Early Greek astronomy

Early cosmologies were based heavily on man's earthly experiences and owed little to accurate astronomical observation. They fulfilled a well-documented psychological need, providing a stage for the drama of daily life, for the actions of the gods and also supplying meaning to man's existence. Before the ancient Greek civilization, cosmologies were constructed that made no attempt to explain any but the most rudimentary of astronomical phenomena. For example, the Egyptians explained the motion of the Sun by having the Sun god Ra take a daily trip through the air and then each night make a passage through the water. The Moon, on the other hand, was attacked on the fifteenth day of each month by a sow and after 2 weeks of agony the Moon died and was reborn.

Gradually, as the Greek civilization progressed, cosmologies became more sophisticated, and the idea that the way to find out about the nature of the Universe is through accurate astronomical observation, now a well-established tenet of Western thought, developed. One of the earliest examples we have showing Greek interest in the heavens comes from the writings of Homer in which the Earth is a flat circular disc surrounded by the Ocean river and covered by the vault of heaven. The Homeric epics show that in the eighth century BC the Greek understanding of the natural world was very primitive, and their awareness of astronomical phenomena was considerably less than that of their Babylonian contemporaries. Further evidence of early Greek astronomical knowledge can be obtained from Hesiod's poem 'Works and Days', written about a century after Homer's time, which among other things, contains what might be described as an agricultural calendar. Hesiod described how the seasons are related to the heliacal risings and settings of certain stars and, in contrast to Homer, connected astronomical events to the lives of ordinary people.

Up until about 300 BC, Greek astronomy was almost entirely qualitative. Indeed, there is no evidence that the accurate prediction of heavenly phenomena was even thought of as a desirable goal. This situation changed when the Greeks came into contact with the quantitative methods of Babylonian astronomers during the expansion of their empire under Alexander the Great. Before then, however, celestial phenomena were the subject of a great deal of philosophical debate that provided the basis on which later astronomers could build. Four major schools of philosophical thought existed during the 300 or so years prior to the construction of the first mathematical model of the Universe by Eudoxus.13 There were the Ionians, a group founded by Thales of Miletus (in Asia Minor, modern-day Turkey) in about 600 BC. During the sixth century BC the region of Asia Minor went through a considerable upheaval due to the expansion of the Persian empire and many philosophers travelled to other parts of the Greek empire. One member of the Ionian School, Xenophanes of Colophon, migrated to southern Italy and eventually settled in Elea, which became an important philosophical centre. Perhaps the most famous of the early Greek philosophical schools was that set up by Pythagoras (who is said to have been taught by Thales), again in southern Italy, and finally there was the celebrated school centred around Plato's Academy in Athens. It should always be borne in mind

For a modern introduction to ancient Greek philosophy, see Kenny (1998).

that it is difficult to be certain about the specific theories espoused by these early thinkers, since most of our knowledge comes from remarks of later (and not always reliable) authors.

The Ionian philosophers began the process of determining the nature of things but did not progress very far towards a rational description of the Universe. We know very little about the people who made up this School and none of their writings have survived. What little we do know is due to Greek historians and commentators. As far as astronomy was concerned, the philosophers of the Ionian School thought the Earth was flat, and they had a very poor understanding of the nature of the Sun and Moon.

According to Aristotle, the founder of Ionian natural philosophy was Thales, whose ideas (cultivated during travels around Egypt, the Mediterranean and Near East) were built around the fundamental belief that water is the essence of all things. The significance of this is the suggestion that there is an underlying unity to physical phenomena and, hence, that nature is not quite as haphazard as our senses would have us believe. In terms of the structure of the Universe, however, Thales' ideas were about as primitive as those found in Homer. Later authors have ascribed knowledge of the sphericity of the Earth and of the causes of eclipses to him, though it is extremely doubtful that Thales actually possessed this knowledge. He is also credited with predicting a solar eclipse in 585 BC

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