of astrophysics was Galileo’s discovery that stars varied enormously in brightness, with some being a few hundred times brighter than others. About three centuries later, astronomers created diagrams in which the stellar lum
inosity was displayed against the stellar color, and the patterns observed in those diagrams have led to the realization that the stars themselves evolve. They are born out of clouds of gas and dust, they spend their lives generating power through nuclear reactions, and they die, sometimes explosively, after running out of energy sources. In some sense, this could be regarded as the final nail in the coffin of the Aristotelian notion of unchanging heavens. Still, the most surprising result concerning stars came when Galileo pointed the telescope to the Milky Way. That apparently smooth, luminous, and mysterious band across the sky broke up into countless faint stars packed closely in clusters.

  These findings had significant implications for the Copernican-Ptolemaic debate. Some years earlier, the famous Danish astronomer Tycho Brahe pointed out what he perceived as a serious difficulty for the heliocentric theory. If the Earth was really orbiting around the Sun, he contended, then in observations taken six months apart (when the Earth is at two diametrically opposed points along its orbit), the stars should have shown a detectable displacement in position—a parallax—against the background, in the same way that trees observed through the window of a moving train appear to move with respect to the horizon. For such a shift not to be detected, Brahe argued, necessitated that the stars should be at very great distances. However, one could then estimate the size that the stars had to be in order for them to be seen with their apparent dimensions observed with the naked eye. Those expanses turned out to be even larger than the diameter of the entire solar system, which seemed highly implausible. Consequently, Brahe concluded that the Earth could not be orbiting the Sun. Instead, he proposed a revised, hybrid geocentric-heliocentric system in which all the other planets orbited the Sun, but the Sun itself orbited the Earth.