Astronomy? Impossible to understand and madness to investigate.
—SOPHOCLES, 496-406 B.C.
Don’t confuse progress with perfectibility. A great poet is always timely; a great philosopher is an urgent need. There’s no rush for Isaac Newton. We were quite happy with Aristotle’s cosmos. Personally I preferred it. Fifty-five crystal spheres geared to God’s crankshaft is my idea of a satisfying universe.
—BERNARD NIGHTINGALE,
the literary Byron-lover in
Tom Stoppard’s Arcadia
THE EARLIEST REFERENCES TO ASTRONOMY AMONG THE GREEKS APPEAR around 800 B.C. in the poems of Homer and Hesiod. By that time their countrymen, like the Babylonians and the Egyptians, had put names to various stars and quantified the risings and settings of the Sun.1 However, they were not content with merely recording the movements of the heavens: once they took to astronomy in earnest, they inquired into what the skies were made of; the shapes and sizes of the Sun, the stars, the planets, and the Earth itself; how far these were from one another, and what revolved around what, in orbits of what form; and the number of stars, and whether they could be located with systematic accuracy. To this they added a raft of further questions about their immediate world: Exactly how long was the year, or the month? When were the equinoxes, and could one pinpoint the exact moment of the solstices? The one matter on which almost everyone agreed was that the Earth itself was stationary and central—a belief that would bedevil science for millennia.
Despite that crucial misapprehension, a formidable line of Greek students of the stars extends for over a thousand years, from the time of Hesiod until at least the death of Ptolemy. The muster of ancient Chinese philosophers may have stretched twice as far, but the Greek roll call is the more impressive. The great German scholar Otto Neugebauer lists 121 astronomers of note—and even that tally excludes figures such as Pherekydes of Syros (the teacher of Pythagoras), Zeno of Elea, Plato, and Epicurus, who, while not primarily astronomers, made significant contributions. Many familiar names appear—Aristotle, Euclid, Archimedes, Ptolemy—and others who were giants in their time but less well known now, such as Parmenides, Anaxagoras, Eudoxus, Heraclitus, and Aristarchus.
Of course, they built in part on the foundations laid by the Babylonians. Herodotus (c. 484–c. 425 B.C.), in his only book, The Enquiries (more often transliterated as The Histories), describes how his countrymen not only copied the Babylonian records and calculations but also borrowed several of their measuring devices, such as one for reckoning the motion of the Sun along the ecliptic. And the first glimpses of an astronomy that went beyond pure observation came from the Egyptians. The early Greek practitioners did not even seem to consider the prediction of heavenly phenomena particularly valuable: Plato—in the Dialogue of Phaedrus—blames his countrymen for not being interested enough in the planets. This mind-set did not change until the Greeks came into contact with the observation tablets of the Babylonians during the creation of the Macedonian/Greek Empire under Alexander, who dispatched literally camel-loads of Babylonian astronomical tablets to the Greek cities along the Adriatic coast.2
By around 650 B.C., the old aristocratic order had been overthrown, supplanted by a succession of tyrants (then meaning rulers who had seized absolute authority, not necessarily cruel despots) who encouraged trade and economic reform, so that for the first time there was an opportunity throughout the Greek states for a different style of life—enjoying the means to reflect, rather than having to be solely concerned with survival. It was from this point on that, to quote the great scholar of the period D. R. Dicks, “the Greek passion for rational investigation along mathematical lines … converted a mass of crude, observational material into an exact science.”3
The first great practitioner was Thales (c. 625–c. 547 B.C.) of Miletus, a prosperous port serving the whole Aegean. A nobleman by birth, he was also a statesman, engineer, and merchant, canny enough, the story goes, quietly to corner the olive oil presses of Miletus and neighboring Chios just ahead of what he knew would be a bumper harvest, thereby amassing a considerable fortune: proof, he said, that philosophers could make money if they chose.
Thales had traveled to Egypt to learn the elementary principles of practical geometry, a lesson that would inspire him to make astronomy a deductive science. He was the first to posit that the Moon eclipses the Sun when it moves into a direct line between it and Earth, and the first Greek to maintain that it shone by reflected sunlight. He also is said to have established the times and sequence of the equinoxes. He is amply quoted in Plutarch, Pliny, Cicero, and Diogenes Laertius—which is fortunate, as none of his writings survive, and indeed both ancient and modern scholars have built him up into a scientific hero on very thin evidence.
