Get to Know Claudius Ptolemy & His Geocentric Model of the Universe

Claudius Ptolemy of Alexandria is best known for his geocentric model of the universe. His pivotal work, the Almagest influenced astronomy for almost 1500 years.

Dec 10, 2023By Mirjana Uzelac, PhD Anthorpology, MA Ethnology & Anthropology
claudius ptolemy geocentric model universe

 

Claudius Ptolemy was a scientist from Alexandria who lived in the 2nd century CE. His main contribution to astronomy was a detailed Ptolemaic model of the universe, a geocentric system that has Earth in the center and planets revolving around it. While geocentrism was the leading scientific system in Ancient Greece and Rome, Ptolemy made important improvements to the system, with detailed predictions of planetary motions. His book, the “Almagest,” was a pivotal work that set geocentrism as the leading scientific model for almost 1500 years. It was only challenged by Copernicus’ heliocentric system in the 16th century.

 

Claudius Ptolemy (c.100-170 CE)

ptolemy
Claudius Ptolemy by Justus van Gent and Pedro Berruguete, 1476. Source: Louvre, Paris

 

Claudius Ptolemy was a mathematician, astronomer, and geographer. Born in Alexandria during Roman rule, Ptolemy is best known for his work on the geocentric model of the universe.

 

During the time of his birth, Egypt was under Roman rule. There are different opinions on Ptolemy’s background: he was probably a Greek living in Alexandria, although there is a possibility that he was an Egyptian. In any case, Ptolemy’s culture and education were Hellenistic, and he wrote in Koine Greek.

 

Ptolemy’s work focused on mathematics and astronomy and also wrote works on geography and astrology. Regarding the last one, Ptolemy viewed astrology as a science, albeit an imperfect one.

 

Get the latest articles delivered to your inbox

Sign up to our Free Weekly Newsletter

In developing his ideas, Ptolemy meticulously examined the works of previous astronomers, particularly those from Babylon and Greece. Drawing upon observations and measurements spanning hundreds of years, Ptolemy developed his own improved geocentric model of the universe, with the Earth in the center. In fact, his model, was so refined that it became the most famous of geocentric models, known as Ptolemaic.

 

Claudius Ptolemy died in Alexandria around 168-170 CE, but his work continued to live on for centuries.

 

Ancient Greek Geocentrism

geocentric model aristotle
Geocentric model based on Aristotle and Ptolemy. Source: facultysites.vassar.edu/brvannor/Asia350/ptolemy.html

 

The Ptolemaic model was based on earlier Greek geocentric systems. Geocentrism was an established doctrine in Ancient Greece. It was rarely doubted or questioned. The geocentric model was supported by Plato and Aristotle, as well as the majority of scientists and philosophers throughout Ancient Greek and Hellenistic times. However, there were exceptions, notably the heliocentric model offered by Aristarchus of Samos in the 2nd century BCE.

 

Geocentrism established the Earth in the center of the universe, with the Sun, Moon, and planets orbiting around it. This concept originated in Ancient Greece during the pre-Socratic era. For example, Anaximander believed that the Earth was cylindrical, one-third as tall as it is wide, and situated at the center of the universe. The Greeks believed in the perfection of planetary motions. This was reflected in the way in which planets orbited around the Earth, in perfect circles.

 

Before Ptolemy, the dominant geocentric view was Aristotle’s model who described the celestial bodies as attached to their own transparent spheres, each revolving around the Earth. The stars, positioned on the outermost sphere, also revolved around Earth in circular orbits. This model highlighted the perfection of planetary motions while emphasizing the Earth’s central position in the universe.

 

mars retrograde
Mars, retrograde motion, 2022. Source: Wikimedia Commons

 

However, despite the intended perfection, this geocentric model had its share of issues. The Greek geocentric models of the universe failed to fully explain the retrograde motion of the planets. The retrograde motion refers to the way in which planets periodically move on the night sky. Most of the time, planets move in direct motion, as predicted by the geocentric model. However, periodically, the planets slow down their movement, stop, and theThis reversal of the direction is known as the retrograde motion. Even though the planets resume their direct movement after a while, the retrograde motion remained a mystery for the Ancient Greeks in conflict with their belief in the perfect circular motions of the celestial bodies. Ptolemy focused a lot of his work on explaining why this was happening.

 

Today, we know that the retrograde motion is just an illusion. The planets revolve around the Sun, not the Earth; their movements are observed from the Earth, which also revolves around the Sun. Retrograde motion arises due to the varying speeds at which planets orbit the Sun. It is an illusion created by the complex movements of planets and the Earth itself. In reality, the planets never change the direction of their movement.

 

However, this was impossible to explain using a standard Greek geocentric model. Ptolemy’s own model was set to explain retrograde motion and provide a more accurate prediction of celestial movements.

 

Ptolemaic Geocentric System

epicycle
Geocentric system with an epicycle and deferent. Source: sciencedemonstrations.fas.harvard.edu



Ptolemy’s main contribution to the geocentric system was the refinement of the earlier models. His fundamental assumption is that the seemingly irregular motions of celestial bodies are, in fact, combinations of regular motions. In his opinion, these could be explained, calculated, and predicted. The Ptolemaic system presented a unified model in which each celestial body was connected to its own sphere, forming a nested arrangement that extended from Earth.

