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Early cosmology

  In their many travels the early Greeks came into contact with older civilizations and learned their mathematics and cosmologies. Early sailors relied heavily on the celestial bodies for guidance and the observation that the heavens presented very clear regularities gave birth to the concept that these regularities resulted, not from the whims of the gods, but from physical laws. Similar conclusions must have been drawn from the regular change of the seasons. This realization was not sudden, but required a lapse of many centuries, yet its importance cannot be underestimated for it is the birth of modern science.

The earliest of the Greek cosmologies were intimately related to mythology: earth was surrounded by air above, water around and Hades below; ether surrounded the earth-water-Hades set (Fig. 2.6),


 

Figure 2.6: The universe according to Greek mythology.  
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This system was soon replaced by more sophisticated views on the nature of the cosmos. Two interesting examples were first the claim of Anaxagoras of Clazomenae that the Moon shines only through the light it reflects from the sun, and that that lunar eclipses are a result of the earth blocking the sunlight in its path to the moon; he also believed the Sun to be a ball of molten iron larger than the Peloponesus.

Another remarkable feat was the prediction of a solar eclipse by Thales in 585 B.C. (for which he used the data obtained by Babylonian astronomers). During this period other ideas were suggested, such as the possibility of an infinite, eternal universe (Democritus) and a spherical immovable Earth (Parmenides).




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Despite these strikingly ``modern'' views about the sun and moon, the accepted cosmologies of the time were not so advanced. For example, Thales believed that the Earth floats on water (and earthquakes were the result of waves in this cosmic ocean), and all things come to be from this cosmic ocean. In particular the stars float in the upper waters which feed these celestial fires with their ``exhalations''. The motion of the stars were assumed to be governed by (then unknown) laws which are responsible for the observed regularities.

A good example of the manner in which the Greeks drew logical conclusions from existing data is provided by the argument of Anaxagoras who pointed out that meteors, which are seen to fall from the heavens, are made of the same materials as found on Earth, and then hypothesized that the heavenly bodies were originally part of the Earth and were thrown out by the rapid rotation of the Earth; as the rapid rotation of these bodies decreases they are pulled back and fall as meteors. This conclusion is, of course, wrong, but the hypothesis proposed does demonstrate imagination as well as close adherence to the observed facts.

The early Greek cosmological theories did explain all the data available at the time (though they made no predictions). And, even with these deficiencies, this period is notable for the efforts made to understand the workings of Nature using a rational basis. This idea was later adopted by Plato and is the basis of all modern science.


There are many other early cosmologies, for example, Anaximander believed the Earth to be surrounded by a series of spheres made of mist and surrounded by a big fire; the Sun, Moon and stars are glimpses of this fire through the mist. In a different version of his cosmology he imagined the Earth to be a cylinder floating in space. In a more poetical vein, Empedocles believed the cosmos to be egg-shaped and governed by alternating reigns of love and hate.




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The early Greeks also considered the composition of things. It was during these times that it was first proposed (by Anaximines of Miletus, c. 525 B.C.) that everything was supposed to be made out of four ``elements'': earth, water, air and fire. This idea prevailed for many centuries. It was believed that earth was some sort of condensation of air, while fire was some sort of emission form air. When earth condenses out of air, fire is created in the process. Thus we have the first table of the elements (see Fig. 2.7)


 

Figure 2.7: The earliest table of the elements.  
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This, however was not universally accepted. The most notable detractor was Democritus who postulated the existence of indestructible atoms (from the Greek a-tome: that which cannot be cut) of an infinite variety of shapes and sizes. He imagined an infinite universe containing an infinite number of such atoms, in between the atoms there is an absolute void.




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next up previous contents
Next: The Pythagoreans Up: Early Greeks Previous: Mythology

Jose Wudka
9/24/1998