According to Diogenes (born c. 412 B.C.), Thales was the first to gauge the Sun’s diameter in relation to its apparent orbit around the Earth with any accuracy, estimating it as 1/720 thereof, a ratio that is an exceptionally close estimate of Earth’s actual orbit around the Sun. Less impressively, he argued that water was the prime element of the universe, that the Earth floated like a cork on a great sheet of this water, and that the Sun was made up of a burning earthly substance. He was on firmer ground when postulating that the duration of the Sun’s circuit between solstices was never even and that the solar year lasted 365 days; but again we know too little of his writings to be certain what he actually said. Ironically, he is best remembered for something he never did: predict the solar eclipse of May 28, 585 B.C.*
ONE GIANT OF THIS time was Heraclitus (535-475 B.C.), a mystic poet who rejoiced in the nickname of “He who rails at the people” for his arrogant misanthropy. The 130 fragments of his writings that survive emphasize two aspects of the cosmic order: its continuity and its periodicity, especially that of the Sun. Aspects of his thinking can still be seen in the writings of Darwin, Spencer, and other masters of the nineteenth century. He could be absurdly wrong: for instance, he suggests that the Sun is just a foot across, about the size of a shield, and that there is a new Sun every day. But his belief that the universe is composed of worlds that are being created and destroyed in a continuous cycle would have modern physicists nodding in agreement. He understood that nothing is constant—hence his dictum “One cannot step twice into the same river.”
Another in the line of great Greek mathematician-astronomers was Pythagoras of Samos, who flourished during the second half of the sixth century B.C. He is said to have spent twenty-two years traveling in Arabia, Syria, Chaldea, Phoenicia, Greek Gaul (better known today as the French Riviera), and possibly India before settling in southern Italy when over fifty years old. (He also spent some time in Egypt studying astronomy, geometry, “and,” as the popular historian Will Durant puts it, “perhaps a little nonsense.”)5 An astronomer of exceptional originality, once in Italy he invested his energy in founding an influential religious brotherhood and studying mathematics and theoretical geometry, where he discerned mystical properties in specific numbers. He created a new word to cover such inquiries, which translates as “love of wisdom”—in Greek, “philosophy.” In the sixth century, “philosopher” and “Pythagorean” were synonyms.
Pythagoras saw number as the essence of all things, harmony as the most beautiful, and the universe as fundamentally well organized. He pioneered the science of acoustics, which he extended to a hypothetical “harmony of the spheres,” believing that, as with the strings on an instrument, each orbiting planet sounded a characteristic note, its pitch determined by its size and speed. The five planets then known, together with the Sun, the Moon, and the Earth, formed an octave. (No one seemed worried by the fact that the Earth, believed stationary, would not therefore be vibrating.)
He also believed that those bodies in the sky moved in two separate circular motions, so that the Sun circled the Earth every twenty-four hours in one revolution, but also made a second journey around the Earth along a different circle, at an angle to the first, once every year. He was attempting to rationalize the apparent motions of the stars in a universe deemed geocentric, and his theory broke down when applied to the planets; but this was the first serious attempt to explain their motions. All of modern science stems from Pythagoras’ core insight: that there are patterns in nature and that those patterns can be described mathematically.
A French artist’s depiction of Pythagoras (c. 580–c. 490 B.C.) discussing the heavens with Egyptian priests during his prolonged studies in Alexandria Illu(6.1)
Pythagoras quickly passed into myth, and legends about him were already circulating by Aristotle’s time. He had many followers, several of whom advanced our knowledge of the Sun. We know that one of them, Parmenides, worked out that the bright side of the Moon shone because it was turned toward the Sun and was also the first to pronounce that the Earth was spherical. Once this idea had established itself, Pythagoreans went on to reason that the heavens, too, must be spherical, and so spherical coordinates made their appearance. Philolaus (c. 470–c. 385 B.C., a contemporary of Socrates) even preempted Copernicus by moving the Earth from the center of the cosmos, making it a planet. However, in his system, it still did not orbit the Sun but rather circled an unseen “central fire.” All else was spread out across space—including the Sun, which was not a body with its own light but a reflector of this central fire. The universe centered on fire because this was the noblest of the elements, and the center the most honorable place—a theory adopted in most of the leading schools in Athens.
Some of the finest solar astronomers, however, came from outside the main city. Among them were the three “Anaxes”—Anaximander, Anaximenes, and Anaxagoras—who, like Thales, all hailed from Miletus. As indicated by the prefix “Anax,” which means “lord,” each was wellborn. Anaximander’s pupil was Anaximenes, who taught some thirty years before Anaxagoras, the greatest of the three.
