Cartographer of the Heavens

By Matt McKeown

The Ptolemaic system has become a byword for backwardness, yet in truth, Ptolemy is arguably the father of science.

Still a respected field, astronomy today is simply one of a wide variety of sciences; historically, it was far more illustrious, enjoying the status of one of the seven liberal arts. These subjects (the other six being grammar, logic, rhetoric, arithmetic, geometry, and music) formed the backbone of all western education right down to the end of the eighteenth century; and until the Copernican Revolution, one ancient man was held to be he greatest master of the knowledge of the heavens. That man was Claudius Ptolemy.

His personal history is somewhat shadowy. He lived in Alexandria in the second century, which at that time was one of the most respected intellectual centers of the Roman Empire. Whether he was of Greek or Egyptian stock is not clear; judging from his forename, he was likely a Roman citizen, a privilege that he probably inherited from an ancestor who may have received it from the Emperor Claudius. Beyond this, direct information about his life is extremely murky—yet he was one of the only scholars whose work never ceased to be copied and commented upon (“never out of print,” so to speak), even during the chaos of the barbarian invasions and the collapse of the Empire in western Europe.

Ptolemy’s writings were not limited to astronomy. His scholarly interests were broad: more minor works on optics, philosophy, and music survive, and just as he made star-charts, he also wrote a good deal on cartography, including detailed instructions for projecting the earth’s curved surface onto a two-dimensional map (a challenge that had been recognized by map-makers for centuries).

We consider it a good principle to explain the phenomena by the simplest hypothesis possible.

The accepted system of astronomy at the time was geocentric, with the sun, moon, and planets orbiting the earth. (A heliocentric system had been proposed as early as the Pythagoreans; however, it had been generally rejected on the grounds that no one had observed any sign of stellar parallax, which remained undetectable until advances in telescope technology in the nineteenth century.) Since, however, the planets all exhibit apparent retrograde motion—a phenomenon in which they seem to travel in the reverse of their normal movement in the night sky—a series of sub-orbits called epicycles was devised, a little like a spirograph pattern. The mathematics required to describe this system was therefore highly complex, and indeed, though we know Ptolemy’s principal work under the Arabized title of Almagest, its original Greek title was the Mathēmatikē Syntaxis, the “mathematical treatise.”

All this work on astronomy was vital, for two reasons. One was that astrology was widely accepted (to varying degrees—Epicureans and, later, Christians allowed stellar influence on man but insisted on the fundamental freedom of the will), so that predicting the influence of the planets on daily affairs was as prudent then as checking the weather forecast is today. The other, which is still true now though we rarely think about it, is that the stars are ultimately what we use to tell time. Days and years are determined by the sun, i.e. by the earth’s rotation and orbit around it; months on the Jewish and Islamic calendars still follow the course of the moon, and, though ours have come unmoored from it, the fossilized name of what were once “moonths” survives.

But perhaps Ptolemy’s greatest claim to fame are the principles he bequeathed to all later astronomers and to scientists in general. One is that the scientist’s business is to construct a theory which will “save the appearances” (a phrase borrowed from Aristotle), or as we would put it today, one that explains the data. The other is often called Ockham’s razor: of the possible theories that explain a given set of data, since there are usually many, the simplest one is to be preferred. All science, right down to our own day, operates on exactly these principles.


Every week, we publish a profile of one of the figures from the CLT author bank. For an introduction to classic authors, see our guest post from Keith Nix, founder of the Veritas School in Richmond, VA.

If you liked this post, take a look at some of our other author profiles, like these ones of St. Augustine, Fyodor Dostoevsky, and John Steinbeck; you might also enjoy our series on “the Great Conversation,” on topics from emotion to life and death to prophecy. And be sure to check out our weekly podcast, Anchored.

Note: This author was included in a previous version of the Author Bank, but is not present on the current edition (though passages from his work may still appear on CLT exams). A discussion of the latest revisions to the Bank, courtesy of Dr. Angel Adams Parham, can be found here.

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