by Dan Falk
Philosophy is written in this grand book, the universe, which stands continually open to our gaze. But the book cannot be understood unless one first learns to comprehend the language and read the letters in which it is composed. It is written in the language of mathematics, and its characters are triangles, circles, and other geometric figures without which it is humanly impossible to understand a single word of it; without these, one wanders about in a dark labyrinth.
Nature displayed an underlying order, and through careful observation and mathematical representations—what we would today call mathematical modeling—that order can be understood and investigated, and predictions can be made. This combination of experiment and mathematical analysis, taken together, would serve as the backbone of science in the centuries ahead.
* * *
Galileo moved to the northeastern city of Padua in 1592, and would teach at the university there for the next eighteen years. He was involved with, but did not marry, a woman named Marina Gamba, who bore him two daughters and a son.* He would later look back on that time as the happiest and most productive of his life. Even so, he struggled financially, especially following the death of his father. Happily, his teaching duties left ample time for tinkering. He was desperate to invent something he could patent—a machine or instrument whose utility would guarantee him some measure of financial security, perhaps by winning the patronage of a prince or duke. “… I have many diverse inventions,” he wrote, “only one of which could be enough to take care of me for the rest of my life … if I can only find a grand Prince who would like it.… Then he could do with this invention and with its inventor whatever he likes. I would hope that he would accept not only the stone but the quarry.”
Galileo invented a primitive thermometer and, more lucratively, a military compass designed to aid artillery officers in battle. (He made money not only by having his assistants mass-produce the instrument in his workshop, but by offering tutorials in its use.) By his early thirties, Galileo had taken an interest in astronomy. When a new star appeared in the sky in 1604—the supernova now known as “Kepler’s star”—he delivered a series of lectures on the remarkable object, speculating on its significance; the university’s auditorium was filled to capacity for each of his talks. He also corresponded with Kepler, after receiving a copy of the German scientist’s book Mysterium Cosmographicum (1596). The book was a blend of science, mysticism, and numerology—but it also praised the Copernican model, which Galileo approved of. “It is really pitiful that so few seek the truth,” Galileo wrote to Kepler. He noted that he himself had been “an adherent of the Copernican system for many years. It explains to me the causes of many appearances of nature which are quite unintelligible within the commonly held hypothesis.” It was the first sign that Galileo, too, saw the Ptolemaic model as outdated.
“A CERTAIN FLEMING HAD CONSTRUCTED A SPYGLASS”
Up to this point, Galileo—like all skywatchers since the dawn of history—had only the unaided eye with which to observe the heavens.* That would soon change. It was in Padua that he first learned of a curious optical device from Holland—an instrument that was said to make distant objects appear nearby:
About ten months ago a report reached my ears that a certain Fleming had constructed a spyglass by means of which visible objects, though very distant from the eye of the observer, were distinctly seen as if nearby. Of this truly remarkable effect several experiences were related, to which some persons gave credence while others denied them. A few days later the report was confirmed to me in a letter from a noble Frenchman at Paris, which caused me to apply myself wholeheartedly to inquire into the means by which I might arrive at the invention of a similar instrument.
As his own testimony shows, Galileo did not invent the telescope. In fact, we can’t be sure who did, although credit usually goes to a Dutch spectacle maker named Hans Lipperhey (sometimes spelled Lippershey), who applied for a patent for a telescope-like device in October, 1608. It was an instrument, he claimed, “by means of which all things at a very great distance can be seen as if they were nearby.” It apparently magnified distant objects threefold. That may not sound like much, but even so, the military applications must have been obvious. Still, his application was turned down on the grounds that the design was already well known, and indeed two other Dutchmen are thought to have independently come up with a similar device at about the same time.
Before long, Galileo had improved on the original Dutch invention. Soon he had a telescope—or as he called it, a “perspicillum”—that could magnify twenty or even thirty times, compared with what one would see with the unaided eye.* Galileo had managed, as Owen Gingerich has put it, “to turn a popular carnival toy into a scientific instrument.” Galileo immediately recognized the potential of this new instrument. But his first thoughts had nothing to do with astronomy; instead, he saw the telescope’s value as a military tool. He arranged a meeting with senior Venetian statesmen, leading them to the top of the bell tower at the Piazza San Marco. Galileo urged them to aim the telescope at ships in the harbor. The device worked so well that they could identify ships a full two hours before they arrived in port. The officials were suitably impressed, and offered to double Galileo’s salary at the university. In the end, he used their offer as a bargaining chip to land an even better job in his native province of Tuscany. Galileo would soon head for Florence with a lofty new title: In July 1610, he was appointed Chief Mathematician and Philosopher to the Grand Duke of Tuscany. But he was still in Padua when he aimed his telescope skyward, and began to scrutinize the night sky with the new device. What he saw would change the world forever.
