Ancient world models

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First theories were necessarily simple. The Earth was a flat plane with rivers, hills, seas and land, fixed, eternal. The heavenly bodies revolved, passing from east to west. But if the land continued indefinitely, how could the Sun that set in the west be the same Sun that rose in the east the next morning? Perhaps, the Babylonians reasoned, the Earth was flat but finite with a circle of ocean beyond which a ring of mountains supported the heavens, the firmament. Then, if doors were provided in the base of this great solid half-sphere on the eastern and western sides, the celestial bodies would be able to slip through the western doors on setting and be transported in some miraculous way to the east to reappear as ordained.

The Babylonians were skilled astronomers though their world-picture was naive. They observed the positions of the Sun, Moon, planets and stars for many centuries with great accuracy. They found that they could predict eclipses. Their observations were motivated by their belief that the future of human beings could be predicted from celestial configurations and events such as eclipses or the appearance of comets. Because of this, kings kept court astrologers and the wealthy paid for horoscopes. This belief in astrology, found in all nations, should have withered away with alchemy and the search for the philosopher's stone but even today there are many who set great faith in this pseudo-science. It is perhaps needless to say that modern astronomy demonstrates how ludicrous such beliefs are.

The Egyptians, astronomers almost as skilled as the Babylonians, had equally simple world-pictures. They noticed that the yearly inundation of the Nile valley coincided with the days when the star Sirius could be seen best in the morning twilight. This linking of celestial and earthly events spurred on their development of astrology and brought religion into the picture. The Sun-god descended at night, passing beneath the Earth to visit the dead.

Farming people were more interested in the solar cycle since it was linked with seed time and harvest. Seafaring peoples like the Phoenicians and the Minoans used the rising and setting directions of the stars as navigational aids. It may well have been as an aid to memory that the stars were grouped in constellations, embodying myths current at that time.

As is to be expected, the ancient Chinese civilizations produced schools of astronomy and cosmological theories. Serious Chinese astronomy probably began prior to 2000 BC although details of events in that era are largely legendary. The story of the two Chinese astronomers, Ho and Hi, executed for failing to predict an eclipse of the Sun in 2137 BC is possibly apocryphal and may refer to two astronomical colleges of a much later date destroyed in civil strife. Reliable historical details begin about 1000 BC. A farming people required a calendar and so the lengths of month and year were quickly ascertained. A year of 365^ days was certainly used by 350 BC.

By that date, the Chinese constellation figures, 122 in number and quite different from those handed down to us by the Greeks, had been mapped out, the Sun's path—the ecliptic—being divided into 12 regions. The size of a region was not only connected with the heavenly arc inhabited by the Sun each month but also with the yearly journey of the planet Jupiter. The other planetary motions were also studied. As in the west, a pseudo-science of astrology developed from such studies. China was the centre or hub of the flat Earth with heavenly and human events in close harmony: not only did celestial events guide and control men, in particular the Emperor and his court but the decisions and actions of such powerful rulers influenced the state of Heaven.

As mathematical knowledge grew and more accurate astronomical instruments for measuring altitudes and angles were developed in succeeding centuries, the movements of the Sun, Moon and planets were systematized in remarkably accurate tables for prediction purposes. Cometary appearances were noted, among them several apparitions of Halley's comet, and by the 14th century AD the state of Chinese astronomy compared favourably with that of the Arabs in the West.

In various other places where a civilization had developed, astronomical schools flourished. The ravages of time and barbarism have sadly destroyed most of the works of such schools, though happily some traces remain to tell us of the heights of thought their practitioners achieved. For example, we shall see later how ingenious were the steps megalithic man took to keep track of the Sun and Moon. This remarkable civilization flourished in Western Europe in the third and second millennia BC.

Observations of eclipses were also recorded by early American Indians as, for example, by Mayans. A sundial remaining in the 'lost city', Macchu Piccu, provides us with evidence that the Incas of Peru used solar observations to some purpose. The 'Puerta del Sol' at Tiahuanaco, Bolivia, tells us of solar observations prior to the Incas.

However, very few of the ideas and notions of astronomy and cosmology from any of these civilizations have had an influence on the development of our understanding of the astronomical Universe. Our starting points find their origins mainly in ancient Greece.

A completely new departure in mankind's contemplation and interpretation of the heavens came with the flowering of Greek civilization. Many of their thinkers had extraordinarily original minds, were mentally courageous and devoted to rational thought. They were not afraid of questioning cherished beliefs and of following unsettling, disturbing trains of thought.