Anaximander was already sixty-four when, around 560 B.C., he wrote one of the first works of natural philosophy. He may also, following the Egyptians, have introduced the gnomon, and appears to have marked down in some form the motions of the planets, the angle of the ecliptic, and the dates of solstices, equinoxes, and seasons. He engraved the first “world” map into a tablet of brass, and with his geometric model of Earth, Sun, Moon, and stars sought to depict the heavens in nonmythological terms.
Anaximenes (d. 528 B.C.) was less original, and is best known for his doctrine that air is the source of all things, in contrast to Thales’ view that everything comes from water. Although eminent in his time, he had little to say about the Sun—only that it didn’t travel under the Earth but circled around it, hidden by mountains at night—in sharp contrast to the third member of the group, Anaxagoras (494-428 B.C.). Although his name translates as “lord of the marketplace”—his parents evidently hoped he would turn out a merchant prince—when Anaxagoras moved to Athens from Miletus, at the age of twenty, he gave away his inheritance in order to devote himself to studying the skies.
One of his early pupils—and later close friend—was Pericles (c. 495-429 B.C.), who grew up to become the most influential person in Athens at the height of Greece’s golden age. This period began after the Greeks had repulsed the Persians in 479 B.C. and endured until around 399 B.C. During this time, Athens became famous, evolving under the guidance of Pericles into the great intellectual center of the Mediterranean world, a place where the work of Anaxagoras and his fellow astronomers could flourish. Skywatchers of that era still had only the most primitive technology to help them, and even their observatories were probably no more than flat-roofed buildings. What they did have was mathematical geometry, even then closely allied with astronomy, and the same men were usually active in both fields. Yet solar astronomy continued to be a mix of the utterly wrong (as would come to be seen) and the formidably acute. For an instance of the latter, Oenopides of Chios (active around 460 B.C.) not only came up with the idea of the ecliptic, but went on to measure the angle of the Earth’s axis against the ecliptic plane and figured it to be 23°45’—less than three-tenths of a degree off from the presently accepted value of 23°27’.
Among his many accomplishments, Anaxagoras was even able to explain lunar eclipses, but this insight was not taken up; he was too far ahead of his time. Around 440 B.C., a meteorite over a foot in diameter fell on Thrace in broad daylight. Anaxagoras, visiting and seeing it on display, concluded that it had fallen from the Sun, which must, he reckoned, be a mass of red-hot iron; thus heavenly bodies were not divine beings but material objects. The authorities charged him with atheism, and he retired into exile at Lampsacus, where he died, much honored by his new neighbors, in 428 B.C.6
About 432 B.C., Meton of Athens, a leading geometer and engineer, made a detailed study of the summer solstice, probably employing a gnomonlike device known as a heliotropion (“Sun turner”), a pillar fixed on a level platform, which enabled him to measure the ratio of the noon shadow. He also constructed a parapegma, or simple engraved list of astronomical events, and merged the traditional agricultural calendar into one for city use. He became famous—indeed, a celebrity of sorts, his enthusiasm for geometric astronomy such that he found himself publicly made fun of in Aristophanes’ comedy The Birds:
PITHETAERUS: In the name of the gods, who are you?
METON: Who am I? Meton, known throughout Greece and at Colonus.
PITHETAERUS: What are these things?
METON: Tools for measuring the air. In truth, the spaces in the air have precisely the form of a furnace. With this bent ruler I draw a line from top to bottom; from one of its points I describe a circle with the compass. Do you understand?
PITHETAERUS: Not in the least.
METON: With the straight ruler I set to work to inscribe a square within this circle; in its center will be the marketplace, into which all the straight streets will lead, converging to this center like a star, which, although only orbicular, sends forth its rays in a straight line from all sides.*
The same summer that Meton was making his solstice calculations, Athens became embroiled in a struggle against a coalition led by its archrival, Sparta, a conflict that endured, with one brief interlude, for twenty-seven years. When it finally wore itself out, the political structure of the “tyrant-city” was in ruins. A plague that lasted for three years had killed over one-third of the population. Into this turmoil, around 428 B.C., a great figure was born. He was named Aristocles—“best and renowned”—but his prowess as a wrestler at the Isthmian Games led to the nickname Platon, or “broad.” Unlike the fathers of Pythagoras and Socrates, the one a merchant seaman and the other a stonemason, both Plato’s parents were wealthy, each coming from one of the most ancient and aristocratic families in Greece.
