Coming of Age in the Milky Way (9 page)

Copernicus’s model of the solar system is generally portrayed in simplified form, as in this illustration based upon one in his
De Revolutionibus
. In its details, however, it was as complex as Ptolemy’s geocentric model.

This, in retrospect, was the tragedy of Copernicus’s career—
that while the beauty of the heliocentric hypothesis convinced him that the planets ought to move in perfect circles around the sun, the sky was to declare it false. Settled within the stone walls of Frauenburg Cathedral, in a three-story tower that afforded him a view of Frisches Haff and the Gulf of Danzig below and the wide (though frequently cloudy) sky above—“the most remote corner of the earth,”
¹¹
he called it—Copernicus carried out his sporadic astronomical observations, and tried, in vain, to perfect the heliocentric theory he had outlined while still a young man. For decades he turned it over in his thoughts, a flawed jewel, luminous and obdurate. It would not yield.

As Darwin would do three centuries later, Copernicus wrote and privately circulated a longhand sketch of his theory. He called it the “ballet of the planets.” It aroused interest among scholars, but Copernicus published none of it. He was an old man before he finally released the manuscript of
De Revolutionibus
to the printer, and was on his death bed by the time the final page proofs arrived.

One reason for his reluctance to publish was that Copernicus, like Darwin, had reason to fear censure by the religious authorities. The threat of papal disapproval was real enough that the Lutheran theologian Andreas Osiander thought it prudent to oil the waters by writing an unsigned preface to Copernicus’s book, as if composed by the dying Copernicus himself, reassuring its readers that divine revelation was the sole source of truth and that astronomical treatises like this one were intended merely to “save the phenomena.” Nor were the Protestants any more apt to kiss the heliocentric hem. “Who will venture to place the authority of Copernicus above that of the Holy Spirit?” thundered Calvin,
¹²
and Martin Luther complained, in his voluble way, that “this fool wishes to reverse the entire science of astronomy; but sacred Scripture tells us that Joshua commanded the sun to stand still, and not the earth.”
^13
*

The book survived, however, and changed the world, for much the same reason that Darwin’s
Origin of Species
did—because it was too technically competent for the professionals to ignore it. In addition to presenting astronomers with a comprehensive, original, and quantitatively defensible alternative to Ptolemy,
De Revolutionibus
was full of observational data, much of it fresh and some of it reliable. Consequently it was consulted regularly by astronomers—even by non-Copernicans like Erasmus Reinhold, who employed it in compiling the widely consulted
Prutenic Tables
—and thus remained in circulation for generations.

To those who gave it the benefit of the doubt, Copernicanism offered both a taste of the immensity of space and a way to begin measuring it. The minimum radius of the Copernican sphere of stars (given the unchanging brightnesses of the zodiacal stars) was estimated in the sixteenth century to be more than 1.5 million times the radius of the earth. This represented an increase in the volume of the universe of at least 400,000 times over al-Farghani’s Ptolemaic cosmos. The maximum possible size of the Copernican universe was indefinite, and might, Copernicus allowed, be infinite: The stars, he wrote, “are at an immense height away,” and he expressed wonderment at “how exceedingly vast is the godlike work of the Best and Greatest Artist!”
¹⁴

Interplanetary distances in Ptolemy were arbitrary; scholars who ventured to quantify them did so by assuming that the various orbits and epicycles fit snugly together, like nested Chinese boxes. The Copernican theory, however, precisely stipulated the relative dimensions of the planetary orbits: The maximum apparent separation of the inferior planets Mercury and Venus from the sun yields the relative diameters of their orbits, once we accept that both orbit the sun and not the earth. Since the relative sizes of all the orbits were known, if the actual distance of any one planet could be measured, the distances of all the others would follow. As we will see, this advantage, though purely theoretical in Copernicus’s day, was to be put to splendid use in the eighteenth century, when astronomical technology reached the degree of sophistication required to measure directly the distances of nearby planets.

The immediate survival of Copernicanism was due less to any compelling evidence in its favor than to the waning fortunes of the Ptolemaic, Aristotelian model. And that, as it happened, was
prompted in large measure by changes in the sky—by the apparition of comets, and, most of all, by the fortuitous appearance of two brilliant
novae
, or “new stars,” during the lifetimes of Tycho, Kepler, and Galileo.

Integral to Aristotle’s physics was the hypothesis that the stars never change. Aristotle saw the earth as composed of four elements—earth, water, fire, and air—each of which naturally moves in a vertical direction: The tendency of earth and water is to fall, while that of fire and air is to rise. The stars and planets, however, move neither up nor down, but instead wheel across the sky. Aristotle concluded that since objects in the sky do not partake of the vertical motion characteristic of the four terrestrial elements, they must be made of another element altogether. He called this fifth element “aether,” from the Greek word for “eternal,” and invested it with all his considerable reverence for the heavens. Aether, he argued, never ages or changes: “In the whole range of time past,” he writes, in his treatise
On the Heavens
, “so far as our inherited records reach, no change appears to have taken place either in the whole scheme of the outermost heaven or in any of its proper parts.”
¹⁵

Aristotle’s segregation of the universe into two realms—a mutable world below the moon and an eternal, unchanging world above—found a warm welcome among Christian theologians predisposed by Scriptures to think of heaven as incorruptible and the earth as decaying and doomed. The stars, however, having heard neither of Aristotle nor of the Church, persisted in changing, and the more they changed, the worse the cosmology of Aristotle and Ptolemy looked.

