Cosmology

Chapter 2

Greek cosmology
The first “cosmologies” were based on creation myths in which one or more deities made the universe out of sheer will, or out of their bodily fluids, or of the carcass of some god they defeated, etc. A few examples of such “theories” of the universe are provided in this chapter. These are hardly scientific theories in the sense that they have almost no support formobservation and in that they predict very few things outside of the fact that there is a world (if everything is due to the whims of the Gods then there is very little one can predict). It is an interesting comment on the workings of the human mind that quite different cultures produced similar creation myths. The first scientific cosmology was created by the Greeks more than 2000 years ago, and thischapter also describes these ideas and their origin. The Greeks used some of the knowledge accumulated by earlier civilizations, thus this chapter begins with a brief description of the achievements of the Egyptians and Babylonians. We then consider the highlights of Greek cosmology culminating with Ptolemy’s system of the world.

2.1
2.1.1

Egypt and Babylon
Babylon

The Babylonians lived inMesopotamia, a fertile plain between the Tigris and Euphrates rivers (see Fig. 2.1). They developed an abstract form of writing based on cuneiform (wedge-shaped) symbols. Their symbols were written on wet clay tablets which were baked in the sun; many thousands of these tablets have survived to this day; an example is shown in Fig. 2.1. 1

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Figure 2.1: Left: Region dominated by theBabylonian civilization. Right: example of a cuneiform tablet containing Pythagorean triples. The Babylonian apparently believed the Earth to be a big circular plane surrounded by a river beyond which lies an impassable mountain barrier, with the whole thing resting on a cosmic sea. No human may cross the river surrounding the Earth. The mountains support the vault of heaven, which is made of a verystrong metal. There is a tunnel in the northern mountains that opens to the outer space and which also connects two doors, one in the East and one in the West. The sun comes out through the eastern door, travels below the metallic heavens and then exits through the western door; he spends the nights in the tunnel. The creation myth is more lively than the Egyptian version. It imagines that the cosmicocean Apsu mixed with chaos Tiamat and eventually generated life. For a while life was good for the gods but there came a time when Tiamat felt her domain was too small and made war against the other gods. All but Marduk were afraid of her, so Marduk, after getting all the powers from the frightened gods, fought Tiamat. When Tiamat opened her mouth to swallow him he thrust a bag full withhurricane winds into her so that she swelled and, taking advantage of her indisposition, Marduk pierced her with his lance and killed her. Then he split Tiamat’s carcass making the lower half the earth and the upper the heavens. Finally Marduk mixed his bloodown blood with the earth to make men for the service of the gods. Babylonians and Chaldeans observed the motion of the stars and planets from theearliest antiquity (since the middle of the 23rd century B.C.). They cataloged the motion of the stars and planets as well as the occurrence of eclipses and attempted to fit their behavior to some numerical theories. Many of these observations were used for astrological prophesying and, in fact, they were the originators of astrology. They believed that the motions

3 and changes in the stars andplanets determine (or so they believed) what occurs on this planet. The Babylonians excelled in computational mathematics, they were able to solve algebraic equations of the first degree, understood the concept of function and realized the truth of Pythagoras’ theorem (without furnishing an abstract proof). One of the clay tablets dated from between 1900 and 1600 B.C. contains answers to a problem...