Athens was by now a state of about a quarter of a million people, half citizens and their families, the rest slaves and resident foreigners, its capital numbering about seventy-five thousand. The city was never more than a narrow margin away from serious hunger, and devoid of almost any medical services—a place where even the people who owned other human beings had a standard of living greatly below the working class of modern industrial democracies. (The term for “burglar” was “wall-piercer,” because houses were so fragile.) Having survived his home city’s crushing defeat and a brutal oligarchic revolution in 404 B.C., a confused restoration of democracy, then the trial and death of Socrates in 399 B.C., Plato fled to Megara, a trading port some twenty-six miles to the northwest, thence to Egypt. After various adventures—at one point he was sold into slavery, although quickly ransomed—by 395 B.C. he had returned to his home city. Nine years later, following further travels in Italy and Sicily, he borrowed money from friends to purchase a patch of land on the outskirts of Athens named after the local hero, Academos. This he would make his Academy, where he was to teach for forty years: little could he have imagined that it would remain the intellectual center of Greece for the next five hundred.
Plato made astronomy a branch of applied mathematics at his Academy, the inscription over its doorway famously reading, “Let no one enter who does not know geometry.” No record exists of the Greeks’ using algebra before the time of Christ.7 Astronomy, which Plato sometimes called Spheric, occupies a comparatively small place in his writings, except in the dialogue Epinomis, which describes the work of an astronomer. Having embraced the concept of a spherical Earth, stationary at the center of a spherical universe, he encouraged his students to investigate this system more carefully, but this soon raised some potentially disturbing questions. Plato believed the stars and planets to be visible images of immortal deities whose movements were part of a transcendent order. However, some celestial bodies did not move with the same regularity as the rest, but “wandered” (the Greek word for planet, planetos, originally meant “wanderer”). How to explain these motions within the context of his belief in that transcendent order?
For Plato, the task of philosophers was to comprehend the reality underlying creation. If direct observation conflicted with understanding of that reality, he cautioned that things were not necessarily what they seemed—a valuable enough axiom in scientific study generally, but a cold shower for would-be astronomers in the Academy. But Plato’s pupils were not put off, because even though he never understood the discipline, his Academy made their deliberations possible. When he died in 347 B.C., one of his students erected an altar to him and gave him a funeral almost befitting a god. The student’s name was Aristotle. He didn’t even like Plato very much.
ARISTOTLE (WHOSE NAME translates as “best objective”) was born in Stagira, to the north of Greece, in 384 B.C. By the time he was ten both his parents were dead, and at seventeen he was sent to the Academy, staying for almost twenty years and becoming so much its star that Plato called him “the mind of the school.” Possibly inspired by his father, who had been court physician to the king of Macedon, he first concentrated on medicine, biology, and zoology, soon advancing from pupil to teacher and lecturing on almost every subject. When Plato died, he was succeeded by his nephew Speusippus, whose intellect Aristotle disdained. Possibly piqued, maybe keen to widen his horizons, and certainly aware of the mounting animosity toward the Macedonians, who had so recently conquered Athens, he chose exile at a school on the Aegean island of Assos. When his patron, the ruler of Assos, was betrayed and crucified by the Persians, he fled to nearby Lesbos, until he was invited to tutor the unruly son of Philip II of Macedon, the future Alexander the Great. About 335 B.C. he returned to Athens and, possibly with money from Alexander, founded his own school, the Lyceum, near the temple of Apollo Lyceius, god of shepherds.
His followers were known as the “Peripatetics,” taken from their master’s habit of strolling up and down the covered walkway, the peripatos, while he lectured. The students came from prosperous families—merchant and landowning families—and keen rivalries developed between the Lyceum, the Academy (with its largely aristocratic student body), and the school of the political theorist Isocrates, which catered to Greeks new to the city. Isocrates’ strength was rhetoric, the Academy’s mathematics, politics, and metaphysics, the Lyceum’s natural science.
By this time, the Pythagorean notion of two sources of light, a central fire with the Sun its mere reflector, had been discredited, but its sister idea of several interconnecting crystalline spheres revolving around the Earth had been seized on and developed. Eudoxus of Cnidus (in what is now southwest Turkey), who was teaching in Athens when Aristotle arrived, argued that the universe comprised not two spheres but twenty-seven. This elaboration enabled him to explain why four of the planets sometimes came to a stop, then reversed course and moved westward (the phenomenon of “retrograde motion”). Each planet’s path, he argued, was that of a hippopede—a name taken from the figure-eight-shaped rope used for hobbling horses’ hooves. This theory also made sense of the Moon and the other planets’ traveling along approximately the same path as the Sun but then deviating north and south.