* * *
By modern standards, Galileo’s telescope was a joke: Today you can go to any department store and get a beginner’s telescope that has vastly superior optics to anything Galileo would have been able to construct. Modest as it was, however, Galileo accomplished a great deal with it. Beginning in the fall of 1609, Galileo aimed his telescope at the night sky—and was amazed at what it revealed. He found the moon to be covered with mountains and craters, and calculated the size of these features by carefully observing their shadows.
He showed that the stars visible to the unaided eye are outnumbered perhaps ten to one by dimmer stars, too faint to show themselves without a telescope. He went on to deduce that the Milky Way itself must be made up of countless faint stars, too dim to see with the unaided eye. But the biggest surprise came when he aimed his telescope at the planet Jupiter:
On the seventh day of January in this present year 1610, at the first hour of night, when I was viewing the heavenly bodies with a telescope, Jupiter presented itself to me; and because I had prepared a very excellent instrument for myself, I perceived (as I had not before, on account of the weakness of my previous instrument) that beside the planet there were three starlets—small indeed, but very bright. Though I believed them to be among the host of fixed stars, they aroused my curiosity somewhat by appearing to lie in an exact straight line parallel to the ecliptic, and by their being more splendid than others of their size. There were two stars on the eastern side and one to the west.
The following night he found “a very different arrangement”: The three little stars were now all to the west of Jupiter, and closer together than they had been the previous night. A couple of days later, “there were but two of them, both easterly, the third (as I supposed) being hidden behind Jupiter.”
Fig. 9.1 Aided by the newly invented telescope, Galileo sketched the moon (these images date from November and December of 1609), showing it to be covered with mountains and craters. Engravings based on these drawings would help make his book Siderius Nuncius (The Starry Messenger) a bestseller. Scala/Art Resource, NY
Galileo was mesmerized by this dynamic display, this string of little stars that seemed to follow Jupiter across the sky from night to night. Eventually he discovered that there were not three but four of these peculiar stars. He observed every night in which the skies were clear
, and gradually came to understand what he was seeing. “There was no way in which such alterations could be attributed to Jupiter’s motion,” he would later write, “yet being certain that these were still the same stars I had observed, my perplexity was now transformed into amazement.” Soon he had “decided beyond all question” that there were four stars “wandering about Jupiter as do Venus and Mercury about the sun, and this became plainer than daylight from observations on similar occasions which followed.” There was no doubt that “four wanderers complete their revolution about Jupiter.” He named them the “Medicean stars” in honor of his new patron, Grand Duke Cosimo II de’ Medici, but it didn’t stick; today we call them the Galilean moons after their discoverer.
* * *
Today, anyone who’s been to a public observing night at their local planetarium or observatory knows what Jupiter and its four bright moons look like (in fact, they can be seen with a good pair of binoculars, if the conditions are favorable and one has steady hands). But until the winter of 1610, no one knew these moons existed. No one had imagined that such objects could exist. The Earth, being at the center of the universe, was presumed also to be the center of rotation. Bodies can revolve around the Earth, and only the Earth. But Galileo now had incontrovertible evidence that there was at least one other body that could also serve as such a center: The planet Jupiter was now seen to have not just one moon, like the Earth, but four moons, circling around it; no doubt they had been doing so since the beginning of time.
Fig. 9.2 In January of 1610, Galileo aimed his telescope at Jupiter —and found it accompanied by four previously unknown “stars.” It soon became clear that these were moons revolving around Jupiter, just as our own moon revolves around the Earth. In this page from his notebook, Galileo tracks their positions over several weeks. The Granger Collection, New York
Galileo knew that before long, someone else would duplicate his observations—so he rushed to get his ideas into print. The result was a slim book called Siderius Nuncius (The Starry Messenger), published in Venice on March 13, 1610. Although written in Latin, this twenty-four-page pamphlet conveyed a remarkably straightforward message: Here was a blow-by-blow account of the wonders of the night sky revealed by Galileo’s telescope, sights “never seen from the creation of the world up to our own time.” Finally there was observational evidence that would lay to rest “all the disputes that have vexed philosophers through so many ages.” The book was an instant best seller. The initial print run of five hundred copies sold out at once, with additional orders pouring in from across Europe. Within five years, Galileo’s discoveries were being discussed as far away as Peking, where a Jesuit missionary had published a summary of Galileo’s findings in Chinese.
