Ptolemy, Astronomy and Almagest

The Ancient Greek Ptolemaic Geocentric Astronomical System

Ptolemy’s geocentric astronomical system placed Earth at the center of the universe and he was considered the ultimate authority in astronomy for about 1400 years.


Ptolemy (aka Claudius Ptolemaeus) lived and worked in Alexandria, Egypt during the second century. We know few details of his life, but he made important lasting contributions to astronomy, geography, optics, mathematics, and music. Although he was born and lived in Egypt, Ptolemy was of Greek ancestry, therefore he is considered an ancient Greek rather than Egyptian. He lived in Alexandria, where the famous ancient library was located, so Ptolemy had access to the best library of his time.


The Almagest was Ptolemy’s major work in astronomy. It consisted of 13 books in which Ptolemy details his geocentric system of the Cosmos. For this work, Ptolemy built heavily on the foundation laid by Hipparchus, but Ptolemy also added many new and original ideas.

Books 1 and 2 of the Almagest are an introduction. Ptolemy outlines the observed motions of celestial objects and the underlying postulates of his theory. These postulates include the requirements that all heavenly bodies move in uniform circular motion and that the spherical Earth is at the center of the universe.

Ptolemy gives a catalog of 1028 stars in the seventh and eighth books. The sixth book is about eclipses and the third is about the Sun’s motions. Ptolemy deduces the ratio of the solar and lunar distances and describes the astronomical instrumentation of the time, the astrolabe, in book 5. None of these works add anything significant to the previous works by Hipparchus.

Ptolemy’s Astronomical Contributions

The remaining books of the Almagest are where Ptolemy adds significant contributions to previous works. The fourth book is about the motions of the moon. Unlike Hipparchus, Ptolemy based his theory of lunar motions on observations made outside of eclipses. Because he used more observations Ptolemy’s theory of the Moon’s motions was more accurate than Hipparchus’ theory.

Books 9 to 13 of the Almagest are Ptolemy’s greatest triumph. Ptolemy explains his theory of planetary motion, which was able to predict the positions of the planet Ptolemy’s geocentric theory was considered the final authority in astronomy for 14 centuries.

Hipparchus explained the observed planetary motions with the deferent and epicyle. Ptolemy made new observations of the planets and found that these two devices were not accurate enough. Ptolemy invented two new geometric devices, the equant and eccentric, to accurately predict the positions of the planets.

Ptolemy used these geometrical devices to calculate the positions of the planets. He however may have simply considered them as computational tools rather than real physical objects.

Ptolemy’s geocentric system is incorrect, but it survived for so long because it was good science. It predicted positions of the planets to within the accuracy of the most accurate observations available at the time. It also met the requirements of Aristotle’s physics, which we now know is incorrect but was thought correct at the time. The uniform circular motions were also aesthetically pleasing to the ancient Greeks who considered circles and spheres as perfect geometrical figures.

The Ptolemaic system met the requirements of a good scientific theory and was a crowning intellectual achievement, despite the fact that it turned out to be wrong.

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