Galileo

by Mario Livio

  Galileo had a much simpler explanation for the absence of parallaxes. As we saw earlier, he concluded that the apparent dimensions of stars as seen with the naked eye did not represent real physical sizes—they were just artifacts. Stars were indeed at such large distances, he claimed, that shifts in their positions could not be detected, even with the then-available telescopes. Galileo was right: the detection of parallaxes had to await the development of higher-resolution telescopes. A stellar parallax was first observed only in 1806 by the Italian astronomer Giuseppe Calandrelli. The first successful measurements of a parallax were made by the German astronomer Friedrich Wilhelm Bessel in 1838. As of 2019, the European Space Agency’s Gaia space observatory, launched in 2013, has determined the parallaxes to more than a billion stars in the Milky Way and nearby galaxies.

  Galileo also rejected Brahe’s “compromise” solar system model for two main reasons: First, the model appeared to him to be extremely contrived—in today’s language, it had “too many moving parts.” Second, in later years, Galileo relied on a moving Earth to explain the phenomenon of sea tides (discussed in chapter 7). He therefore could no longer accept a scenario in which the Earth had to be at rest. Galileo’s intuition on rejecting a hybrid model proved correct.

  The picture that emerged from Galileo’s observations of the stars was very different from Aristotle’s ancient concepts. Rather than being studded on a solid celestial sphere positioned just beyond the orbit of Saturn, stars were now much smaller in apparent size than previously thought, numerous beyond counting, and spanning huge ranges in both brightness and distance. In fact, this star system was beginning to look dangerously similar to the speculative cosmos depicted by mathematician and philosopher Giordano Bruno, in which multiple worlds existed in an infinite universe. Being well aware of Bruno’s tragic end—he was burned alive on February 17, 1600—Galileo was very careful in describing and interpreting his observations of the stars, even in his later book the Dialogo. Careful or not, however, Galileo’s observations of distant stars in the Milky Way galaxy can definitely be regarded as humanity’s first peek at the vast universe that exists beyond the solar system.

  Today we know that the Milky Way contains between 100 billion and 400 billion stars. Based primarily on data from the Kepler and Gaia Space Telescopes, recent estimates of the number of roughly Earth-sized planets in the Milky Way, which are orbiting Sun-like stars in that just right, not-too-hot and not-too-cold “Goldilocks” zone (known as the habitable zone) that allows for liquid water to exist on the planetary surface, put it in the billions.

  A COURT HAS BEEN FOUND FOR JUPITER

  On the evening of January 7, 1610, Galileo observed the planet Jupiter through his twenty-power telescope and noticed that, in his words, “three little stars were positioned near him—small but yet very bright.” Galileo added that these stars intrigued him “because they appeared to be arranged exactly along a straight line and parallel to the ecliptic.” Two of the stars were on the east of Jupiter and one on the west. The following night, he again saw the three stars, but this time they were all west of the planet and equally spaced, which made him think that perhaps Jupiter was moving to the east, contrary to expectations based on the astronomical tables that existed at the time.

  Clouds prevented Galileo from observing on the ninth, but on the tenth he saw only two stars to the east. Guessing that the third star was hidden behind Jupiter, he was beginning to suspect that Jupiter was not moving much after all; rather, it was those stars that were moving. This celestial dance continued, with only two stars appearing on the east on January 11, and the third star reappearing on the west (and two on the east) on the twelfth. On the thirteenth, a fourth star appeared (three on the west and one on the east), and on January 15 all four were on the west. (It was cloudy again on the fourteenth.)

