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Astronomy owes its heritage to many historical strands, foremost among them being the studies of the heavens by the ancient Babylonians, the Chinese and other Far Eastern cultures, the early Egyptians, and the Mesoamerican civilizations. It was the ancient Greeks, however, who set in place a vision of the cosmos that influenced European thinking until the Renaissance. The ancient Greek thinkers are the natural ancestors of current cosmologists, hence the attention that we give to them here. Their studies would impact human understanding of the size and nature of the cosmos for almost two millennia, and their philosophy and observational creativity remain a source of fascination. Ancient Greek tradition saw the Earth as a flat disk, with the heavens as a dome encompassing the disk. The stars were fixed to the dome. But some celestial objects were found to wander among the fixed stars: they were called the planets (the "wandering stars"). The ancient cosmological worldview was dominated by beliefs about the gods. This applied especially to the great "why?" questions. When any wise man or woman of an ancient tribe was asked a question such as "Why does the rain fall?" he or she would answer with some myth-perhaps explaining how the rain was the result of the will of the sky god and his gift to humankind. Myths were handed down from generation to generation, although there was ample freedom for the embellishment of old myths and the creation of new ones. Modern anthropologists believe that humanity began using such myths at the time of the ancient cave paintings, and that these myths were the motivation behind the development of art and much else of ancient culture. Such "why" myths have had a great influence on human development, but their fundamental weakness was that they lacked the power of prediction. A solar eclipse may be explained as the grief of the Sun god. However, this makes the phenomenon unpredictable-and remote from human experience. In 585 b.c.e. the Greek astronomer Thales of Miletus forever changed human experience by correctly predicting the date of a solar eclipse. Thus the ancient Greeks were presented with two different explanations of this event, one mythological and the other scientific. While the mythmakers had centuries of tradition on their side, only Thales had the power of prediction.
Greek astronomical research dates back well before Thales, and Thales himself must have relied on calculations produced by the Babylonians, who were gifted observers of the heavens. In fact Thales of Miletus got very lucky. If his calculations were indeed based on the Babylonian method, he would have been able to predict accurately lunar, but not solar, eclipses. He was particularly fortunate that the eclipse of 585 b.c.e. was total as viewed from the scene of a reasonably significant battle between the Lydians and the Persians. And he was lucky that the first Greek historian, Herodotus, recorded his prediction for prosperity.
The war [between the Lydians and the Persians] was equally balanced, until in the sixth year an engagement took place in which, after battle had been joined, the day suddenly turned to night. This change in the day had been foretold to the Ionians by Thales of Miletus, who had fixed as its term the very year in which it actually occurred.
Despite his good fortune, only the mean-spirited would wish to rob Thales of his scientific immortality. His dramatic prediction demonstrated the value of systematic measurement of the heavens, and it places Thales at the beginning of the story of astronomical prediction.
Another great contribution that Thales made to the history of science is that he was the first Greek philosopher that we know to have referred to the concept of "an element." Certainly he is at the start of the tradition of the search for the underlying substance of all matter, which led in time to the practice of alchemy in the Middle Ages- and would lead to the production of the periodic table when the falsehoods of alchemy were eventually replaced by the scientific method. Thales' imagination led him to propose that the underlying substance of the whole cosmos was water. He believed that the Earth had been produced by the condensation of water and the air had been produced from water by rarefaction.
Thales was a citizen of the city of Miletus, and one estimate of his life is 624 -547 b.c.e. In fact, the first three famous Greek philosophers, Thales, Anaximander, and Anaximenes, all came from Miletus, and the location and circumstances of the city were very significant to the birth of some of the fundamental concepts of science.
Miletus was in modern-day Turkey, on the coast of the Aegean Sea. Not far north was Ephesus, home of the Temple of Artemis, one of the seven wonders of the ancient world. Within a few miles to the south was Didyma, the site of one of the major oracles of the Greek world, and farther south was Halicarnassus, the home city of Herodotus, the first historian. The area was a crucible of philosophical reasoning.
What was particularly important about Miletus, from the point of view of our story, is that the city was open to ideas from the East, and in particular from Babylon. Although Babylon has been remembered in folklore for its immorality (based on its widespread ritual prostitution in honor of the goddess Ishtar), it should perhaps be more charitably remembered for its contributions to the origins of astronomy. Hundreds of years before Thales, the Babylonians had plotted the background stars to the setting Sun. Clearly identifiable groups of stars were known as constellations, and myths and legends were assigned to them. The constellations through which the path of the Sun passed were noted. This path defined what was called the zodiac, which the Babylonians divided into twelve "signs" of 30 degrees each. The signs of the zodiac took on particular significance in prognostication. It supposedly made a difference to the ancient diviners which sign of the zodiac one was born under (an ancient myth that is fed to a gullible public to this day by the tabloid press). The Roman historian Pliny records that Cleostratus of Tenedos, a Greek, recognized the signs of the zodiac.
Modern scholars have placed Cleostratus to the second half of the sixth century b.c.e. and have speculated that he learned from the Babylonians about the signs of the zodiac and about some of the constellations the Babylonians had defined by star patterns. The Babylonians had also discovered the lunar cycle of 223 lunar months, which Thales must have learned about and used as the basis of his eclipse prediction.
The Babylonians might have been able to describe "what" and predict "when." But Thales' achievement, with his fellow Greek philosophers, was to ask the question "why?" It was the transition in reason from "what" and "when" to "why" that set the Greek cultures above all others-even before their contributions to art, music, literature, democracy, and architecture are acknowledged.
Although we have no evidence for Thales' view on the size of the cosmos, we do know that he applied his view about what the cosmos was made of to its structure. In On the Heavens, Aristotle attributes to Thales the view that the Earth .oats on water like a log in a stream. Simplicius, a much later commentator on Aristotle (who, writing in the sixth century c.e., is one of the most important sources of information on the early Greek cosmologists), suggested that Thales had derived his cosmological beliefs from knowledge of ancient Egyptian mythology. This has received some support from modern scholars of Egyptian beliefs. Certainly Thales' beliefs .t into the wider pattern of cosmological ideas put forward in this period by the civilizations of the Middle East, such as the Hebrews and the Babylonians. However, Thales inspired successors, who continued to speculate about the visible world based on their own observations. It was his belief in the possibility of rational explanation of complex visible phenomena that makes Thales such a giant in the history of natural science.
Of all the ancient natural philosophers, Anaximander is the one who most naturally falls into a chapter titled "Ingenious Visions." Anaximander also lived in Miletus, and the historical tradition records that he was a pupil of Thales. Certainly there are obvious links between the ideas attributed to both men. The dates suggested for Anaximander's life are 611-546 b.c.e. Unlike Thales, Anaximander is recorded as having written a book, titled On Nature.
Book writing was rare even by the time of Aristotle and Theophrastus in the fourth century b.c.e. Tragically Anaximander's work, although referenced much later, was subsequently lost. Anaximander appears to have written many cosmological theories in his book On Nature, but we have to rely on secondary sources, such as the Christian apologist and historian of philosophy Hippolytus (180 -235 c.e.), to reconstruct them.