 

To explain the retrograde motion, Ptolemy refined an earlier idea of epicycles. Epicycles were smaller circles that the planets made, while also revolving around the Earth on a large circular path (called deferent). The combination of movements along the epicycle and deferent created a complex arrangement of direct and retrograde motions.

 

Ptolemy was not the first to this idea. In fact, he was preceded by Apollonius of Perga in the 3rd century BCE and Hipparchus in the 2nd century BCE. However Ptolemy refined these existing ideas and supported them with extensive mathematical calculations and predictions of planetary motions. This proved to be immensely influential as Ptolemy’s system of epicycles was used to support the geocentric theory of the universe for more than 14 centuries.

 

The “Almagest”

the almagest
A Greek edition of the Almagest, 1549: Source: Galileo’s World/University of Oklahoma

 

The Almagest is Ptolemy’s astronomical manual written around 150 CE. It was originally titled Mathematike Syntaxis. The name Almagest emerged later, with Arabic scientists and translators. It is said that the word comes from an Arabic take on the Greek word for “the greatest”. The work is best known as the Almagest because of a 12th-century Latin translation Almagestum, done directly from the Arabic translations. The Almagest is a pivotal work establishing Ptolemy’s geocentric model. It was an immensely influential book that remained popular with Islamic scientists and throughout the European Middle Ages.

 

The strength of the Almagest is that it combines explanations of astronomical phenomena with predictions of planetary motions. On one hand, the geocentric system offers a robust explanation of the movements of celestial bodies. On the other, detailed charts and calculations were useful in predicting the movements of the celestial bodies. Using mathematics and astronomy, it was possible to accurately “predict” where the celestial bodies will be located in the night sky in the future.

 

The Almagest is divided into 13 books. Book 1 presents arguments for a geocentric model of the universe, drawing heavily from Aristotle’s model. Book 2 focuses on stellar cartography and the daily motions of the celestial bodies. Book 3 describes the motion of the Sun. This is also the book in which Ptolemy explains the epicycles. Books 4 and 5 focus on the complex motions of the Moon. Book 6 details the eclipses of the Sun and Moon.

 

night sky
Night Sky, 2019. Source: NASA

 

Books 7 and 8 deal with the fixed stars. Ptolemy heavily relied on the star catalog of Hipparchus, written in the 2nd century BCE for this part. Finally, books 9-11 present detailed geometric models for the motion of the five planets visible to the naked eye, along with tables predicting their positions on the night sky. Book 12 explains the retrograde motion of the planets using a geocentric model of the universe. Book 13 focuses on the motion in latitude.

 

The “Planetary Hypothesis”

Bartolomeu Velho map
Figure of the Heavenly Bodies, by Bartolomeu Velho, 1568, via Bibliothqeu Nationale de France, Paris

 

The Almagest played a pivotal role in establishing the prevailing geocentric cosmology. Ptolemy’s later work, Planetary Hypothesis, expanded this model even further explaining the laws of planetary motions and offering a more concrete physical model of geocentrism. In this work, Ptolemy envisioned the universe as a system of nested spheres with the Earth in the center. The smallest sphere around the Earth is the one carrying the Moon. The next largest is Mercury’s, followed by the sphere of Venus. The Sun’s sphere follows, and then the spheres of Mars, Jupiter, and Saturn. The final, largest sphere is the sphere of the fixed stars.

 

In his books, Ptolemy not only presented his geocentric model but also provided mathematical calculations and detailed charts of planetary motions. The accuracy of the model solidified Ptolemy’s work and the geocentric system for almost 1500 years.

 

The Legacy of the Ptolemaic System

geocentric legacy
Ptolemaic system in the star atlas Harmonia Macrocosmica by Andreas Cellarius, 1660. Source: Encyclopedia Britannica

 

Ptolemy’s geocentric planetary models, like those of his predecessors, were widely accepted until the invention of heliocentric models during the scientific revolution. Initially, the predictions from Ptolemy’s work were very accurate, which contributed to its popularity. The Almagest served as the fundamental guide for Islamic and European astronomers until the early 17th century.

 

However, the errors in predictions of planetary movements accumulated over time. By the 13th century, the model’s predictions deviated significantly from the observed situation in the night sky. This opened up numerous issues for scientists. Astronomers had to introduce increasingly intricate adjustments to obtain accurate results, including the addition of smaller epicycles upon larger epicycles. The system became more and more complex, but it could not eliminate errors or provide adequate explanations.

 

These issues with geocentric models eventually led to the development of a different system: the heliocentric. Nicolaus Copernicus’ work, On the Revolutions of the Heavenly Spheres, published in 1543, marks the beginning of the scientific shift towards heliocentrism, with the Sun in the center and Earth revolving around it.

 

Eventually, the Ptolemaic geocentric model was proven incorrect and became dated. However, it is important to note the model’s long-lasting legacy. Also, while its assumptions were incorrect, Ptolemy’s model included important calculations and predictions of the planetary motions, many of which were used in astronomy for centuries. Even though it was eventually abandoned, the Ptolemaic model of the universe marks an important episode in the history of science.

Author Image

By Mirjana UzelacPhD Anthorpology, MA Ethnology & AnthropologyMirjana is a socio-cultural anthropologist and archaeologist with a keen interest in history of science, gender, and constructions of legacies. She has a PhD in anthropology from the University of Alberta, a MA in ethnology and anthropology, and a BA in archaeology from the University of Belgrade.