Claudius Ptolemy wrote the Almagest, the work that defined astronomy for over 1,000 years. He was an iconic scholar at the Library of Alexandria, Egypt, living from A.D 90 – 168. He lived and worked in Alexandria.

The Almagest included a catalogue of over a thousand stars, recording their positions, constellations, and relative brightness’s; and a mathematical model predicting the movements of the planets. The predictive power of Ptolemy’s planetary model was unsurpassed for almost 1,500 years, making him a notorious figure in the world of science. Ptolemy presented his model’s output in the form of data tables. Using his tables, it became possible to predict eclipses.

To create the Almagest, Ptolemy assembled observations of the heavens spanning many hundreds of years, beginning with data compiled in Babylon in 747 BC. He used groundbreaking mathematics to analyse and interpret the data to create his model.

For the modern observer, with modern equipment, the predictions made by Ptolemy’s model are inadequate. In antiquity, however, all observations were made with the naked eye, meaning errors could be rather large. Given these circumstances, Ptolemy’s 150 AD model was rather good. It took almost 1,500 years for a clearly superior model to be found.

Some believe however that Ptolemy was a cheat, claiming he didn’t actually make any observations himself. Some feel Ptolemy’s star catalogue was actually all made by Hipparchus 300 years earlier and simply updated. The stance that was Ptolemy was a contemptible scientific cheat was championed by the physicist Robert Newton in his 1977 book The Crime of Claudius Ptolemy. Newton believed Ptolemy made up a lot of the data in the Almagest to support his mathematical model of planet movements. Other researchers believe Ptolemy used genuine observations, but used them selectively, discarding any observations that did not support his model. The case still stands though, it took over 1500 for a clearly superior model to surface.

Ptolemy also wrote major works on the earth’s geography and optics. In Geography he used unreliable data to, not surprisingly, produce rather unreliable maps of the world.

Very little is known about Claudius Ptolemy’s life other than his works. Whether he married, whether he had children, and where and when he died are unknown. It is also unclear what Ptolemy looked like, existing images of him were first created over a thousand years after he lived.