Invited to lecture at the Lyceum, Eudoxus postulated that the stars and the planets were fixed to the shells of these twenty-seven spheres, which, transparent and thus invisible, encircled the Earth. One carried the Sun, which completed its transit every twenty-four hours, to bring about the day. Another slowly rotated the Earth about its axis, by which it was attached to a larger sphere, this rotation establishing the year. The axis of the newly posited sphere, tilted relative to its outer neighbor, was what moved the Sun down the sky in winter and up it in summer.
Aristotle went even further, proposing that the heavens were made up of fifty-five such bodies, all rotating around the Earth at different but constant speeds—which makes one think of some circus high-wire acrobat spinning plates. His idea was that since there could never be an empty space in the heavens (“Nature abhors a vacuum”), and the spaces between the spheres in this universe had to be filled, the greater number, each sphere nestling with those immediately above and below it, would do the job nicely. The Earth was completely surrounded by this rotating nest of spheres, like the core of an onion. Beyond all these spheres Aristotle envisioned yet one sphere more, the Prime Mover—not to be confused with the creator of the universe, but still the dynamic force that drove all things. This tortured web of elaboration satisfied the world’s astronomers for centuries—until the invention of the telescope.
The whole structure was based more on a priori philosophic speculation than empirical observation, so Aristotle underpinned it with logic as best he could. The universe is spherical, he reasoned, because, of all shapes, a sphere, which looks the same no matter from where it is viewed, is the most perfect. Earth, too, has to be a sphere, as proved by the Moon’s face being overrun by a curve of darkness during an eclipse. Furthermore, travelers north or south do not see the same stars at night as those who stay at home, nor do the stars appear to occupy the same positions in the sky: so terrestrial journeys must be over a curved surface.
Why the Earth should be at the center of the universe was explained by its uniqueness and immobility. While an understanding of gravity was millennia away, Aristotle argued that the Earth’s heavier elements, rock and water, were of their nature drawn toward the center of the universe, where they came together in the shape of a sphere. The lighter elements—air and fire—intrinsically moved up and away. Echoing Plato, he added that heavenly bodies were made up not of these four elements but of ether, which he named (because it was the fifth) “quintessence … more divine, and prior to … the four in our uniquely changeable world that lies under the wandering Moon.”8 This ether was neither heavy nor light but “ageless, unalterable and impassive” (i.e., free from suffering), and—key to the construct of Greek astronomy—passed in an unceasing circular motion, so bringing each heavenly body back to its starting point, enforcing an eternal recurrence.
There were obvious flaws in this explanation. For example, each planet—and the Sun—attached to its sphere, must always be the same distance from the Earth; yet the Moon appeared to change in size by up to almost one-sixth. Did not this indicate that its distance from the Earth was changing? Another indication that the planets’ distance from the Earth was not constant was that their brightness—especially that of Mars—was variable. Transient mutable phenomena such as comets and meteors were assumed to be atmospheric occurrences, taking place below the orbit of the Moon. Eudoxus and Aristotle for some reason ignored all this, as well as the fluctuation in the apparent velocity of the Sun. But what Eudoxus did discover, and Aristotle appreciated, was that the motion of the planets could be explained by combining the uniform rotations of concentric spheres on inclined axes. At least in respect of geometrical model-building, this marks the beginning of scientific astronomy.
Following Alexander’s death in 323 B.C., anti-Macedonian agitation drove Aristotle from the city a second time, and he died the following year in Chalcis, on Euboea, leaving behind the prototype of a modern university library, a zoological garden, a museum of natural history, and an unparalleled body of work (he is said to have spent his honeymoon collecting specimens of marine life). His strengths were not in mathematics or physics, and he undertook no astronomic observations, yet his influence on this as on the other sciences was profound. He systematized the whole approach to learning and settled the principles governing research, collecting a mass of data from which deductions could be drawn.
Even as he advanced his beliefs, however, his almost exact contemporary Heracleides of Pontus (390-322 B.C.—they died within days of each other), who had studied under Plato but lived on the southern coast of the Black Sea, is said to have suggested that Mercury and Venus might revolve around the Sun, not the Earth, and also that the Earth might rotate daily on its axis, despite appearances to the contrary. He went on to assert that the Sun caused winds, which in turn brought about high and low tides. A younger astronomer, Theophrastus (“speaking like a god,” a nickname given him by Aristotle) of Eressos, on Lesbos, discovered sunspots, though we have no idea how. Aristotle’s will designated him his successor.
