Similar trends are apparent throughout the world. The high point in indigenous Chinese astronomy occurred around 1280, with the work of Kuo Shou-ching, who used an observational baseline of 1,500 years and improved both astronomical instruments and mathematical techniques for computation. It is generally thought that Chinese astronomy thereafter underwent a steep decline. Nathan Sivin believes that the reason lies at least partly ‘in increasing rigidity of elite attitudes, so that the educated were less inclined to be curious about techniques and less willing to value science as an appropriate pursuit for a gentleman.’ The occupation of astronomer became a hereditary office, a practice inconsistent with the advance of the subject. Additionally, ‘the responsibility for the evolution of astronomy remained centered in the Imperial Court and was largely abandoned to foreign technicians,’ chiefly the Jesuits, who had introduced Euclid and Copernicus to the astonished Chinese, but who, after the censorship of the latter’s book, had a vested interest in disguising and suppressing heliocentric cosmology. Perhaps science was stillborn in Indian, Mayan and Aztec civilizations for the same reason it declined in Ionia, the pervasiveness of the slave economy. A major problem in the contemporary (political) Third World is that the educated classes tend to be the children of the wealthy, with a vested interest in the status quo, and are unaccustomed either to working with their hands or to challenging conventional wisdom. Science has been very slow to take root.
Plato and Aristotle were comfortable in a slave society. They offered justifications for oppression. They served tyrants. They taught the alienation of the body from the mind (a natural enough ideal in a slave society); they separated matter from thought; they divorced the Earth from the heavens – divisions that were to dominate Western thinking for more than twenty centuries. Plato, who believed that ‘all things are full of gods,’ actually used the metaphor of slavery to connect his politics with his cosmology. He is said to have urged the burning of all the books of Democritus (he had a similar recommendation for the books of Homer), perhaps because Democritus did not acknowledge immortal souls or immortal gods or Pythagorean mysticism, or because he believed in an infinite number of worlds. Of the seventy-three books Democritus is said to have written, covering all of human knowledge, not a single work survives. All we know is from fragments, chiefly on ethics, and secondhand accounts. The same is true of almost all the other ancient Ionian scientists.
In the recognition by Pythagoras and Plato that the Cosmos is knowable, that there is a mathematical underpinning to nature, they greatly advanced the cause of science. But in the suppression of disquieting facts, the sense that science should be kept for a small elite, the distaste for experiment, the embrace of mysticism and the easy acceptance of slave societies, they set back the human enterprise. After a long mystical sleep in which the tools of scientific inquiry lay moldering, the Ionian approach, in some cases transmitted through scholars at the Alexandrian Library, was finally rediscovered. The Western world reawakened. Experiment and open inquiry became once more respectable. Forgotten books and fragments were again read. Leonardo and Columbus and Copernicus were inspired by or independently retraced parts of this ancient Greek tradition. There is in our time much Ionian science, although not in politics and religion, and a fair amount of courageous free inquiry. But there are also appalling superstitions and deadly ethical ambiguities. We are flawed by ancient contradictions.
The Platonists and their Christian successors held the peculiar notion that the Earth was tainted and somehow nasty, while the heavens were perfect and divine. The fundamental idea that the Earth is a planet, that we are citizens of the Universe, was rejected and forgotten. This idea was first argued by Aristarchus, born on Samos three centuries after Pythagoras. Aristarchus was one of the last of the Ionian scientists. By this time, the center of intellectual enlightenment had moved to the great Library of Alexandria. Aristarchus was the first person to hold that the Sun rather than the Earth is at the center of the planetary system, that all the planets go around the Sun rather than the Earth. Typically, his writings on this matter are lost. From the size of the Earth’s shadow on the Moon during a lunar eclipse, he deduced that the Sun had to be much larger than the Earth, as well as very far away. He may then have reasoned that it is absurd for so large a body as the Sun to revolve around so small a body as the Earth. He put the Sun at the center, made the Earth rotate on its axis once a day and orbit the Sun once a year.
