To Explain the World: The Discovery of Modern Science (9 page)

Much more important, it seems to me, was the widespread view among the early Christians that pagan science is a distraction from the things of the spirit that ought to concern us. This goes back to the very beginnings of Christianity, to Saint Paul,
who warned: “Beware lest any man spoil you through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ.”
¹¹
The most famous statement along these lines is due to the church father Tertullian, who around the year 200 asked, “What does Athens have to do with Jerusalem, or the Academy with the Church?” (Tertullian chose Athens and the Academy to symbolize Hellenic philosophy, with which he presumably was more familiar than he was with the science of Alexandria.) We find a sense of disillusion with pagan learning in the most important of the church fathers, Augustine of Hippo. Augustine studied Greek philosophy when young (though only in Latin translations) and boasted of his grasp of Aristotle, but he later asked, “And what did it profit me that I could read and understand all the books I could get in the so-called ‘liberal arts,’ when I was actually a slave of wicked lust?”
¹²
Augustine was also concerned with conflicts between Christianity and pagan philosophy. Toward the end of his life, in 426, he looked back at his past writing, and commented, “I have been rightly displeased, too, with the praise with which I extolled Plato or the Platonists or the Academic philosophers beyond what was proper for such irreligious men, especially those against whose great errors Christian teaching must be defended.”
¹³

Another factor: Christianity offered opportunities for advancement in the church to intelligent young men, some of whom might otherwise have become mathematicians or scientists. Bishops and presbyters were generally exempt from the jurisdiction of the ordinary civil courts, and from taxation. A bishop such as Cyril of Alexandria or Ambrose of Milan could exercise considerable political power, much more than a scholar at the Museum in Alexandria or the Academy in Athens. This was something new. Under paganism religious offices had gone to men of wealth or political power, rather than wealth and power going to men of religion. For instance, Julius Caesar and his successors won the office of supreme pontiff, not as a recognition of piety or learning, but as a consequence of their political power.

Greek science survived for a while after the adoption of
Christianity, though mostly in the form of commentaries on earlier work. The philosopher Proclus, working in the fifth century at the Neoplatonic successor to Plato’s Academy in Athens, wrote a commentary on Euclid’s
Elements
, with some original contributions. In
Chapter 8
I will have occasion to quote a later member of the Academy, Simplicius, for his remarks, in a commentary on Aristotle, about Plato’s views on planetary orbits. In the late 300s there was Theon of Alexandria, who wrote a commentary on Ptolemy’s great work of astronomy, the
Almagest
, and prepared an improved edition of Euclid. His famous daughter Hypatia became head of the city’s Neoplatonic school. A century later in Alexandria the Christian John of Philoponus wrote commentaries on Aristotle, in which he took issue with Aristotle’s doctrines concerning motion. John argued that the reason bodies thrown upward do not immediately fall down is not that they are carried by the air, as Aristotle had thought, but rather that when they are thrown bodies are given some quality that keeps them moving, an anticipation of later ideas of impetus or momentum. But there were no more creative scientists or mathematicians of the caliber of Eudoxus, Aristarchus, Hipparchus, Euclid, Eratosthenes, Archimedes, Apollonius, Hero, or Ptolemy.

Whether or not because of the rise of Christianity, soon even the commentators disappeared. Hypatia was killed in 415 by a mob, egged on by Bishop Cyril of Alexandria, though it is difficult to say whether this was for religious or political reasons. In 529 the emperor Justinian (who presided over the reconquest of Italy and Africa, the codification of Roman law, and the building of the great church of Santa Sophia in Constantinople) ordered the closing of the Neoplatonic Academy of Athens. On this event, though Gibbon is predisposed against Christianity, he is too eloquent not to be quoted:

The Gothic arms were less fatal to the schools of Athens than the establishment of a new religion, whose ministers superseded the exercise of reason, resolved every question by an article of faith, and condemned the infidel or skeptic to eternal
flames. In many a volume of laborious controversy they espoused the weakness of the understanding and the corruption of the heart, insulted human nature in the sages of antiquity, and proscribed the spirit of philosophical inquiry, so repugnant to the doctrine, or at least to the temper, of a humble believer.
¹⁴

The Greek half of the Roman Empire survived until AD 1453, but as we shall see in
Chapter 9
, long before then the vital center of scientific research had moved east, to Baghdad.

PART II

GREEK ASTRONOMY

The science that in the ancient world saw the greatest progress was astronomy. One reason is that astronomical phenomena are simpler than those on the Earth’s surface. Though the ancients did not know it, then as now the Earth and the other planets moved around the Sun on nearly circular orbits, at nearly constant velocities, under the influence of a single force—gravitation—and they spun on their axes at essentially constant rates. The same applied to the Moon in its motion around the Earth. In consequence the Sun, Moon, and planets appeared from Earth to move in a regular and predictable way that could be and was studied with considerable precision.

The other special feature of ancient astronomy is that it was useful, in a way that ancient physics generally was not. The uses of astronomy are discussed in
Chapter 6
.

