Claudius Ptolemy

AD100 - AD170 (aged 69-70)
Controversial astronomer who reinforced the idea that Earth was at the centre of the universe
Representation of Claudius Ptolemy with a rule and stars

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Claudius Ptolemy was a Greek mathematician, astronomer and geographer who lived in the Egyptian city of Alexandria while under the rule of the Roman Empire. Much of medieval astronomy and geography were built on his ideas: his world map, published as part of his treatise Geography in the 2nd century, was the first to use longitudinal and latitudinal lines. This idea of a global coordinates system was highly influential, and we use a similar system today.

However, he is most known for refining the cycles and epicycles that made the geocentric theory of the universe tenable for 14 centuries, as established in his book The Almagest on the motions of the stars and planets. This is the theory which Galileo Galilei and Isaac Newton eventually overthrew more than a thousand years later. For this reason Ptolemy is a controversial figure in the history of science. Robert Newton argues in his book The Crime of Claudius Ptolemy, that despite his skill as an astronomer, Ptolemy was simply an astronomical fraud.

In Ptolemy’s great 13 volume work, The Syntaxis, (otherwise known as The Almagest), Ptolemy made observations about the planets that are surprising. Compared with modern astronomical tables, some are so accurate that, Newton claims, he simply couldn’t have made them with the instruments he describes. Others are extraordinarily error prone: Ptolemy’s sighting of the autumn equinox at 2PM on 25 September AD132 — a measurement that he said he made “with greatest care” — was wrong by more than a day.

Newton says that Ptolemy simply fitted his measurements to his theories, rather than vice versa, often adapting observations made centuries before his time. “We can say that all of Ptolemy’s observations that can be tested are fabricated,” says Newton. “Further, we can say that all of his theories depend heavily on fabricated data, and some of them seem to depend completely upon such data.


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“Overall, The Syntaxis has done more damage to astronomy than any other work ever written, and astronomy would be better off if it had never existed … Ptolemy is not the greatest astronomer of history, but he is something still more unusual: he is the most successful fraud in the history of science.”

That assessment hardly endeared Newton to other historians of ancient science. One refutal by Owen Gingrich ruefully admits that The Syntaxis contains some fishy data, but he suggest that Ptolemy merely followed the practice of his time by selecting only those observations that supported his theory.

“Certainly,” Gingerich says, “Ptolemy did not commit fraud. And his great contribution of the geocentric theory,” he adds, “was the true testament to Ptolemy’s greatness as an astronomer.” More recent studies of his work suggest that a “combination of observational, calculation and rounding errors, plagiarism of existing data and selection of the best examples… could account for the appearance of fraud. But, in the absence of categorical evidence, the verdict must remain ‘not proven.’”

Key facts

Full name: Claudius Ptolemy

Born: 100AD, Egypt, Roman Empire

Died: 170 AD, Alexandria, Egypt, Roman Empire

Claudius Ptolemy was a 2nd century Greek mathematician, astronomer and geographer famous for his controversial geocentric theory of the universe, which would form the basis of our understanding of the motions of stars and planets for over than a thousand years.


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