Early in the third century, one of the most remarkable of all Greece’s astronomers, Aristarchus of Samos (310-230 B.C.), was the first to discover precession. He also calculated that the ratio of the Sun’s diameter to the Earth’s was between 19:3 and 43:6. How could so large a body revolve around one so much smaller? A few years later he made the world-shattering—or at least Earth-demoting—assertion that our globe must go around the Sun, not the other way around, and that, apart from the Moon, the other planets did too. The Earth, not the heavens, turned daily, and made a complete circle of the Sun in the course of a year. The Sun, along with the fixed stars, did not move.
A Stoic astronomer in Athens promptly branded him an atheist and circulated a tract charging him with having set “the hearth [sic] of the cosmos in motion.” Otherwise, this new theory, so momentous in its implications, was buried. This may seem odd, given that Aristarchus’ description not only built on discoveries and investigations that had been in train since the sixth century B.C. but also resolved a whole array of problems, such as retrograde movement. However, if the Earth truly moved, then Aristotle’s theory of falling bodies would be undermined, without any compensating theory to take its place; and, if we were living on just another planet, what would become of the terrestrial celestial dichotomy implicit in the majesty of the heavens?
As Aristarchus himself had to admit, there was nothing to be seen in the world around us to suggest that the Earth moved: surely if it did it would hurl people against one another, clouds and birds would be left behind, things would just be pitched off. Common sense dictated that he was wrong, reinforced by the prejudice that man must surely live at the center of creation. Then there were the doctrines of astrology—at that time still respected as a science—which also required a fixed, central Earth. All in all, there seemed many good reasons for keeping the Earth at the center.
Whether or not the hypotheses of this solitary astronomer, working on his own, ever reached them, the so-called Hellenistic philosophers (i.e., those who practiced in the centuries after Aristotle’s and Alexander’s deaths) decided to leave matters (and the Earth) where they were. The Sun was important, but the Earth much more so. If difficulties emerged to which science had no answer, they should not threaten the status quo. One wonders whether some harbored doubts. For most astronomers of that time, science existed to support the conviction that humanity inhabited a universe of manifest order and design. But, as Durant writes,
Hipparchus (c. 190–c. 120 B.C.) at the Alexandria Observatory. At his left is the armillary sphere he invented, while he is looking not through a telescope but through a sector-defining tube. Illu(6.2)
Since the organization of a religious group presumes a common and stable creed, every religion sooner or later comes into opposition with that fluent and changeful current of secular thought that we confidently call the progress of knowledge. In Athens the conflict was not always visible on the surface, and did not directly affect the masses of the people; the scientists and the philosophers carried on their work without explicitly attacking the popular faith, and often mitigated the strife by using the old religious terms as symbols or allegories for their new beliefs; only now and then, as in the indictment of Anaxagoras … did the struggle come out into the open, and become a matter of life and death.9
The next figure of substance was Hipparchus of Nicaea, who has been called the greatest astronomer of antiquity (even though he stayed firmly within geocentric orthodoxy). Operating around 130 B.C. from his observatory on the island of Rhodes,* he determined the length of the solar year to within six minutes of accuracy and made numerous calculations of the Sun’s diameter. He mapped some 850 stars (crucial, as without defining positions in the sky, mathematical astronomy is impossible), and devised a scale, divided into six ranges of intensity, that measured their brightness: even today, no one has been able to improve on it.
So accurate was his mapping that he had to accept Aristarchus’ discovery of precession when his calculations indicated that the stars were not quite fixed in relation to the Sun. Ever the committed Aristotelian, he set out to explain the apparent circular motions of the main celestial bodies in a way that would make sense without upsetting the geocentric theory. Aware that the seasons were of unequal length, he had the Sun revolve at a uniform speed, but moved the Earth from the center of its path, then suggested that it was how the plane of the Sun’s motion lined up with the Earth’s axis that determined the timing of both the solstices and equinoxes. He remains the best early example of how astronomers tortured their reasoning to keep the Earth where the ancient world wanted it.
Hipparchus’ long and productive life ended around 120 B.C., and with him the long Greek tradition of astronomical observation and speculation. Rome was in the ascendant, and uninterested in the heavens. It would take over two hundred years for another outstanding astronomer to appear.