Many of them dismissed the 'common-sense' picture of solid, flat Earth and god-controlled Heaven. They saw that a spherical Earth poised in space solved a lot of problems. Those stars and planets not seen during the night were simply on the other side of the Earth. Stars were not seen during the day because the dazzling bright Sun blotted out their feeble light. The Moon caused solar eclipses. Pythagoras, in the 6th century BC, taught that the movements of all the heavenly bodies were compounded of one or more circular movements.

In the next century, Philolaus, a follower of Pythagoras, suggested the bold idea that the Earth was not the centre of the Universe and, indeed, that it moved. At the centre of the Universe there was a gigantic fire. Around this fire revolved the Earth, Moon, Sun and planets in that order, in circles of various sizes. He also postulated a body called the Anti-Earth to bring the total of moving bodies up to the sacred number of ten. This Anti-Earth revolved about the central fire within the Earth's orbit and was never seen from the Earth because the Earth faced outwards towards the home of the gods— Olympus—situated beyond the sphere of the fixed stars. Philolaus also believed that the Sun was not self-luminous but shone by the light it absorbed from Olympus and the central fire.

In contrast to this, Anaxagoras taught that the Sun was a mass of glowing metal comparable in size with Greece itself. Aristarchus, in the 3rd century BC, agreed with Philolaus that the Earth moved and taught that it rotated on its axis, thus explaining the diurnal motion of the heavens. Moreover, he said, the Sun is a star and the Earth revolves round it, all other stars being very much farther away.

Aristarchus, like Anaxagoras, had ideas about the relative sizes of Sun, Moon and Earth. The Sun's diameter had to be about seven times the diameter of the Earth, a figure far removed from the modern one but embodying the right idea, namely that the Earth is much smaller than the Sun.

Eratosthenes of Alexandria, living about 230 BC, used solar observations and a knowledge of geometry and geography to calculate the circumference of the Earth, obtaining a value within a few per cent of today's accepted figure.

He knew that at the summer solstice the Sun passed through the zenith at Syene in Upper Egypt,

Figure 2.1. The observations of Eratosthenes.
Figure 2.2. The interpretation of the measurements of Eratosthenes.

being reflected at the bottom of a well. At Alexandria, at the same longitude as Syene, the obelisk at the same solar solstice, cast a shadow at noon, showing by its length that the Sun's altitude was 82^ degrees (figure 2.1). He also knew the distance between Syene and Alexandria. Eratosthenes then made the assumptions that the Sun was very far away and that the Earth was spherical. The Sun's rays arriving at Syene and Alexandria could then be taken to be parallel and the angle the Sun's direction made with the vertical at Alexandria (75°) would, therefore, be the angle subtended at the Earth's centre C by the arc from Syene to Alexandria (figure 2.2). It was then a simple calculation to find the length of the Earth's circumference by asking what distance would subtend an angle of 360° if the distance from Alexandria to Syene subtended an angle of l\° at the Earth's centre.

Other outstanding Greek astronomers and mathematicians such as Hipparchus, Thales, Apollonius, Aristotle and Ptolemy also put forward world-pictures, or cosmologies, that arouse admiration for the way their minds managed to successfully break free from their environment and catch glimpses of the truth. For example, Hipparchus discovered the precession of the equinoxes, noted by the secular change in position of the solar crossing point of its ecliptic path over the celestial equator at the times of the spring and autumnal equinox. He measured the Sun's distance and went a considerable way towards providing theories to account for the motions of Sun and Moon.

Finally, as Greek civilization decayed, the last and perhaps the most influential thinker of them all embodied the work of many of the predecessors in the Almagest. Ptolemy, who lived during the second century AD, not only collected and discussed the work of Greek astronomers but carried out original researches himself in astronomy, geography, mathematics, music, optics and other fields of study. His great astronomical work, the Almagest, survived the Dark Ages of Western civilization, influencing astronomical thought right up to and beyond the invention of the telescope in the early years of the seventeenth century. The Ptolemaic System describing the apparent motions of the Sun, Moon and planets is discussed in section 12.2.

During the Dark Ages astronomy flourished within the Islamic Empire, once the latter had been stabilized. Ptolemy's Almagest was translated into Arabic in 820 AD and thereafter guided the researches of Muslim scientists. They measured astronomical phenomena more precisely than ever before, amassing a wealth of information that proved of inestimable value to Western astronomers when Europe emerged from the Dark Ages. Many of the terms used in modern astronomy come from the Arabic, for example 'zenith', 'nadir', 'almanac', while the names of well-known stars such as Algol, Aldebaran, Altair and Betelgeuse are also of Arabic origin. In addition, the Muslim mathematicians introduced spherical trigonometry and Arabic numerals, including a sign for zero—'algebra' is another Arabic word.

They do not seem, however, to have left us new cosmologies. They were content to accept the world-pictures of the Greeks into their custody until the Western world awoke intellectually once again and began anew the study of natural science, including astronomy.

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