Claudius Ptolemais (Ptolemy) Nicholaus Copernicus Johannes Kepler
Tycho Brahe Galileo Galilei Isaac Newton

Unit 3: Revolutions of the Heavenly Orbs

During the the second century A.D. the Alexandrine astronomer Claudius Ptolemais elaborated a system of the heavens that adequately described all of the observed motions of the Sun, Moon, stars, and planets. The Earth sat unmoving at the center of an unchanging universe while the "heavens wheeled above". Based on Aristotelian ideals little changed since the days of classical Greece, Ptolemy's version of the Geocentric Model, despite its considerable complexity, was to be the accepted explanation for the motions of the heavens for the next 14 centuries.

During the great social upheaval that followed the breakup of the late Roman Empire, much of this knowledge was lost to Europe for centuries. Beginning around the 10th century A.D., the works of Aristotle, Ptolemy, and others began to slowly return to Europe via a tortuous path that led from the outposts of the late Roman Empire, through the Arabs who, after the rise of Islam, nurtured these works and added their own stamp to them, and finally through the hands of Jewish scholars working between the Christian and Islamic worlds who translated them into Latin. In rediscovering their lost "golden past", European scholars of the Age of Faith embraced the explanations of the heavens offered by Aristotle and others and incorporated them into their thinking, reconciling the logic of Aristotle with the faith of Christ. The world of Augustine gave way to the world of Aquinas and Dante. But while they revered the old knowledge, only very rarely did they do more than elaborate upon it. To minds accustomed to viewing knowledge as revealed and received, these works were not a starting point for new explorations but an end in themselves. They, too, found the Aristotelian system of Ptolemy adequate to explain the appearances of the motions in the sky.

By the beginning of the 16th century this received view of the world began to change irrevocably. The calendar of Julius Caesar no longer kept time against the seasons. The role of the Catholic Church of Rome in the social and political order of Europe was being openly questioned by Luther, Calvin, and others. And, in 1493, a Genoese mariner in the employ of the crowns of Castille and Aragon named Christopher Columbus returned from a remarkable voyage across the sea to a country unknown to Aristotle and his disciples. While Columbus himself believed until his death that he had reached the outlying islands of eastern Asia, it quickly became apparent to others that he had landed somewhere else entirely. Soon the unthinkable was being openly discussed. If Aristotle and the ancient philosophers knew nothing about this "New World", what else did they not know?

This unit traces the revolution in astronomical thought that began uncertainly with Nicolaus Copernicus in the 16th century, through its development by Kepler and Galileo during the turbulent Renaissance and Reformation of the 17th century, culminating in the grand synthesis of Isaac Newton who explained the motions of the Sun, Moon, Earth, and planets with three simple mathematical Laws of Motion and his Law of Gravity. But, unlike the knowledge of Aristotle, Newton's synthesis is not an ending but a beginning. It is the start of a journey that leads from Newton to our own time.


The Harmony of the Spheres: Greek Astronomy (Oct 8)

The Revolutions of Nicolaus Copernicus (Oct 9)

The Watershed: Tycho Brahe & Johannes Kepler (Oct 10)

The Starry Messenger: Galileo Galilei & The Telescope (Oct 11)

On the Shoulders of Giants: Isaac Newton & The Laws of Motion (Oct 12)

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Updated: 2007 October 13
Copyright Richard W. Pogge, All Rights Reserved.