LONG BEFORE HIPPARCHUS, the great successor states of the Macedonian Empire, from Greece to Iraq, had been conquered by the Roman Empire.
Most members of the Roman elite were suspicious of Greek scientific learning (its medicine aside). By the last years of the Roman Empire, a measure of astronomical knowledge was at least considered a basic aristocratic educational requirement, but its study was accepted only insofar as it could be applied to literature: Did it help toward a better understanding of a poem? In an empire of some fifty million people, the number of natural philosophers in Rome itself became too small for any fruitful collaboration,10 leading to a decline in scientific knowledge generally.11 There was virtually no solar astronomy of great value by an Italian-born scientist for over nine hundred years. The historian of science Timothy Ferris says of the Romans,
Theirs was a nonscientific culture. Rome revered authority; science heeds no authority but that of nature. Rome excelled in the practice of law; science values novelty over precedent. Rome was practical, and respected technology, but science at the cutting edge is as impractical as painting and poetry.… Roman surveyors did not need to know the size of the sun in order to tell time by consulting a sundial; nor did the pilots of Roman galleys concern themselves overmuch with the distance of the moon, so long as it lit their way.12
IT IS AN IRONY of history that while Rome was turning its back on the heavens, one part of its empire was celebrating the last major figure of this period. Ptolemy—Claudius Ptolemaeus (c. A.D. 90-168)—was an Egyptian geographer and astronomer who flourished for some forty years at Canopus, some fifteen miles east of Alexandria, nicely rendered by that iconoclastic historian Dennis Rawlins as “an infamously licentious town which was an ancient combination in one of Hollywood, Lourdes and Las Vegas.”13 He left four works, any one of which would place him among the most important authors of the ancient world: Syntaxis mathematica, better known by its Arabic title, the Almagest, a thirteen-volume “book” of data about the stars; Tetrabiblos (the “Bible of the astrologer”), which he begins by distinguishing between the two modes of studying the heavens, mathematical astronomy and horoscopic astrology; Harmonics, which relates musical harmonies to the properties of mathematical proportions derived from the harmonies he considered to exist throughout the universe; and Geographia, a compilation of what was known about the world at that time.*
In the Almagest, Ptolemy suggested that the planets moved in circles within other circles, with the Earth at their very center, all the while assuming that their actual movements were unknowable. He concluded that the Sun was about 1,200 Earth radii from us (one-nineteenth of the actual figure), essentially the same estimate accepted during the whole Middle Ages. At one point he does consider a Sun-centered universe, but discards it: After all, where was the evidence? But he does hold that one of the two main composite motions of each planet is governed by its relation to the Sun, which made the conversion to heliocentricity much easier when it came.
When the Almagest appeared, critics charged that he had lifted large sections from Hipparchus and that several observations were fabricated—at one point he does unwittingly assign two quite different dates thirty-seven days apart to the same celestial event, and there are many other instances of sloppy research and borrowed ideas.14 But if he was a fraud, he did a hopeless job of covering his tracks, and his achievement is better seen from a different perspective.
Colin Ronan, in his history of astronomy, points out that during these years, scholarship was “primarily in a reminiscent mood, collating and assessing the achievements of previous generations.”15 Ptolemy was the supreme collator; although the Almagest contains errors not corrected until the seventeenth century, the huge number of tables he created was sufficiently accurate to be taken up by Copernicus (never a sophisticated observer of the skies). The Almagest shares with Euclid’s Elements—which laid down the basis for geometry—the distinction of being one of the two mathematical texts longest in use. It was Ptolemy’s thinking that set the course of astronomy for the next fifteen hundred years.
