Ptolemy and the Ptolemaic System

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  • 0:04 Ptolemy
  • 0:34 Ptolemy's Early Life
  • 1:57 Ptolemaic System
  • 4:10 Impact
  • 5:10 Lesson Summary
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Lesson Transcript
Instructor: Christopher Sailus

Chris has an M.A. in history and taught university and high school history.

In this lesson, we explore the work of the ancient Greco-Egyptian philosopher, mathematician, and astronomer, Ptolemy, whose system explaining the structure of the universe was believed for centuries.


Many of us spend most of our lives trying to make sure we leave a mark on this world. Whether it's through funding a building with our name on it or simply by raising healthy, productive children, most of us seek to leave something good behind in this world after it's our time to go. If this is what drove the Greek mathematician and astronomer Claudius Ptolemy, he certainly succeeded; for over a millennium after his death, his theories concerning matter, the solar system, and the universe defined how man viewed the structure of the universe.

Ptolemy's Early Life

We know very little about the life of Ptolemy. He was likely born around 100 A.D. and died around 170 A.D. His name suggests he was of Greek descent, though his family likely held citizenship in the Roman Empire. Despite these roots, he was born, lived, and died in the vicinity of Alexandria in Egypt.

Ptolemy worked feverishly throughout his life, observing the stars and making careful notations. He built upon the work of others, like Hipparchus and Aristotle, and he likely was taught by the great Theon of Smyrna, an accomplished mathematician and astronomer in his own right. Though Ptolemy's astronomical work was his most important as far as we are concerned, Ptolemy also wrote on various topics, including geography, optics, and music.

What sets Ptolemy apart from many of his contemporaries is the book he wrote, likely completed around 150 A.D. The book, originally titled The Mathematical Compilation, is more commonly known by its Latinized Arab title: The Almagest. In The Almagest, Ptolemy compiled a list of all the known stars in the night sky and proposed a fundamental structure to explain the motion of the stars and the heavenly bodies. This structure, known as the Ptolemaic system, remained the prevailing theory concerning the universe among many intellectuals and astronomers for over a thousand years until the late Middle Ages.

Ptolemaic System

According to the Ptolemaic system, the universe was necessarily geocentric. This was due in large part to the Greek theory that explained what we know today as gravity. According to the Greeks, all matter naturally fell to the center of the universe. As birds, arrows, and people all fall to the Earth if dropped from the sky, Earth must necessarily be at the center of the universe.

But what then, you might ask, keeps the moon, the sun, and the stars from crashing down from the sky? According to the Ptolemaic system, the heavenly bodies remained suspended in the sky as they were attached to crystalline spheres. Ptolemy built upon the theories first proposed by his predecessor Aristotle, who claimed that the heavens contained 55 of these crystalline, translucent spheres, with each larger one encompassing all the others.

Upon these concentric spheres, the planets, the moon, and the sun were all attached. Upon the largest and furthest away sphere were pasted the faintest stars. These spheres all rotated around the Earth at separate speeds. The furthest of these spheres was imparted with a speed by a being Aristotle called the 'Prime Mover.' All of the other spheres rotated due to the angular momentum produced by the force first imparted on this last sphere.

The Aristotelian theory explained some of the motion of the stars but not all, and it was often incorrect. For example, it often failed to account for retrograde motion, when some planets and stars appeared to move backward in the night sky after a long period of moving in one direction. To tackle this problem, Ptolemy invented epicycles and deferent points.

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