It is the same idea we associate with the name of Copernicus, whom Galileo described as the ‘restorer and confirmer’, not the inventor, of the heliocentric hypothesis.* For most of the 1,800 years between Aristarchus and Copernicus nobody knew the correct disposition of the planets, even though it had been laid out perfectly clearly around 280 B.C. The idea outraged some of Aristarchus’ contemporaries. There were cries, like those voiced about Anaxagoras and Bruno and Galileo, that he be condemned for impiety. The resistance to Aristarchus and Copernicus, a kind of geocentrism in everyday life, remains with us: we still talk about the Sun ‘rising’ and the Sun ‘setting’. It is 2,200 years since Aristarchus, and our language still pretends that the Earth does not turn.
* Copernicus may have gotten the idea from reading about Aristarchus. Recently discovered classical texts were a source of great excitement in Italian universities when Copernicus went to medical school there. In the manuscript of his book, Copernicus mentioned Aristarchus’ priority, but he omitted the citation before the book saw print. Copernicus wrote in a letter to Pope Paul III: ‘According to Cicero, Nicetas had thought the Earth was moved . . . According to Plutarch [who discusses Aristarchusl… certain others had held the same opinion. When from this, therefore, I had conceived its possibility, I myself also began to meditate upon the mobility of the Earth.’
The separation of the planets from one another – forty million kilometers from Earth to Venus at closest approach, six billion kilometers to Pluto – would have stunned those Greeks who were outraged by the contention that the Sun might be as large as the Peloponnesus. It was natural to think of the solar system as much more compact and local. If I hold my finger before my eyes and examine it first with my left and then with my right eye, it seems to move against the distant background. The closer my finger is, the more it seems to move. I can estimate the distance to my finger from the amount of this apparent motion, or parallax. If my eyes were farther apart, my finger would seem to move substantially more. The longer the baseline from which we make our two observations, the greater the parallax and the better we can measure the distance to remote objects. But we live on a moving platform, the Earth, which every six months has progressed from one end of its orbit to the other, a distance of 300,000,000 kilometers. If we look at the same unmoving celestial object six months apart, we should be able to measure very great distances. Aristarchus suspected the stars to be distant suns. He placed the Sun ‘among’ the fixed stars. The absence of detectable stellar parallax as the Earth moved suggested that the stars were much farther away than the Sun. Before the invention of the telescope, the parallax of even the nearest stars was too small to detect. Not until the nineteenth century was the parallax of a star first measured. It then became clear, from straightforward Greek geometry, that the stars were light-years away.
There is another way to measure the distance to the stars which the Ionians were fully capable of discovering, although, so far as we know, they did not employ it. Everyone knows that the farther away an object is, the smaller it seems. This inverse proportionality between apparent size and distance is the basis of perspective in art and photography. So the farther away we are from the Sun, the smaller and dimmer it appears. How far would we have to be from the Sun for it to appear as small and as dim as a star? Or, equivalently, how small a piece of the Sun would be as bright as a star?
An early experiment to answer this question was performed by Christiaan Huygens, very much in the Ionian tradition. Huygens drilled small holes in a brass plate, held the plate up to the Sun and asked himself which hole seemed as bright as he remembered the bright star Sirius to have been the night before. The hole was effectively* 1/28,000 the apparent size of the Sun. So Sirius, he reasoned, must be 28,000 times farther from us than the Sun, or about half a light-year away. It is hard to remember just how bright a star is many hours after you look at it, but Huygens remembered very well. If he had known that Sirius was intrinsically brighter than the Sun, he would have come up with almost exactly the right answer: Sirius is 8.8 light-years away. The fact that Aristarchus and Huygens used imprecise data and derived imperfect answers hardly matters. They explained their methods so clearly that, when better observations were available, more accurate answers could be derived.