Coming of Age in the Milky Way

Timothy Ferris

• Éditeur: HarperCollins e-books
• Année: 2010
• ISBN:
  9780062006547
• Sujet: Physical Sciences
• Sous-sujet: Astronomy & Astrophysics

From the second-century celestial models of Ptolemy to modern-day research institutes and quantum theory, this classic book offers a breathtaking tour of astronomy and the brilliant, eccentric personalities who have shaped it. From the first time mankind had an inkling of the vast space that surrounds us, those who study the universe have had to struggle against political and religious preconceptions. They have included some of the most charismatic, courageous, and idiosyncratic thinkers of all time. In Coming of Age in the Milky Way, Timothy Ferris uses his unique blend of rigorous research and captivating narrative skill to draw us into the lives and minds of these extraordinary figures, creating a landmark work of scientific history.

PART ONE

SPACE

The self shines in space through knowing.

—The Upanishads

1
THE DOME OF HEAVEN

You may have heard the music of Man but not the music of Earth. You may have heard the music of Earth but not the music of Heaven.

—Chuang Tzu

Had we never seen the stars, and the sun, and the heaven, none of the words which we have spoken about the universe would ever have been uttered. But now the sight of day and night, and the months and the revolutions of the years, have created number, and have given us a conception of time, and the power of enquiring about the nature of the universe; and from this source we have derived philosophy, than which no greater good ever was or will be given by the gods to mortal man.

—Plato

The skies of our ancestors hung low overhead. When the ancient Sumerian, Chinese, and Korean astronomers trudged up the steps of their squat stone ziggurats to study the stars, they had reason to assume that they obtained a better view that way, not, as we would say today, because they had surmounted a little dust and turbulent air, but because they had got themselves appreciably closer to the stars. The Egyptians regarded the sky as a kind of tent canopy, supported by the mountains that demarked the four corners of the earth, and as the mountains were not all that high, neither, presumably, were the heavens; the gigantic Egyptian constellations hovered close over humankind, as proximate as a mother bending to kiss a sleeping child. The Greek sun was so nearby that Icarus had achieved an altitude of only a few thousand feet when its heat melted the wax in his wings, sending the poor boy plunging into the uncaring Aegean. Nor were the Greek stars significantly more distant; when Phaethon lost control of the sun it veered into the stars as suddenly as a swerving chariot striking a signpost, then promptly rebounded to earth (toasting the Ethiopians black on its way down).

But if our forebears had little notion of the depths of space, they were reasonably well acquainted with the two-dimensional motions of the stars and planets against the sky, and it was by studying these motions that they were led, eventually, to consider the third dimension as well. Since the days of the ancient Sumerians and probably before, there had been students of the night sky willing to devote their evening hours to the lonely business of squinting and straining to take sightings over aligned rocks or along wooden quadrants or simply across their fingers and thumbs, patiently keeping records of what they saw. It was a lot of trouble. Why did they bother?

Part of the motive may have had to do with the inchoate longing, mysterious but persistent then as now, to express a sense of human involvement with the stars. As Copernicus noted, reverence for the stars runs so deep in human consciousness that it is embedded in the language itself. “What is nobler than the heavens,” he wrote, “the heavens which contain all noble things? Their very names make this clear: Caelum (heavens) by naming that which is beautifully carved; and Mundus (world), purity and elegance.”¹ Even Socrates, though personally indifferent toward astronomy, conceded that the soul “is purified and kindled afresh” by studying the sky.

There were obvious practical incentives as well. Navigation, for one: Mariners could estimate their latitude by measuring the elevation of the pole star, and could tell time by the positions of the stars, and these advantages were sufficiently appreciated that seafaring peoples codified them in poetry and mythology long before the advent of the written word. When Homer says that the Bear never bathes, he is passing along the seafarer’s knowledge that Ursa Major, the constellation that contains the Big Dipper, is circumpolar at Mediterranean latitudes—that is, that it never sinks beneath the ocean horizon.

