Published in: Philip’s

ISBN: 978-0-540-08791-4

File Type: pdf

File Size:  53 MB

Language: English


Astronomy is certainly the oldest of all the sciences. Our remote cave-dwelling ancestors must have looked up into the sky and marveled at what they saw there, but they can have had no idea what the universe is really like, or how vast it is. It was natural for them to believe that the Earth is flat, with the sky revolving around it once a day carrying the Sun, the Moon and the stars. Early civilizations in China, Egypt, and the Middle East divided the stars up into groups or constellations, and recorded spectacular phenomena such as comets and eclipses; a Chinese observation of a conjunction of five bright planets may date back as far as 2449 BC. Probably the earliest reasonably good calendars were drawn up by the Egyptians. They paid great attention to the star Sirius (which they called Sothis), because its ‘heliacal rising’, or date when it could first be seen in the dawn sky, gave a reliable clue as to the annual flooding of the Nile, upon which the whole Egyptian economy depended. And, of course, there is no doubt that the Pyramids are astronomically aligned. The first really major advances came with the Greeks.

The first of the great philosophers, Thales of Miletus, was born around 624 BC. A clear distinction was drawn between the stars, which seem to stay in the same positions relative to each other, and the ‘wanderers’ or planets, which shift slowly about from one constellation to another. Aristotle, who lived from around 384 to 325 BC, gave the first practical proofs that the Earth is a globe, and in 270 BC Eratosthenes of Cyrene measured the size of the globe with remarkable accuracy. The value he gave was much better than that used by Christopher Columbus on his voyage of discovery so many centuries later. The next step would have been to relegate the Earth to the status of a mere planet, moving around the Sun in a period of one year. Around 280 BC one philosopher, Aristarchus of Samos, was bold enough to champion this idea, but he could give no firm proof, and found few supporters. The later Greeks went back to the theory of a central Earth. Ptolemy of Alexandria, last of the great astronomers of Classical times, brought the Earth-centred theory to its highest state of perfection. He maintained that all paths or orbits must be circular, because the circle is the ‘perfect’ form, but to account for the observed movements of the planets he was forced to develop a very cumbersome system; a planet moved in a small circle or epicycle, the center of which – the deferent – itself moved around the Earth in a perfect circle. Fortunately, Ptolemy’s great work, the Almagest, has come down to us by way of its Arab translation. Ptolemy died in or about the year AD 180. There followed a long period of stagnation, though there was one important development; in AD 570 Isidorus, Bishop of Seville, was the first to distinguish between true astronomy and the pseudo-science of astrology (which still survives, even though no intelligent person can take it seriously).

The revival of astronomy at the end of the Dark Ages was due to the Arabs. In 813 Al-Ma’mun founded the Baghdad school, and during the next few centuries, excellent star catalogs were drawn up. In 1433 Ulugh Beigh, grandson of the Oriental conqueror Tamerlane set up an elaborate observatory at Samarkand, but with his murder, in 1449, the Baghdad school of astronomy came to an end. The first serious challenge to the Ptolemaic theory came in 1543 with the publication of a book by the Polish churchman Mikol ́aj Kopernik, better known by his Latinized name Copernicus. He realized the clumsiness and artificial nature of the old theory could be removed simply by taking the Earth away from its proud central position and putting the Sun there. He also knew there would be violent opposition from the Church, and he was wise enough to withhold publication of his book until the end of his life. His fears were well-founded; Copernican theory was condemned as heresy, and Copernicus’ book, De Revolutionibus Orbium Coelestium (Concerning the Revolutions of the Celestial Orbs) was placed on the Papal Index. It remained there until 1835.
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