  The earliest surviving record of Galileo’s observations of Jupiter and what turned out to be its satellites is on the bottom half of a draft letter to the Doge of Venice (Figure 7 in the color insert). That page is now in the Special Collections of the University of Michigan—Ann Arbor. Fascinatingly, Galileo’s drawings in the document reveal that at least until January 12, it had not occurred to him that the satellites could be orbiting Jupiter. Rather, he assumed that the three objects were moving along a straight line, in a very non-Copernican fashion. When the fourth satellite appeared on the thirteenth, however, Galileo realized that his assumption could not be correct, since it required one satellite to literally pass through another. Only after the fifteenth did the correct explanation dawn upon him. The conclusion from these meticulous observations now seemed inescapable:

  Since they sometimes follow and at other times precede Jupiter by similar intervals, and are removed from him toward the east as well as the west by only very narrow limits, and accompanying him equally in retrograde and direct motion, no one can doubt that they complete their revolutions about him while, in the meantime, all together they complete a 12-year period about the center of the world [referring to Jupiter’s orbit around the Sun].

  In plain English, Galileo discovered that Jupiter had four satellites, or moons, orbiting it, and, like our Moon, they were revolving approximately in the same plane as other planetary orbits. Jupiter was exhibiting a miniature Copernican system. On January 30 he informed the Tuscan state secretary, Belisario Vinta, that the four satellites move around a larger “star” (planet) “like Venus and Mercury, and perhaps other known planets, do around the Sun.” He confirmed this fact through diligent, methodical observations of the satellites on every clear night until March 2. During this period, he also determined the distances of the moons from Jupiter and from one another and measured their brightness. To convince everybody else of what he saw, he presented no fewer than sixty-five diagrams showing the different configurations of satellites he had observed.

  The discovery of Jupiter’s four satellites was not only of historical significance—these were the first new bodies revealed in the solar system since antiquity—it also demolished one of the serious objections to the heliocentric model. Aristotelians maintained that the Earth could not keep possession of its Moon while orbiting the Sun. They also raised a legitimate question: If the Earth is a planet, why is it the only planet to have a Moon? Galileo decisively silenced both of these demurrals by showing that Jupiter—which was clearly moving, since it was orbiting either the Sun (in the Copernican view) or the Earth (in the Ptolemaic system)—was still able to retain not just one but four orbiting moons! He put it very clearly in The Sidereal Messenger:

  We have moreover an excellent and splendid argument for taking away the scruples of those who, while tolerating with equanimity the revolution of the planets around the Sun in the Copernican system, are so disturbed by the attendance of one Moon around the Earth while the two together complete the annual orb around the Sun that they conclude that this constitution of the universe must be overthrown as impossible. For here we have only one planet revolving around another while both run through a great circle around the Sun: but our version offers us four stars wandering around Jupiter like the Moon around the Earth while all together with Jupiter traverse a great circle around the Sun in the space of 12 years.

  After the publication of The Sidereal Messenger, Galileo continued to observe Jupiter’s satellites for almost three more years, until he was satisfied that he had accurately determined the periods of their revolutions around Jupiter. He referred to this gigantic observational and intellectual endeavor as an “Atlantic labor,” alluding to Atlas, who was ordered by the god Zeus to support the sky on his shoulders. Even the great astronomer Johannes Kepler had believed it impossible to determine the periods, since he saw no obvious way to unambiguously identify and distinguish among the three inner moons. Astonishingly, Galileo’s results for the periods agree with modern values to within less than a few minutes.

  Today there are seventy-nine known moons of Jupiter (fifty-three are named and the others awaiting official names). Eigh
t of these are believed to have formed in orbit about the planet, and the others were probably captured. Out of the four Galilean satellites, as they are now called, two, Europa and Ganymede (the latter of which is larger than the planet Mercury), are each thought to contain a large ocean underneath a thick crust of ice. Both moons are considered potential candidates for harboring simple life forms under the ice, a fact that no doubt would have delighted Galileo. The innermost of the four Galilean satellites, Io, is the most geologically active body in the solar system, with more than four hundred known active volcanoes. The fourth Galilean moon, Callisto, is the second largest of the four.

