The Calendar by David Ewing Duncan Read Free Book Online

fleece; the anatomist Herophilus of Chalcedon, who performed one of the first systematic autopsies; and Euclid and Archimedes, whose ideas form the core of Western mathematics. But perhaps the greatest achievements in this city on the western edge of the Nile delta, hard by the Libyan Desert, were a long line of discoveries in astronomy, some of which became the basis for the new calendar born of Caesar’s tryst with Cleopatra.
    The stargazers of Alexandria started with the patrimony left them by earlier Greek astronomers and mathematicians. Since at least the sixth century BC, they had been looking up in the sky and postulating about what they saw. The earliest of these postulated that the sun is one foot wide and is renewed afresh each day, and that the earth either floats on water or is supported on air. But they also realized that ‘moonshine’ is really reflected sunlight, that the moon is closer to the earth than to the sun, and that eclipses are caused by the shadow of the earth and other celestial bodies.
    These speculations gave way to more solid science with Pythagoras (sixth century BC), who developed some of the early geometry and mathematics used by later astronomers to analyse the respective positions of the sun, moon, earth and stars. Then came the Athenian astronomer Meton, discoverer of the Metonic cycle in 432 BC. At roughly the same time the astronomer Euctemon estimated the length of the seasons, though he got them wrong. A century later Callippus of Cyzicus calculated the correct lengths to round figures--90 days for summer, 90 for autumn, 92 for winter and 93 for spring. Also working in the fourth century BC, the astronomer Eudoxus of Cnidus devised a mathematical theory involving spheres that he used to try to explain the motions of the planets and the moon, and what appeared to be the motion of the sun in an earth-centred universe. Aristotle (384-322 BC) also weighed in, working in the years immediately leading up to the founding of Alexandria. His writing in astronomy expands on Eudoxus’s theory of the planetary spheres by suggesting that the stars, planets and sun literally are encased in invisible spheres that orbit the earth in a series of concentric circles.
    One of the greatest of the early astronomers in Alexandria itself was Aristarchus (fl. c. 270 BC), who constructed a modified sundial called a skaphe-- a spherical bowl with a needle standing up in the centre like a miniature obelisk to cast shadows against lines marked off on the bowl’s surface. Using this device he could measure the height and direction of the sun. This allowed him to figure out that the sun shines light against a half moon, as seen on earth, at an angle of 87 degrees. From this he surmised that the sun is many times the size of the earth and must be very far away.
    Aristarchus also deduced that the earth circles the sun, an astronomic theory that ran counter to the accepted orthodoxy that the sun orbited a stationary earth. He argued that the sun seems to move across the sky because the earth spins on its axis. But lacking a telescope and accurate star charts, Aristarchus could not prove something considered ludicrous by an earth-centred world, one that would remain convinced the sun was subservient to our little planet for another eighteen centuries, until the age of Copernicus and Galileo.
    A generation after Aristarchus, the Alexandria-based mathematician, philosopher, geographer and astronomer Eratosthenes (276-194 BC) deduced within a tenth of a degree the tilting of the earth’s axis of rotation, which causes the seasons. He also measured the circumference of the earth to within 250 miles of the true value. A few years later Ctesibius of Alexandria constructed an elaborate water clock using floats, a chain winch, cog shaft, dial and a sundial system that linked the path of the sun astronomically and geometrically with levels of its shadow.
    In about 130 BC the astronomer Hipparchus (fl. 146-127 BC) discovered