If he expected laughter he was right. When Martin Luther, the Protestant reformer, heard about his theory he complained, “That fool wants to turn the whole art of astronomy upside down.” Luther pointed out that the Bible, the word of God, tells us that when the Israelite general Joshua needed light by which to continue slaughtering Canaanites, he commanded the setting sun to stand still. According to Luther, that proved that the sun is normally in motion. How could that knucklehead say that it’s at rest?
A generation after old Copernicus had glimpsed his book and died, Johannes Kepler was born. If anyone had heard that this particular baby would one day chart the courses of the planets he would have thought it unlikely. Kepler’s father was a German ne’er-do-well and wife-beater, who one time “ran the risk of hanging” for some unrecorded crime. He would later leave his family and die in exile. Much later, Johannes’s mother was indicted as a witch, and narrowly escaped burning at the stake. In small ways, though, this couple introduced Johannes to the heavens. When he was six years old his mother took him up a hill to see a comet, and when he was nine his parents once called him outdoors to see the moon eclipsed.
Johannes seemed like poor material for greatness. He nearly died from smallpox and was always sickly. Worst of all for a future astronomer, he suffered from multiple vision. Nothing was easy for him. At the age of twenty-one, he writes, he “was offered union with a virgin; on New Year’s Eve. I achieved this with the greatest possible difficulty, experiencing the most acute pains of the bladder.”
Everything was against him but this: he was brilliant. As a result, he had, at government expense, a good high school and university education. He tells us that he wrote plays and learned long poems by heart, read the works of Aristotle in Greek, and “argued with men of every profession for the profit of [my] mind.” He “explored various fields of mathematics as if [I] were the first man to do so [and discovered things] which [I] later found had already been discovered.”
He also learned about Copernicus’s “hypotheses,” which put the sun at the center of the universe. Decades later Newton would show that neither Copernicus nor the old astronomy had it right. The sun does not go around the earth, or the earth around the sun; they go around each other. But Copernicus’s explanation enraptured Kepler. He was deeply pious, and believed Copernicus had glimpsed a beauty in the heavens that was worthy of God, who had made them.
Kepler’s big opportunity came in 1600 when a Danish astronomer invited Kepler to join his research staff. Tycho Brahe was widely known not only as a man who gazed at stars but also because, having lost the bridge of his nose in a duel, he had replaced it with another made of gold and silver. For twenty years he had made many thousands of observations of the movements of the planets. That was his great achievement: those many, many fixings of the planets as they journeyed through the skies. His measurements were not only precise but continuous; they showed where the planets went, and how long it took them to get there. It was as if Tycho had replaced still photos with a movie. Before his time astronomers had had only a limited number of observations to work with, but now, providing Tycho made his measurements available, they would have long series of them.
When Kepler joined him, Tycho’s working years were over; he had little time to live. He probably sensed that what he needed was a greater scientist than he. He needed someone who could take that raw material he had gathered, that wealth of observations, and discover in it the structure of the universe. Probably he guessed that Kepler was that person. But he hated to give his life’s work to another person, so he only occasionally threw Kepler scraps of information. On his deathbed, though, he bequeathed him all his data. Just before his death Brahe kept repeating, “Let me not seem to have lived in vain.”
Kepler focused on Tycho’s data, especially what the older man had learned about the movements of Mars, the planet that was never where it should be. He started by assuming that the planets traveled around the sun, as Copernicus had said, but without those complicated epicycles.
In 1609 Kepler published his discovery that Mars (like other planets, he assumed) does not orbit the sun in a perfect circle. It moves in an ellipse, the shape you get if you slice a cone not straight but at an angle. If the planets orbit in ellipses, not in circles, that means that they are not attached to spheres. (A sphere is the result of rotating a circle, not an ellipse, around one of its diameters.) Circular motion and invisible spheres had been key features of the older view of the universe, but out they had to go. Kepler was unhappy with this finding because he had grown up with the old idea of spheres and perfect circles. The ellipse, he felt, had nothing to recommend it except truth. He compared it to a load of dung that he had to bring into the heavens as the price for ridding them of a vaster amount of dung.
Later Kepler also found that a planet travels faster in its elliptical orbit when it’s closer to the sun than when it’s far away. And he found that the shorter a planet’s mean distance from the sun the faster it completes its orbit. All these findings he expressed precisely, using numbers.
Kepler knew how much he had accomplished. He had described the structure of the universe, and he was ecstatic about it. “The die is cast,” he wrote, “and I am writing the book [about his discoveries] — to be read either now or by posterity, it matters not.” His book, he said, could wait a century for a reader, just as God had waited ever since he created the universe for a Kepler to see the wonder of it.
While Kepler was finding laws about the motion of the planets, Galileo Galilei was studying the heavens in a different way. Galileo was a mathematics professor at the University of Padua, in Italy, and a maker of compasses and surveying instruments. In 1609 he heard that a Dutchman who made eyeglasses had found that if he held two lenses in a certain way and looked through them at the same time, they greatly magnified distant objects. The Dutchman had, in short, invented the telescope. Galileo at once began to make one for himself.
