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# 2.19 Aristotle and Geocentric Cosmology

The earliest Greek thinkers developed the tools of geometry, allowing them to distinguish between apparent size and true size. These tools were used to determine the Earth’s place in the universe. Aristotle (384-322 B.C.) was the most famous and influential Greek philosopher. He founded a school at Lyceum, near Athens, with a library, zoo, and lavish research equipment bought by his one-time pupil, Alexander the Great, who ruled Greece and conquered much of the Mediterranean world. Aristotle applied his prodigious brain to many subjects. He developed the rules of logic that are the basis of the scientific method. He wrote books on botany, anatomy, economics, politics, and meteorology. He is also responsible for a cosmological model that lasted for 2000 years, even though it proved to be wrong!

Aristotle and his colleagues made few new observations. In fact they were painfully aware of the limitations of the human senses. They believed that the universe could be understood by the power of reason alone. This is truly audacious. We can see how far it is from the old idea that humans are subject to capricious and supernatural forces. The goal of these philosophers was to unify the diversity of the natural world with a few elegant ideas.

For example, Democritus imagined the process of dividing a grain of sand in half again and again. He supposed that this could not go on forever and that there must be a must be a tiny indivisible unit of matter, much smaller than the eye could see. This is the idea of an atom. Empedocles imagined that nature was composed of only four primordial substances: earth, water, fire, and air. He proposed that all the material in the world is made of combinations of these four substances. This is the idea of elements. The Greeks did not have the tools to understand the microscopic world of the atom, but in their applications of logic and their search for simplicity they came up with some strikingly modern ideas.

The? Ptolemaic geocentric model?of the?Universe?according to the Portuguese cosmographer and cartographer?Bartolomeu Velho?(Biblioth?que Nationale de France, Paris). Click here for original source URL.

Aristotle's cosmology had several essential features. The Earth was a sphere. Aristotle followed Pythagoras in believing that a sphere was the most perfect shape. He was also aware of the powerful evidence provided by the shape of the Earth’s shadow during a lunar eclipse. The Earth was stationary. To Aristotle this was just common sense, since we do not feel any motion of the Earth and objects fall straight down when dropped. We call this a geocentric cosmology or Earth-centered cosmology, where all the other celestial bodies travel around the Earth in circular orbits. Aristotle borrowed from the idea of crystalline spheres from Eudoxus. The Sun, the Moon and each of the planets have a crystalline sphere, nested like a set of Russian dolls. The outermost sphere carried all the stars.

Geocentric model and Heliocentrism. Click here for original source URL.

An alternative view came from Aristarchus (310-250 B.C.), who lived on the island of Samos off the coast of present day Turkey. Living in the time just after Aristotle, he boldly proposed that the Earth and the planets orbited the Sun. This is a heliocentric cosmology. Few of his writings survive, but we have descriptions from other Greek authors. Think of the irony that Aristarchus was measuring the relative distances of a spherical Earth, Moon, and Sun, yet 1000 years later, many Dark Age citizens of Europe still thought that the Earth was flat! This raises an important point. Why, in the history of science, do the correct ideas not always prevail? Usually it has to do with a lack of compelling evidence. Aristarchus’ followers couldn’t prove that his hypothesis of an orbiting Earth was correct. Aristotle's followers couldn’t prove that the Earth stood still. Aristotle argued that if Earth was really rushing though space, we should be able to detect its motion. This was considered a strong argument.

We can recognize a powerful psychological reason for favoring a geocentric cosmology. Humans are intelligent enough to consider our place in the universe. The Greek philosophers were convinced that humans were the pinnacle of creation and therefore must be at the center of the universe. It was unthinkable for them to relegate the Earth to just another object flying through space. There is natural resistance to displacing the Earth in its importance in the scheme of things. The discovery of the Earth's true place in a vast universe is a story that has unfolded throughout the history of astronomy.