THROUGH THE EYEPIECE
Galileo continued his examination of the night sky, soon discovering that Venus goes through phases, just like our own moon. Something was odd about Saturn, too, though it was hard for Galileo to say exactly what. He could tell the planet was elongated in a peculiar way, but his telescope lacked the resolution needed to show its rings. He also found that the sun, like the moon, was imperfect, its surface peppered with dark spots (a discovery that, by now, others had also made).*
Everything Galileo saw through the telescope seemed to accord with the Copernican model of the universe. As we’ve seen, Galileo was leaning toward Copernicanism even before he began his telescopic observations; what he saw through the eyepiece, however, seemed to clinch it. Consider the planet Venus: If the Ptolemaic model were correct, with Venus circling the Earth in a “lower” shell, or sphere, than the sun, then it would always display a crescent phase.† The fact that Venus displayed a full set of phases, from “new” to “full,” only made sense if Venus was actually revolving around the sun. (By extension, the same argument must apply to Mercury.) Upon making the discovery, he sent a coded message to Kepler: Unscrambled, it read “The mother of love emulates the figures of Cynthia”—that is, Venus emulates the moon. A short time later he wrote to Christopher Clavius, a Jesuit astronomer working in Rome, that the sun is “without any doubt the center of the great revolutions of all the planets.” In 1613 he was ready to go public with the discovery. He wrote, “With absolute necessity we shall conclude, in agreement with the theories of the Pythagoreans and of Copernicus, that Venus revolves about the sun just as do all the other planets.”
And then there was Jupiter, behaving like a miniature solar system in its own right. If Jupiter had moons of its own, how could anyone say that the Earth was the center of the universe? One of the old arguments put forward against the Copernican theory had involved the moon: How could the Earth revolve around the sun—presumably at enormous speed—without our planet “losing” its moon in the process? And yet, whether one believed Ptolemy or Copernicus, Jupiter certainly moved around something—and it did so accompanied by four moons! This was “a fine and elegant argument” in support of the Copernican model, Galileo wrote, adding that
now we have not just one planet rotating about around another while both run through a great orbit about the sun; our eyes show us four stars which wander around Jupiter as does the moon around the earth, while all together trace out a grand revolution about the sun in the space of twelve years.
The moons of Jupiter, along with the phases of Venus, seemed to directly contradict Ptolemy’s cosmology, while the imperfections seen on both the sun and moon were an affront to the Aristotelian paradigm on which the Ptolemaic model rested. The ancient Greek model of the cosmos was under attack, first from Copernicus’s chisel, and now from Galileo’s sledgehammer. After fifteen centuries, a worldview that had seemed unassailable was giving way.
“THE STRANGEST PIECE OF NEWS”
Until Galileo’s observations, the appeal of the Copernican model had largely rested on its utility for computing planetary positions. Now there was concrete observational evidence in its favor. The Copernican system, by now almost seventy years old, was clearly more than a mathematical convenience: It could now be regarded as providing a physical description of the cosmos. Galileo’s observations, as Owen Gingerich writes, “made it intellectually respectable to believe in heliocentrism as a physical reality.”
The news spread quickly. In the French court, the queen, Marie de Médicis, cousin to Grand Duke Cosimo, ordered a telescope to be set up at her window; before it was in place, she became so excited that she fell to her knees in delirious anticipation. Meanwhile her husband, King Henri, suggested that Galileo might name his next discovery in honor of himself. Word of the Italian’s discoveries reached England just as quickly. As it happened, an English diplomat, Sir Henry Wotton, was visiting Venice when The Starry Messenger was published. Before the day was over he had made sure that a copy of the book was on its way to his king, James I. He included a cover letter informing the king that Galileo’s discoveries had “overthrown all former astronomy.” The letter reads, in part:
… I send herewith unto his majesty the strangest piece of news (as I may justly call it) that he hath ever yet received from any part of the world; which is the annexed book (come abroad this very day) of the Mathematical Professor at Padua, who by help of an optical instrument (which both enlargeth and approximateth the object) invented first in Flanders, and bettered by himself, hath discovered four new planets rolling about the sphere of Jupiter, besides many other unknown fixed stars.
As mentioned in Chapter 5, the king’s eldest son, Prince Henry, also requested Galileo’s book; and we’ve noted that Sir William Boswell, at Cambridge, corresponded with Galileo and helped to spread word of the telescopic discoveries. Another copy of The Starry Messenger reached Sir William Lower, who wrote, “Me thinks my diligent Galileo hath done more in his three-fold discoveries than Magellan in opening the straits to the South Sea, or the Dutchmen that were eaten by bears on Novaya Zemlya. I am sure with more ease and safety to himself, and more pleasure to me.” Within a year of the publication of The Starry Messenger, students at Oxford were asked to debate the
question “An luna sit habitabilis?” (“Is the moon habitable?”). As Mordechai Feingold writes, “Thus began the Galilean craze. By the end of the decade, almost every prominent man of science in England … was conducting telescopic observations.” From this point on, Galileo’s book “became seminal to the on-going discussion of the heliocentric theory in England.”
You didn’t have to be a scientist to be caught up in “telescope fever.” Thomas Hobbes, the political philosopher, sought out a copy of The Starry Messenger and was disappointed when all the bookshops he visited reported being sold out. (And he sensed that they would not be resold any time soon: “They that buy such bookes, are not such men as to part with them againe,” he noted in a letter to a friend.) Barten Holyday, a clergyman and playwright, mentioned Galileo’s observations in a play published in 1618. And a Scottish poet named Thomas Segett, roughly contemporary with Shakespeare, observed: