## Competing Models of the Universe

Many ancient civilizations developed models of the universe. The Egyptians, for example, believed the universe to be like a long rectangular box, with the Earth being a slightly concave floor at the bottom of the box and the sky being a slightly arched iron ceiling from which lamps were hung. There were four mountain peaks supporting the ceiling, and they were connected by ranges of mountains, behind which flowed a great river. The Sun was a god who traveled in a boat along this river and, of course, could only be seen during the daylight hours.

The models considered by the Greeks at a much later time were a great deal more advanced than this, and reflected considerably more knowledge. They con sidered two types of models, the geocentric or Earth-centered type and the heliocentric or Sun-centered type. In the geocentric models, the Earth in addition to being at the center of the universe was usually stationary. In the heliocentric models, the Earth circled about the Sun or about a central fire, like all the other planets, and usually rotated on its own axis. By and large, the Greeks preferred the geocentric models and rejected the heliocentric models.

In one of the simplest geocentric models, the Earth is a motionless small sphere surrounded by eight other rotating concentric spheres that carry the Moon, the Sun, Venus, Mercury, Mars, Jupiter, Saturn, and the fixed stars, respectively. Because the spheres all have the same center, this is called a homocentric model. Associated with each of these eight spheres are a number of other auxiliary spheres. (This model is credited to Eudoxus, one of Plato's pupils who took up the challenge to determine the true reality.) The Moon and Sun each have two auxiliary spheres, and the planets have three auxiliary spheres each. Counting the stellar sphere, this made a total of 27 spheres.

The purpose of the auxiliary spheres is to help generate the observed motions. For example, the sphere carrying the Moon rotates about an axis whose ends are mounted on the next larger sphere, so that the axis of the rotation of the Moon's sphere can itself rotate with the auxiliary sphere. In turn, the axis of this auxiliary sphere is mounted on the next auxiliary sphere, which can also rotate. By placing these axes at various angles with each other, and adjusting the rotation speeds of the spheres, it is possible to generate a motion for a particular heavenly object such that an observer on Earth will perceive the object, against the background of the fixed stars, following the appropriate path along the ecliptic.

In the early versions of this model the question as to how these spheres obtained their motion was ignored. It was simply assumed that the very nature of a perfect sphere in the heavens is such that it would rotate. The purpose of the model was simply to explain the nonuniform motion of heavenly objects as combinations of uniform circular motion, thereby "saving the appearances." To the extent that anyone might wonder about the causes of the motion, it was said to be due to various "Intelligences." By this time the Greek philosophers had more or less put aside such mythical explanations as gods driving fiery chariots across the sky and so on.

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### Responses

• carola
What are the competing model of the universe?
1 year ago
• isaias
What are the competing models of the universe?
1 year ago
• TAPANI
What are the competing models of the universe by exodus?
1 year ago
• Christina
What world are competing model of the universe?
4 months ago