Chapter 7
discusses what, flawed as it was, can be considered a triumph of Hellenistic science: the measurement of the sizes of the Sun, Moon, and Earth, and the distances to the Sun and Moon.
Chapter 8
treats the problem posed by the apparent motion of the planets, a problem that continued to concern astronomers through the Middle Ages, and that eventually led to the birth of modern science.

6

The Uses of Astronomy
1

Even before the start of history, the sky must have been commonly used as a compass, a clock, and a calendar. It could not have been difficult to notice that the Sun rises every morning in more or less the same direction, that during the day one can tell how much time there is before night from the height of the Sun in the sky, and that hot weather will follow the time of year when the day lasts longest.

We know that the stars were used for similar purposes very early in history. Around 3000 BC the Egyptians knew that the crucial event in their agriculture, the flooding of the Nile in June, coincided with the heliacal rising of the star Sirius. (This is the day in the year when Sirius first becomes visible just before dawn; earlier in the year it is not visible at night, and later it is visible well before dawn.) Homer, writing before 700 BC, compares Achilles to Sirius, which is high in the sky at the end of summer: “that star, which comes on in the autumn and whose conspicuous brightness far outshines the stars that are numbered in the night’s darkening, the star they give the name of Orion’s Dog, which is brightest among the stars, and yet is wrought as a sign of evil and brings on the great fever for unfortunate mortals.”
²
A little later, the poet Hesiod in
Works and Days
told farmers that grapes are best cut at the heliacal rising of Arcturus, and that plowing should be done at the cosmical setting of the
Pleiades constellation. (This is the day in the year when these stars first are seen to set just before sunrise; earlier in the year they do not set at all before the Sun comes up, and later they set well before dawn.) Following Hesiod, calendars known as
paramegmata
, which gave the risings and settings of conspicuous stars for each day, became widely used by Greeks whose city-states had no other shared way of identifying dates.

By watching the stars at night, not obscured by the light of modern cities, observers in many early civilizations could see clearly that, with a few exceptions (about which more later), the stars always remain in the same places relative to one another. This is why constellations do not change from night to night or from year to year. But the whole firmament of these “fixed” stars seems to revolve each night from east to west around a point in the sky that is always due north, and hence is known as the north celestial pole. In modern terms, this is the point toward which the axis of the Earth extends if it is continued from the Earth’s north pole out into the sky.

This observation made it possible for stars to be used very early by mariners for finding directions at night. Homer tells how Odysseus on his way home to Ithaca is trapped by the nymph Calypso on her island in the western Mediterranean, until Zeus orders Calypso to send Odysseus on his way. She tells Odysseus to keep the “Great Bear, that some have called the Wain . . . on his left hand as he crossed the main.”
³
The Bear, of course, is Ursa Major, the constellation also known as the Wagon (or Wain), and in modern times as the Big Dipper. Ursa Major is near the north celestial pole. Thus in the latitude of the Mediterranean Ursa Major never sets (“would never bathe in or dip in the Ocean stream,” as Homer puts it) and is always more or less in the north. With the Bear on his left, Odysseus would keep sailing east, toward Ithaca.

Some Greeks learned to do better with other constellations. According to the biography of Alexander the Great by Arrian, although most sailors in his time used Ursa Major to tell north, the Phoenicians, the ace sailors of the ancient world, used Ursa
Minor, a constellation that is not as conspicuous as Ursa Major but is closer to the north celestial pole. The poet Callimachus, as quoted by Diogenes Laertius,
⁴
claimed that the use of Ursa Minor goes back to Thales.

The Sun also seems during the day to revolve from east to west around the north celestial pole. Of course, we cannot usually see stars during the day, but Heraclitus
⁵
and perhaps others before him seem to have realized that the stars are always there, though with their light blotted out during the day by the light of the Sun. Some stars can be seen just before dawn or just after sunset, when the position of the Sun in the sky is known, and from this it became clear that the Sun does not keep a fixed position relative to the stars. Rather, as was well known very early in Babylon and India, in addition to seeming to revolve from east to west every day along with the stars, the Sun also moves each year around the sky from west to east through a path known as the zodiac, marked in order by the traditional constellations Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. As we will see, the Moon and planets also travel through the zodiac, though not on precisely the same paths. The particular path through these constellations followed by the Sun is known as the “ecliptic.”

Once the zodiac was understood, it was easy to locate the Sun in the background of stars. Just notice what constellation of the zodiac is highest in the sky at midnight; the Sun is in the constellation of the zodiac that is directly opposite. Thales is supposed to have given 365 days as the time it takes for the Sun to make one complete circuit of the zodiac.

One can think of the firmament of stars as a revolving sphere surrounding the Earth, with the north celestial pole above the Earth’s north pole. But the zodiac is not the equator of this sphere. Rather, as Anaximander is supposed to have discovered, the zodiac is tilted by 23½° with respect to the celestial equator, with Cancer and Gemini closest to the north celestial pole, and Capricorn and Sagittarius farthest from it. In modern terms, this tilt, which is responsible for the seasons, is due to the fact that
the axis of the Earth’s rotation is not perpendicular to the plane of its orbit, which is pretty close to the plane in which almost all objects in the solar system move, but is tilted from the perpendicular by an angle of 23½°; in the northern summer or winter the Sun is respectively in the direction toward which or away from which the Earth’s north pole is tilted.