Comets were an old problem for the Aristotelians, since no one could anticipate when they would appear or where they would go once they showed up.
^*
(It was owing to their unpredictability that comets acquired a reputation as heralds of disaster—from the
Latin
dis-astra
, “against the stars.”)
^*
Aristotle swept comets under the rug—or under the moon—by dismissing them as atmospheric phenomena. (He did the same with meteors, which is why the study of the weather is known as “meteorology.”)

But when Tycho Brahe, the greatest observational astronomer of the sixteenth century, studied the bright comet of 1577, he found evidence that Aristotle’s explanation was wrong. He triangulated the comet, by charting its position from night to night and comparing his data with those recorded by astronomers elsewhere in Europe on the same dates. The shift in perspective produced by the differing locations of the observers would have been more than sufficient to show up as a difference in the comet’s position against the background stars, were the comet nearby. Tycho found no such difference. This meant that the comet was well beyond the moon. Yet Aristotle had held that nothing superlunar could change.

The other great empirical challenge to Aristotle’s cosmological hegemony came with the opportune appearance, in the late sixteenth and early seventeenth centuries, of two violently exploding stars—what we today call
Supernovae
. A star that undergoes such a catastrophic detonation can increase a hundred million times in brightness in a matter of days. Since only a tiny fraction of the stars in the sky are visible without a telescope, Supernovae almost always seem to have appeared out of nowhere, in a region of the sky where no star had previously been charted; hence the name
nova
, for “new.” Supernovae bright enough to be seen without a telescope are rare; the next one after the seventeenth century did not come until 1987, when a blue giant star exploded in the Large Magellanic Cloud, a neighboring galaxy to the Milky Way, to the delight of astronomers in Australia and the Chilean Andes. The two Supernovae that graced the Renaissance caused quite a stir, inciting not only new sights but new ideas.

Tycho spotted the supernova of 1572 on the evening of November 11,
while out taking a walk before dinner, and it literally stopped him in his tracks. As he recalled the moment:

The next supernova came only thirty-two years later, in 1604. Kepler observed it for nearly a year before it faded from view, and Galileo lectured on it to packed halls in Padua.

Scrutinized week by week through the pinholes and lensless sighting-tubes of the sixteenth- and seventeenth-century astronomers, the two Supernovae stayed riveted in the same spot in the sky, and none revealed any shift in perspective when triangulated by observers at widely separated locations. Clearly the novae, too, belonged to the starry realm that Aristotle had depicted as inalterable. Wrote Tycho of the 1572 supernova:

The shock dealt to the Aristotelian world view could not have been greater had the stars bent down and whispered in the astronomers’ ears. Clearly there was something new, not only under the sun but beyond it.
^*

Tycho was no Copernican. It was through Ptolemy that his passion for astronomy had crystallized, when, on August 21, 1560, at the age of thirteen, he watched a partial eclipse of the sun and was amazed that it had been possible for scholars, consulting the Ptolemaic tables, accurately to predict the day (though not the hour) of its occurrence. It struck him, he recalled, as “something divine that men could know the motions of the stars so accurately that they could long before foretell their places and relative positions.”
¹⁸

But when Tycho began making observations of his own, he soon became impressed by the inaccuracy of Ptolemy’s predictions. He watched a spectacular conjunction of Saturn and Jupiter on August 24, 1563, and found that the time of closest approach—which in this case was so close that the two bright planets appeared almost to merge—was days away from the predictions of the Ptolemaic tables. He emerged from the experience with a lifelong passion for accuracy and exactitude and a devotion to the verdict of the sky.

To compile more accurate records of the positions of the stars and planets required state-of-the-art equipment, and that cost money. Fortunately, Tycho
had
money. His foster father had saved King Frederick II from drowning, dying of pneumonia as a result, and the grateful king responded with a hefty grant to the young astronomer. With it, Tycho built Uraniburg, a fabulous observatory on an island in the Sund between Elsinor Castle (Hamlet’s haunt) and Copenhagen. He ransacked Europe in search of the finest astronomical instruments, complemented them with improved quadrants and armillaries of his own design, and deployed them atop the turrets of a magnificent castle that he equipped with a chemical laboratory, a printing plant supplied by its own paper mill, an intercom system, flush toilets, quarters for visiting researchers, and a private jail. The grounds sported private game preserves, sixty artificial fishponds, extensive gardens and herbariums, and an arboretum with three hundred species of trees. The centerpiece of
the observatory was a gleaming brass celestial globe, five feet in diameter, on which a thousand stars were inscribed, one by one, as Tycho and his colleagues remapped the visible sky.

No dilettante, Tycho drove himself and his assistants in a ceaseless pursuit of the most accurate possible observations, charting the positions of the stars and the courses of the planets night after night for over twenty years. The resulting data were more than twice as accurate as those of the preceding astronomers—precise enough, at last, to unlock the secrets of the solar system.