Ptolemy regarded his work as part of an ongoing inquiry, but it was treated by his successors as definitive. If, as seems most likely, he came up with his theory of orbits within orbits in order to maintain the Earth at the center of the universe, then in turn had to shape his “observations” to fit his theory, he might almost not have bothered. Rulers and ever more powerful Church dignitaries—their views reinforced by the rising importance of the Christian idea that doctrine counted for more than knowledge—were becoming skeptical of the value of looking into the heavens. With Ptolemy’s death, as Ronan has it, “the light of astronomical research went out [throughout Western Europe] for a thousand years”;16 no astronomers from that civilization made any significant advance on what he had done. Conditions for scientists began to worsen, and not until far on into the Middle Ages were Ptolemy’s ideas revived, as the philosophy of Aristotle was wedded to medieval theology in the great synthesis of Christian faith and ancient reason undertaken by such philosopher-theologians as Thomas Aquinas. The Prime Mover of Aristotle’s universe (while never conceived by him as the cause of all creation) fed into the God of Christian theology; the outermost sphere of the Prime Mover became the cosmological embodiment of the Christian vision of heaven; and the central position of the Earth was interpreted as a sign of the Christian God’s concern for mankind. Thereafter it served the Church not to study the heavens too carefully.17
Astrology was the leading discipline of the time; fortune-telling became an obsession that lasted through the centuries, and its connections to alchemy and number symbolism were to become important elements in both Christian and Muslim Arab thinking. As Nietzsche was to ask, “Do you believe that the sciences would ever have arisen and become great had there not been beforehand magicians, alchemists, astrologers and wizards, who thirsted and hungered after obscure and forbidden powers?”18
Rome’s leaders were no exception. In the days before he accepted the sobriquets of Augustus (“the Increaser”) and Emperor (“Victorious General”), Caesar’s great-nephew and successor, Octavian, was converted to astrology when a leading practitioner examined his horoscope and immediately knelt in worship before the man he saw as his future sovereign. Although the adopted son of Augustus, Tiberius (42 B.C.–A.D. 37), banished all astrologers from the capital, he privately continued to depend on them. Nero (A.D. 37-68), too, was officially skeptical, but had one Barbillus read the heavens to discover his enemies, whom he promptly executed. These years and the centuries that followed were a period of regression, as astronomical observation and investigation became subservient to sun worship and false prophets abounded. Tertullian the Convert (c. A.D. 155-245), a lawyer from Carthage, wrote: “As for us, curiosity is no longer necessary.”
Here astronomy came to theology’s aid. Ptolemy’s theories survived the collapse of the Roman West for their technical quality, their coherent picture of the cosmos, their usefulness to astrology, and their harmony with Christian teaching. The system outlined by Pythagoras, refined by Plato and Aristotle, given final form by Hipparchus, and recorded by Ptolemy promoted, as Franz Cumont says, “a divinity unique, almighty, eternal, universal and ineffable, that revealed itself throughout nature, but whose most splendid and energetic manifestation was the sun.”19 The way these astronomers described the Sun chimed in almost perfectly with how the Church, appropriating solar imagery, wanted its believers to see the Christian God.
This cumulative reading of the universe afforded far more to Christianity than merely a legacy of pagan festivals and symbols. “To arrive at the Christian monotheism,” continues Cumont, “only one final tie had to be broken, that is to say, this supreme being residing in a distant heaven had to be removed beyond the world.” The various cults devoted to the Sun not only made straight the roads for Christianity; they heralded its triumph. Little wonder that for the next fourteen hundred years Church and state should combine so effectively to keep man’s understanding of the universe exactly as the Greeks had left it.
* Plato adds a less dignified anecdote of the astronomer in his youth being so caught up in his stargazing that on one occasion he tripped and fell down a well, to be promptly teased by “a clever and pretty maidservant from Thrace,” who asked, “How do you expect to understand what is going on up in the sky when you don’t even see what is at your feet?”4 The first absent-minded-professor joke.
* Meton got off lightly. Socrates, whose teaching contained a strong holier-than-thou element, so angered Aristophanes that he ridiculed the former’s methods in The Clouds. In one scene an old gentleman finds his way to the “School of Very Hard Thinkers,” where he finds Socrates suspended from the ceiling in a basket, engrossed in thought, while his students are bent down with their noses to the ground. He asks one: “Excuse me, but—their hind quarters—why are they stuck up so strangely in the air?” The student replies: “Their other ends are studying astronomy.”
* The center of a flourishing sun cult, Rhodes devoted to the Sun a quadrennial festival with athletic games, the most important being a chariot race, and erected the famous Colossus, one of the Seven Wonders of the World, in 284 B.C. This figure of the sun god towered over 105 feet (32 meters), only to collapse in an earthquake in 218 B.C. It took nine hundred camels to haul away the wreckage.
* “Books” then were more like scrolls, of necessity brief affairs, corresponding to a modern-day substantial chapter. Liber meant a unit of writing, not a unit of creation (a book in our present-day sense), so it is not impossible for someone to have written many books, totaling several dozen in one lifetime. Livy authored one book of 127 chapters, Caesar’s opponent Varro 490—yet in modern terms just 200 to 250 pages. By contrast, Henry James authored some forty books, few of them slim. But then every generation throws up at least one person who cannot stop writing.