Originally published 29 January 1996
Just back from a sunny sojourn on the Tropic of Cancer. Not far along the beach from where I was staying is a concrete slab with “23° 26′ 22.07″ North of the Equator” in big letters. On midsummer day at noon, the sun stands at the zenith, burning a hole in the top of your head. This is the farthest north the sun ever gets in the sky.
At that time the sun is in the constellation Cancer as seen against the invisible background stars. Or rather, the sun was in the constellation Cancer thousands of years ago when astronomers figured all of this out. Today, the midsummer sun is in the constellation Taurus; it has slipped westward among the stars because of a slow wobble of the Earth’s axis. We should now properly speak of the Tropic of Taurus.
Likewise, the Tropic of Capricorn, marking the sun’s farthest southward excursion, should now be called the Tropic of Sagittarius. However, in these days of electric lights, air conditioning, and satellite navigation, no one pays much attention to the sun and stars, anyway. So let the names of the tropics remain as they were 2,300 years ago when science was invented.
And what an invention! A new way of thinking, involving quantitative observations and mathematical deduction, that gave Western culture an unparalleled power over nature. Electric lights, air conditioning, and satellite navigation, like all other modern technologies, flowed like a river from this powerful new way of making sense of the world.
The ideas sprang from Greek philosophy. They found a welcoming home in the Hellenistic city of Alexandria at the mouth of the Nile River in Egypt. In the 3rd and 2nd centuries B.C., Alexandria became the seat of a magnificent flowering of mathematical and scientific thought. Euclid, Apollonius, Archimedes, Eratosthenes, Aristarchus and many others worked or studied in Alexandria.
It was in Alexandria, for example, that geography and astronomy were made mathematical sciences. The equator, poles, polar circles, and tropics were deduced, although the only one of these places of which the Alexandrians had direct knowledge was the Tropic of Cancer, which passed through the town of Syene in the valley of the Nile, near the site of the present Aswan High Dam.
Latitude and longitude were invented. Eratosthenes measured the size of the Earth with a combination of celestial and terrestrial observations. Perhaps the greatest achievement of Alexandrian science is Aristarchus’ book, On the Sizes and Distances of the Sun and the Moon, a work of skillful observation and flawless logic that compares favorably with anything in modern science.
These spectacular achievements get no more than passing mention in textbooks of Western Civilization, in spite of the fact the Alexandrian invention of science probably had more effect on modern life than any other legacy of antiquity. We learn in school about the Golden Age of Greece and the glory that was Rome, Sophocles and Ovid, the Parthenon and the Pantheon, triremes and aqueducts, but little or nothing of the invention of science that took place in the white city at the mouth of the Nile. Western civilization, as represented by our textbooks, passed Alexandria by.
It is true that what happened in Alexandria had little effect on the ancient world. The scientists and mathematicians who worked there talked mostly among themselves. Alexandria was a sort of private think tank, a pressure cooker where the temperature of human thought was raised to a new level. More than a thousand years would pass before the new way of thinking took firm root and transformed civilization.
I was thinking about these things the other night under an inky dark tropic sky. In the west, Mercury, Venus, and Saturn were strung along the zodiac like clothespins on a line. To the uninformed eye they are indistinguishable from stars; to the Alexandrians, they were wandering stars whose positions were measured to a fraction of a degree and whose motions were given precise mathematical expression.
Later that night, after midnight, the constellation Cancer passed overhead. Cancer is an inconspicuous constellation, with no bright stars, invented to mark the sun’s apparent path among the stars. At the center of the constellation is a blur of light, near were the sun would have been on midsummer day in ancient Alexandria.
The ancients called this blur Praesepe, or “manger.” The two faint nearby stars are the “donkeys” eating from the manger. We now know that the blur is a cluster of hundreds of stars, too faint to be distinguished by the unaided eye. For those of us who live near cities, the blur has long been made invisible by artificial light and haze.
On the Tropic of Cancer, far from city lights, the Praesepe was easily visible exactly at the zenith, as the Alexandrians might have seen it if they had traveled a ways down the Nile. As I watched, I wondered admiringly at those brilliantly creative scientists and mathematicians who first imposed a net of abstract thought upon the Earth and sky.