Another practical motive was timekeeping. Early farmers learned to make a clock and a calendar of the moving sky, and consulted almanacs etched in wood or stone for astronomical guidance in deciding when to plant and harvest their crops. Hesiod, one of the first poets whose words were written down, emerges from the preliterate era full of advice on how to read the sky for clues to the seasons:

When great Orion rises, set your slaves
To winnowing Demeter’s holy grain
Upon the windy, well-worn threshing floor….
Then give your slaves a rest; unyoke your team.
But when Orion and the Dog Star move
Into the mid-sky, and Arcturus sees
The rosy-fingered Dawn, then Perseus, pluck
The clustered grapes, and bring your harvest home….
When great Orion sink, the time has come
To plough; and fittingly, the old year dies.²

The hunter-gatherers who preceded the farmers also used the sky as a calendar. As a Cahuilla Indian in California told a researcher in the 1920s:

The old men used to study the stars very carefully and in this way could tell when each season began. They would meet in the ceremonial house and argue about the time certain stars would appear, and would often gamble about it. This was a very important matter, for upon the appearance of certain stars depended the season of the crops. After several nights of careful watching, when a certain star finally appeared, the old men would rush out, cry and shout, and often dance. In the spring, this gaiety was especially pronounced, for … they could now find certain plants in the mountains. They never went to the mountains until they saw a certain star, for they knew they would not find food there previously.³

Stonehenge is one of thousands of old time-reckoning machines the moving parts of which were all in the sky. The Great Pyramid at Giza was aligned to the pole star, and it was possible to read the seasons from the position of the pyramid’s shadow. The Mayans of ancient Yucatan inscribed stone monuments with formulae useful in predicting solar eclipses and the heliacal rising of Venus (i.e., its appearance westward of the sun, as a “morning star”). The stone medicine wheels of the Plains Indians of North America ticked off the rising points of brighter stars, informing their nomadic architects when the date had come to migrate to seasonal grazing lands. The twenty-eight poles of Cheyenne and Sioux medicine lodges are said to have been used to mark the days of the lunar month: “In setting up the sun dance lodge,” said Black Elk, a priest of the Oglala Sioux, “we are really making the universe in a likeness.”⁴

Political power presumably played a role in early efforts to identify periodic motions in the sky, inasmuch as what a man can predict he can pretend to control. Command of the calendar gave priests an edge in the hardball politics of the Mayans, and Christopher Columbus managed to cow the Indians of Hispaniola into providing food for his hungry crew by warning that the moon otherwise would “rise angry and inflamed to indicate the evil that God would inflict on them.” Writes Columbus’s son Ferdinand, in his journal entry for the night of February 29, 1504:

At the rising of the moon the eclipse began, and the higher the moon rose the more the eclipse increased. The Indians observed it, and were so frightened that with cries and lamentations they ran from every side to the ships, carrying provisions, and begged the Admiral by all means to intercede for them with God that he might not make them feel the effects of his wrath, and promised for the future, diligently to bring all he had need of…. From that time forward they always took care to provide us with all that was necessary, ever praising the God of the Christians.⁵

But the better acquainted the prehistoric astronomers became with the periodic motions they found in the night sky, the more complicated those motions proved to be. It was one thing to learn the simple periodicities—that the moon completes a circuit of the zodiacal constellations every 28 days, the sun in 365¼ days, the visible planets (from the Greek planetes, for “wanderers”) at intervals ranging from 88 days for fleet-footed Mercury to 29½ years for plodding Saturn. It was another and more baffling matter to learn that the planets occasionally stop in their tracks and move backward—in “retrograde”—and that their paths are tilted relative to one another, like a set of ill-stacked dishes, and that the north celestial pole of the earth precesses, wobbling in a slow circle in the sky that takes fully 26,000 years to complete.^*

The problem in deciphering these complexities, unrecognized at the time, was that the earth from which we view the planets is itself a planet in motion. It is because the earth orbits the sun while rotating on its tilted axis that there is a night-by-night shift in the time when any given star rises and sets at a given latitude. The earth’s precessional wobble slowly alters the position of the north celestial pole. Retrograde motion results from the combined wanderings of the earth and the other planets; we overtake the outer planets like a runner on an inside track, and this makes each appear first to advance, then to balk and retreat across the sky as the earth passes them. Furthermore, since their orbits are tilted relative to one another, the planets meander north and south as well as east and west.