Then he did what today seems obvious: he pointed a telescope up and looked at the night sky. He was not the first to do this, but the first top-notch observer. What he saw amazed him. Scientists had thought the moon, our nearest neighbor, was absolutely smooth. Galileo found it “full of hollows and protuberances, just like the surface of the earth itself, which is varied everywhere by lofty mountains and deep valleys.” He saw four moons circling Jupiter, and many new stars — “more than ten times as many” as anyone had ever seen with the unaided eye. He discovered that the Milky Way was not just a vague white smear across the sky but “a mass of innumerable stars.”
Galileo reported on his sightings in 1610 in a pamphlet called The Messenger from the Stars. It immediately made him famous. In the same year a poet in far-off England recorded that Galileo had “summoned the other worlds, the stars to come neerer to him, & give him an account of themselves.” His university offered him a permanent job and a huge raise, but the Grand Duke of Tuscany (in Florence) offered to make him his court philosopher and mathematician. Of course he took the job in Florence, since it freed him from annoying colleagues and dim-bulb students.
Galileo’s findings partly proved that Copernicus and Kepler were right: the earth was not the center of the universe. But The Messenger’s main effect was to excite people about astronomy and stir up arguments. Many astronomers sided with him, but some did not. They had spent their lives learning the old view of the universe and didn’t want to see it challenged.
Galileo loved a fight, and he took to calling his opponents “mental pygmies” and “hardly deserving to be called human beings.” Two professors at his university hadn’t even deigned to peer through his telescope. When one of them died a little later, Galileo wrote that he “did not choose to see my celestial trifles while he was on earth; perhaps he will do so now that he has gone to heaven.”
The Catholic Church began to take an interest. Theologians had never taken a stand about the structure of the universe because the matter hadn’t seemed important. But Galileo championed
a view of the universe, Copernicus’s, that seemed not only new but shocking. Many churchmen who had never even heard of Copernicus now learned that he had fathered these disturbing ideas. An Italian bishop wanted Copernicus thrown in jail and was surprised to learn that he had been dead for seventy years.
The Church’s leading theologian talked with Galileo in Rome and gently warned him that it was all right to discuss the sun-centered universe “hypothetically.” But to say that the sun “in very truth” was at the center would be “a very dangerous attitude.” It would arouse philosophers and theologians and tend to “injure our holy faith by contradicting the Scriptures.” The official had in mind such passages in the Bible as the one Luther had cited against Copernicus, in which Joshua tells the sun to “stand thou still.”
For almost two decades after he was warned, Galileo kept his fingers off his troublemaking pen. In 1632, however, he published a book that was bound to stir up trouble. Intended not for experts but for laymen, it was called A Dialogue on the Two Great Systems of the World. In the Dialogue three men discuss the old and new views of the universe. One of them is a persuasive scientist, clearly Galileo, who explains the new, sun-centered view. The other two are an intelligent layman and a pious, stupid one, who sticks to the old earth-centered universe, and makes himself look foolish. Galileo calls him Simplicio.
Galileo was summoned to Rome and questioned by the Inquisition, the Catholic body that combated “false” opinions. After several months, the Church came down against him. The Dialogue was not to be read by anyone, it said, and it censured Galileo for non-compliance (to the theologian’s warning) in teaching the Copernican view. Told to admit his errors and confess his disobedience, he knelt and did so.
He went home to Florence and spent the rest of his life there under house arrest. During his last four years he was blind, perhaps from having peered at the sun through his telescope. Not long before his death he wrote a friend, “this earth, this universe, which I, by marvelous discoveries and clear demonstrations, have enlarged a hundred thousand times beyond the belief of the wise men of bygone ages, henceforward for me is shrunk into such small space as is filled by my own bodily sensations.”
Despite the Church’s stand, opinions changed. By the time of Galileo’s death, every educated person had heard, and many believed, that the earth was not the center of the universe, and that the heavens seemed to stretch forever into space. Astronomers also knew that Kepler had described how planets moved.
No one knew as yet what force held the planets and the stars together and made them move the way they did. Some scientists had offered explanations. For example, William Gilbert, an Englishman who had studied magnetism, speculated that the same mysterious force that made a compass needle point to north also held the planets in their courses.
In 1642, the year Galileo died, the man was born who would bring this question to a kind of resolution. This scientist was Isaac Newton, the son of an English farmer who couldn’t sign his name and who had died before his son was born. Newton grew up in a country house, went to the village school, and spent much of his boyhood drawing and making water clocks and little windmills, powered by a mouse. Then he went to Cambridge University, where he proved to be astoundingly brilliant. While other students were chasing girls and foxes, Newton invented the well-known binomial theorem, which states that, for any positive integer n, the n th power of the sum of two numbers…well, never mind.
