Claudius Ptolemy: The Almagest, a 30-Hour Equinox Error and Fraud Charges

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Claudius Ptolemy lived in or near Alexandria, Egypt, in roughly 100 to 170 AD, with a Roman family name pointing to citizenship and a Greek name, and he wrote in Koine Greek. A ninth-century Persian astronomer, Abu Ma'shar, claimed he was a member of the Ptolemaic royal family, a story historians reject. In Alexandria he combined Babylonian observational records with Greek geometry in the Mathematical Treatise, known by its Arabic name, the Almagest. Its Earth-centered model used epicycles to explain retrograde motion, supplied tables for predicting planetary positions and eclipses, and catalogued 48 constellations. In the Planetary Hypotheses he placed the Sun about 1,210 Earth radii away, against more than 23,000 in reality.

In 1977 the physicist Robert R. Newton published The Crime of Claudius Ptolemy, accusing him of inventing observations, while Owen Gingrich and Bernard Goldstein defended him. In 2022 multispectral imaging of a palimpsest revealed the lost star catalogue of Hipparchus, whose coordinates did not simply match Ptolemy's, weakening the plagiarism charge. Ptolemy's Geography gave about 8,000 places latitude and longitude, the Tetrabiblos treated astrology as physics, Harmonics tested musical ratios by ear on a monochord, and Optics tabulated refraction. His work survived through Byzantine and Islamic scholars, shaping Ibn al-Haytham and Johannes Kepler.

• An autumn equinox he said he observed with great care at 2 p.m. on September 25, 132 AD actually occurred 30 hours earlier.
• Defenders pointed to bronze instruments warping in the Egyptian sun, atmospheric refraction and the selective publishing of observations.
• He measured latitude by climata, the length of the longest day of the year, and longitude by comparing local times of lunar eclipses.
• He reasoned that red Mars gave off a heating, drying force that caused hot weather and anger, and he rejected numerology for lacking a physical basis.
• In 2023 a deciphered manuscript gave his instructions for building a meteoroscope.