  What would have undoubtedly annoyed Galileo no end is the fact that the Galilean satellites are known today by the names assigned to them by the German astronomer Simon Mayr rather than as the “Medici stars.” Mayr may have independently detected the satellites before Galileo, but he failed to understand that the moons were orbiting the planet. Galileo regarded Mayr as a “poisonous reptile” and an “enemy not only of me, but of the entire human race,” ever since he convinced himself that Mayr was the villain behind Baldessar Capra’s plagiarizing the geometric and military compass. Galileo wrote about Mayr that while in Padua (where Galileo resided at the time), “he set forth in Latin the use of the said compass of mine, appropriating it to himself, had one of his pupils [Capra] print this under his name. Forthwith, perhaps to escape punishment, he departed immediately for his native land [Germany], leaving his pupil in the lurch, as the saying goes.”

  FOR THIS OLD MAN, TWO ATTENDANTS

  The detection of Jupiter’s four satellites was the last of Galileo’s world-changing discoveries made in Padua. It only whetted his appetite for more breakthroughs. No wonder, then, that soon after moving to Florence, he aimed his telescope at the next giant planet in terms of its distance from the Sun: Saturn. The initial observations were disappointing, however, since they did not reveal any satellites. This situation changed when, on July 25, 1610, his inspection revealed something that looked like two motionless stars, one attached to Saturn on each side. To avoid being scooped, and still at a stage where he was making discoveries faster than he could publish them, Galileo sent a jumbled series of letters announcing his discovery to Kepler via the Tuscan ambassador to Prague. This was common practice at the time, to establish priority of discovery using a riddle, thereby not actually disclosing what had been found. Galileo’s coded description was:

  smaismrmilmepoetaleumibunenugttauiras.

  Kepler had initially failed to make any sense of Galileo’s message, and the English astronomer and Kepler’s correspondent Thomas Harriot fared no better. But from the fact that the Earth had one Moon and Jupiter four moons, he concluded that Mars had to have two moons, so as to form the geometric progression 1, 2, 4, and so on. Guided by this mathematical belief and assuming that Galileo had discovered the satellites of Mars, Kepler eventually succeeded in creating a message out of Galileo’s string of letters that differed by only one character from the original jumble: Salve umbistineum geminatum Martia proles, meaning roughly: “Be greeted, twin companionship, children of Mars.”

  Ingenious as Kepler’s solution was, it had nothing to do with Galileo’s intentions. The decoded message in the string of letters was supposed to read: Altissimum planetam tergeminum observavi, which translates into: “I have observed the highest of the planets [Saturn] three-formed.”

  On November 13, 1610, Galileo finally revealed precisely what he meant:

  I have observed that Saturn is not a single star but three together, which always touch each other, they do not move in the least among themselves and have the following shape oOo.… If we look at them with a telescope of weak magnification, the three stars do not appear very distinctly and Saturn seems elongated like an olive.… A court has been found for Jupiter, and now for this old man two attendants who help him walk and never leave his side.

  To Galileo’s amazement, those apparently reliable “attendants” completely disappeared by late 1612. He expressed his bewilderment in a letter to his correspondent, German humanist, historian, and publisher Markus Welser: “Were the two smaller stars consumed like spots on the Sun? Have they suddenly vanished and fled? Or has Saturn devoured his own children?” Despite his puzzlement, Galileo dared to predict that those “stars” would reappear in 1613, which they did, this time resembling ears or handles, one on each side of Saturn.

  While Galileo was able to correctly predict again, in 1616, another vanishing of the “handles” ten years later, his prediction was apparently based on the assumption that these were similar to Jupiter’s moons. A true explanation for the strange ears had to wait until the 1650s, when Dutch mathematician and astronomer Christiaan Huygens identified them as the now-famous Saturn’s rings. Since the rings are flat and relatively thin, they were essentially undetectable when seen edge-on, and appeared like ears when their surface was inclined at a larger angle to the line of sight or when seen face-on.