In 1666, when he was twenty-three, plague broke out in Cambridge. To escape it, Newton went home and stayed there a year and a half. And how did he fill his time? Well, for one thing he invented calculus, a branch of mathematics that deals with the effect of changes in one of several variables. (He would need this tool to analyze the pull of planets on each other.) And he developed his ideas about how forces here on earth act on moving bodies. And he discovered the composition of white light. And, as if all that were not enough, he began to calculate that force that Kepler had described, that glue that holds in place the stars and planets, that drawing power we now call gravitation.
When someone asked him how he made all these discoveries, Newton answered simply, “By thinking upon them.” He was generous enough to admit that he had built upon the work of other scientists, such as Kepler and Galileo. “If I have seen further,” he said, “it is by standing upon the shoulders of Giants.”
Decades later, Newton told someone that he began to think about gravitation one day in 1666 or 1667 when he was lying in his orchard, and saw an apple fall. He asked himself why it fell straight toward the center of the earth. A “drawing power” pulls it, he surmised, and that power must be in proportion to the sizes of the earth and the apple. Newton had no idea what the drawing power was. It might be magnetism, as Gilbert had suggested; or whirlwinds of invisible matter, as the French scientist René Descartes had guessed; or the omnipotent hand of God. Not knowing what the force was, how hard it must have been to figure how it worked!
Newton concentrated on the moon. By rights, the moon should wander off. But earth, which is bigger, attracts it just enough to keep it orbiting our planet. Newton imagined the moon in its orbit as if it were a stone that a farm boy whirls around his head in a sling and then lets fly against a rabbit. Then Newton worked out how to calculate the force needed to keep an object here on earth moving in a circle, like the stone in the sling.
Using this formula, Newton calculated the force that the earth exerts on the moon to keep it in its orbit, that is, to keep the moon from flying away from earth. And then he showed that earth’s pulling-in force on the moon precisely equaled the moon’s flying-away force. This led him to formulate a law describing the gravitational pull between all heavenly bodies, such as the planets and the sun. This force, he said, is inversely proportional to the square of the distance between them. All of the universe depends on this measurable force that moves masses through space and time. (However, physicists now know of major exceptions to this law.)
Perhaps that does not sound like such a big achievement. After all, Newton had not identified the force that holds the universe together. He did not pretend to know what that binding-together force is, and neither do we, to this day. But he had, as he said, demonstrated “the frame of the System of the World.” He had tied together with mathematics the findings of Kepler, Galileo, and other scientists.
What Europeans had learned in the century and a half from Copernicus to Newton was unsettling. We humans are not at the center of the universe. Rather, we live, as the astronomer Carl Sagan once wrote, on an “insignificant planet of a humdrum star lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people.”
Some found it alarming to imagine a universe stretching far beyond what one could see with his eyes, or even with a telescope. That space apparently was endless and nearly empty, and the sun, the planets, and the stars rolled on silently, indifferent to us. In such a universe it was hard to imagine a fatherly God concerned above all with the doings of humans he had created in his image. Blaise Pascal, a French scientist and devout Christian, famously described his anguish at the thought. He felt “engulfed in the infinite immensity of spaces whereof I know nothing, and which know nothing of me. I am terrified by the eternal silence of those infinite spaces.”
Most of those who learned of this new view of the universe, however, felt no sorrow at losing their favored place in the universe. On the contrary, poets acclaimed what had been learned. Alexander Pope wrote:
Nature and Nature’s laws lay hid in night:
God said, “Let Newton be!” and all was light.
An Italian writer charmingly explained the findings of the astronomers in a book called Newtonianism for the Ladies.
The report of the finding that earth revolved around the sun spread around the world. In the 1600s, while Kepler, Galileo, and Newton were at work, Catholic priests from Europe were serving the emperor of China as astrologers, absurdly predicting the future for h
im by studying the positions of the stars and planets. But these men were also missionary teachers, and we know that during most of the 1600s they also taught the Chinese the new, sun-centered view of the universe. They also introduced the telescope, and they printed astronomy books in Chinese.
When their teachings reached Japan, the Japanese quickly accepted the new sun-centered idea. Japanese scholars explained that the sun was really an ancient god of the Japanese, “the god who rules the center of the heavens.” So the brand-new science was really their ancient faith.
Many hailed Newton as the greatest scientist of any age, and Newton, who was anything but modest, probably agreed. But a little before he died he told a friend, “I do not know what I may appear to the world, but to myself I seem to have been only a boy playing on the sea-shore and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, while the great ocean of truth lay all undiscovered before me.”
IT WAS SHOCKING enough to learn that a human being is merely a passenger on a small planet who during his lifetime takes a few dozen trips around a local star.1 Bigger news was on the way.
1I paraphrase from Carl Sagan and Ann Druyan, Shadows of Forgotten Ancestors: A Search for Who We Are (1992), p. 30.
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