These complications, though they must have seemed a curse, were in the long run a blessing to the development of cosmology, the study of the universe at large. Had the celestial motions been simple, it might have been possible to explain them solely in terms of the simple, poetic tales that characterized the early cosmologies. Instead, they proved to be so intricate and subtle that they could not be predicted accurately without eventually coming to terms with the physical reality of how and where the sun, moon, and planets actually move, in real, three-dimensional space. The truth is beautiful, but the beautiful is not necessarily true: However aesthetically pleasing it may have been for the Sumerians to imagine that the stars and planets swim back from west to east each day via a subterranean river beneath a flat earth, such a conception was quite useless when it came to determining when Mars would go into retrograde or the moon occult Jupiter.

Retrograde motion of Mars occurs when Earth overtakes the more slowly moving outer planet, making Mars appear to move backward in the sky.

Consequently the idea slowly took hold that an adequate model of the universe not only should be internally consistent, like a song or a poem, but should also make accurate predictions that could be tested against the data of observation. The ascendency of this thesis marked the beginning of the end of our cosmological childhood. Like other rites of passage into adulthood, however, the effort to construct an accurate model of the universe was a bittersweet endeavor that called for hard work and uncertainty and deferred gratification, and its devotees initially were few.

One was Eudoxus. He enters the pages of history on a summer day in about 385 B.C., when he got off the boat from his home town of Cnidus in Asia Minor, left his meager baggage in cheap lodgings near the docks, and walked five miles down the dusty road to Plato’s Academy in the northwestern suburbs of Athens. The Academy was a beautiful spot, set in a sacred stand of olive trees, the original “groves of academe,” near Colonus, blind Oedipus’ sanctuary, where the leaves of the white poplars turned shimmering silver in the wind and the nightingales sang day and night. Plato’s mentor Socrates had favored the groves of academe, which even Aristophanes the slanderer of Socrates described lovingly as “all fragrant with woodbine and peaceful content.”⁶

Beauty itself was the principal subject of study at the Academy, albeit beauty of a more abstract sort, LET NONE BUT GEOMETERS ENTER HERE, read the motto inscribed above the door, and great was the general enchantment with the elegance of geometrical forms. Geometry (geo-metry, “earth-measurement”) had begun as a practical affair, the method employed by the Egyptian rope-stretchers in the annual surveys by which they reestablished the boundaries of farmlands flooded by the Nile. But in the hands of Plato and his pupils, geometry had been elevated to the status approaching that of a theology. For Plato, abstract geometrical forms were the universe, and physical objects but their imperfect shadows. As he was more interested in perfection than imperfection, Plato wrote encomiums to the stars but seldom went out at night to study them.

He backed this view with an imposing personal authority. Plato was not only smart, but rich—an aristocrat, one of the “guardians” of Greek society, descended on his mother’s side from Solon the lawmaker and on his father’s from the first kings of Athens—and physically impressive; Plato, meaning “broad-shouldered,” was a nickname bestowed upon him by his gymnastics coach when as a youth he wrestled in the Isthmian Games. Eudoxus, we may assume, was suitably impressed. He was, however, a geometer in his own right—he was to help lay the foundations of Euclidean geometry and to define the “golden rectangle,” an elegant proportion that turns up everywhere from the Parthenon to the paintings of Mondrian—and, unlike Plato, he combined his abstract mathematical reasonings with a passion for the physical facts. When he made his way to Egypt (a pilgrimage to the seat of geometrical wisdom that many Greek thinkers undertook, though Plato seems never to have got around to it), Eudoxus not only conducted research in geometry but applied it to the stars, building an astronomical observatory on the banks of the Nile and there mapping the sky. The observatory, though primitive, evinced his conviction that a theory of the universe must answer to the verdict, not only of timeless contemplation, but of the ceaselessly moving sky.

When the mature Eudoxus returned to the Academy, now as a renowned scholar with his own retinue of students, he set to work crafting a model of the cosmos that was meant to be both Platonically pleasing and empirically defensible. It envisioned the universe as composed of concentric spheres surrounding the earth, itself a sph...