  It is interesting that we now know that the rings didn’t always exist, nor will they last forever. The rings are estimated to be no older than about 100 million years, a short time compared with the approximately 4.6-billion-year-old solar system. More surprisingly, however, a study published in December 2018 found that due to “ring rain”—the draining away of the rings onto the planet in the form of a dusty rain of ice particles—the rings will disappear in about 300 million years. Galileo and we are therefore lucky to have lived in the relatively “short” period of time that allowed us to see this spectacular phenomenon.

  Amusingly, Kepler’s anticipation of Mars having two moons turned out to be correct, even though this had nothing to do with a geometric series. Furthermore, in his famous 1726 satire Gulliver’s Travels, the English writer Jonathan Swift (who may have been inspired by Kepler) wrote about two Martian moons. In 1877 American astronomer Asaph Hall discovered the moons, now called Phobos and Deimos.

  THE MOTHER OF LOVE

  One of the major objections raised against the heliocentric model had to do with the appearance of the planet Venus. In the Ptolemaic geocentric model, Venus is always more or less between the Earth and the Sun, so it was expected always to appear as a crescent of varying dimensions (but never as much as half full, Figure 4.4a). In the Copernican model, on the other hand, since Venus was assumed to orbit the Sun, and it is closer to the Sun than the Earth, it was expected to exhibit the complete series of phases like the Moon, appearing fully illuminated as a small, bright disk, when farthest from the Earth and as a dark, large one, when closest (and as a large crescent just before that; Figure 4.4b). Through a series of painstaking observations between October and December 1610, Galileo definitively confirmed the predictions of the Copernican model. His decision to embark on these observations (and the interpretation of the results) may have been inspired and certainly encouraged by a detailed letter he had received from Benedetto Castelli, in which Castelli emphasized the significance of observing the phases of Venus. This was the first clear indication of the superiority of the Copernican view over the Ptolemaic one.

  Figure 4.4. The expected appearance of Venus in the Ptolemaic model (a) and in the Copernican model (b).

  On December 11 Galileo hurried to send Kepler another mysterious anagram: Haec immatura a me jam frustra leguntur oy, meaning roughly “This was already tried by me in vain too early.” (The use of the word oy has humorously been taken to hint at a potential Jewish ancestry for Galileo.) Frustrated by his inability to solve the puzzle, Kepler wrote to Galileo: “I adjure you not to leave us long in doubt of the meaning. For you see you are dealing with real Germans. Think in what distress you place me by your silence.”

  Responding to this plea, on January 1, 1611, Galileo sent Kepler the unscrambled version: Cynthiae figuras aemulatur mater amorum, which meant: “The mother of love [Venus] emulates the figures of Cynthia.” The Greek female name Cynthia was used sometimes as a personification of the Moon.

  Galileo was so confident
in his interpretations of the observations that on December 30, 1610 he had sent a letter to Christopher Clavius—who until that point had objected to Copernicanism on both physical and religious grounds—explaining that all the planets shined only by reflecting sunlight, and that “without any doubt, the center of the great revolutions of all the planets” was the Sun.

  Galileo’s statements didn’t go unnoticed. In the last edition of Clavius’s comments on the influential astronomy textbook The Sphere, by the thirteenth-century astronomer Johannes de Sacrobosco, the Jesuit mathematician, who was then seventy-three years old, admitted:

  Far from the least important of things seen with this instrument [the telescope] is that Venus receives its light from the Sun as does the Moon, so that sometimes it appears to be more like a crescent, sometimes less, according to its distance from the Sun. At Rome I have observed this, in the presence of others, more than once. Saturn has joined to it two small stars, one on the east, the other on the west. Finally, Jupiter has four roving stars, which vary their places in a remarkable way both among themselves and with respect to Jupiter—as Galileo Galilei carefully and accurately describes.

  As he realized that these results all but shattered the Ptolemaic model, Clavius then added cautiously: “Since things are thus, astronomers ought to consider how the celestial orbs may be arranged in order to save these phenomena.”