Ancient Greek Thinking on Astronomy Aristotle’s geocentric model of the universe. 1

Greek Astronomy - Greek astronomy came from a new approach. Greeks moved away from religion and astrology to use reason and logic. - This was a first step towards studying the “heavens” using modern science. - Made possible by two early Greek philosophers. Thales of Miletus ( BC) Taught that the universe was rational and understandable by the human mind. 2

Pythagoras BC He and his students noticed that nature seemed geometric and mathematical. Other Greek philosophers used geometry to explain universe. Plato said that everything was made of perfect spheres rotating at constant rates carrying objects in circles. Plato BC 3

Plato’s ideas would influence astronomy for over 2000 years. His ideas would become known as the principle of uniform circular motion. Eudoxus ( BC ), a student of Plato, devised a system of 27 nested spheres rotating at different rates to explain motions of the universe. Note: the word university is derived from Greek. 4

Aristotle and the Nature of Earth Aristotle was one of the most respected philosophers of his time. he became the greatest authority of antiquity. His astronomical model was accepted for over 2000 years! Although most of what he described was wrong, he attempted to use logic and base his ideas on first principles. 5

First Principles A first principle is something held true, whether it is true or not. – It is an assumption that is treated as fact whether it is or not. “Common sense” as we use it is a first principles. – Since a first principle is “foundational,” it cannot be deduced from any other assumption and is the “point of departure for a logical argument.” 6

First Principles Aristotle’s first principles were: 1.that Earth is the center of the universe. 2.that the heavens rotate in perfect spheres around the Earth – uniform circular motion. 3.that objects in the heavens are made of perfect materials and their intrinsic properties (like brightness) cannot change. 7

First Principles Can you come up with other examples of a first principle? – Legal cases are based on the concept of first principle – based on an initial assumption (which may or may not be supported), which is then the foundation of a logical argument. – Prejudices. – Religious dogma – God created heaven and Earth – and they were perfect. – Patriotism can function as a first principle. 8

First Principles Compare first principles with the scientific method. – How do they compare to one another? What are the differences? 9

Aristotle’s Geocentric Universe His model was of a universe composed of 55 perfect spheres rotating at different rates carrying the sun, moon, and planets across the sky. The spheres revolved around a motionless Earth every 24 hours to produce day and night. The spheres moved at different rates with respect to one another, to produce the motions of the sun, moon, and planets against the background stars. 10

Deferents and Epicycles In order to maintain Earth as the center of the universe, early astronomers had to invent a very complex model to explain the movement of the planets. Epicycles were needed in order to account for the apparent “retrograde (backwards movement)” orbits of the planets around Earth. 11

Deferents and Epicycles Observations did not fit the model so epicycles were created to make the model viable. In modern science, models are based on evidence – they do not replace evidence. Orbit of Saturn (short video) – what they observed. – /unit2_sub1.htm /unit2_sub1.htm 12

Model of Universe with Epicycles 13

Ptolemy Was largely responsible for the enduring acceptance of Aristotle’s universe. – Transformed it into a sophisticated mathematical model. – Was the inventor of the epicycle. – His 13 tomb master work Almagest was saved by Arabic astronomers. – The universe described by Almagest, based on Aristotle, was considered literally considered sacred. – The coming of the enlightenment and science (Copernicus, Kepler, Galileo, and Newton) would quickly change everything. 14

The first Heliocentric Model? About 100 years later a philosopher named Aristarchus devised a model in which Earth rotated on its axis and revolved around the sun. Aristarchus is not credited with the heliocentric model of the universe because the majority of his writings were lost. – Astronomers rejected his ideas due to their “violation” of Aristotle’s first principles. 15

A couple of questions Who is credited with developing the heliocentric model of the universe? Who was the astronomer who killed the idea of epicycles? How did he explain the seemingly backward movements of planets? 16

Eratosthenes used shadows and simple geometry to calculate, with amazing accuracy, the circumference of the earth (within 2% of the true circumference) At the time of Columbus most people knew the Earth was round. They just weren’t sure how large it was. He used Aristotle’s estimate; otherwise he would have never attempted the voyage. 17