Ptolemy (astronomer)

Working in Roman Alexandria in the second century AD, the scholar Claudius Ptolemy wrote the books that defined astronomy, map-making and astrology for well over a millennium. His Almagest set out a geocentric model of the heavens that remained the standard text in Europe, the Middle East and North Africa until the Scientific Revolution, and his Geography taught later ages how to place the known world on a grid.

Almost nothing is recorded about Ptolemy as a person. His birth year and birthplace are unknown; a much later Byzantine writer claimed he came from Ptolemais Hermiou in Upper Egypt, but there is no early evidence for it. He lived and worked in or near Alexandria, in the Roman province of Egypt, and wrote in Greek. His Latin name, Claudius, suggests he held Roman citizenship, perhaps through an ancestor enfranchised under Claudius or Nero. He was not related to the Ptolemaic kings, despite medieval Arabic writers who confused him with them. Scholars place his birth around AD 100 and his death in the 160s or 170s, inferring the date from the range of his writings. Around 146–147 he set up an inscription at Canopus listing the parameters of his astronomical system.

Ptolemy's great astronomical work, originally called the Mathematical Treatise, is the only comprehensive astronomical treatise to survive from antiquity. Building on Hipparchus, he constructed geometric models for the Sun, the Moon and the planets that could actually be used to calculate their positions, and supplied tables for doing so. The book also contained a star catalogue of forty-eight constellations, the ancestor of the modern list. Its familiar title comes from an Arabic rendering of a Greek name meaning "the greatest". The work was translated into Latin twice in the 12th century, in Sicily and in Spain. He also produced the Handy Tables, the model for later Arabic and Latin astronomical tables, and the Planetary Hypotheses, which pictured the cosmos as nested physical spheres.

Modern checks have found systematic errors in observations Ptolemy said he made himself. In 1977 Robert R. Newton accused him of fabricating data, but other historians, among them Owen Gingerich and Bernard Goldstein, rejected the charge of fraud while accepting that some figures are suspect. Fragments of Hipparchus's lost star catalogue identified in 2022 indicate that Ptolemy combined several sources, including his own observations, rather than simply copying his predecessor.

The Geography was a handbook for drawing maps from coordinates. Drawing heavily on the earlier geographer Marinus of Tyre, Ptolemy listed some 8,000 places across the inhabited world, which he knew to be only about a quarter of the globe, from the Atlantic islands to China. The surviving maps date from around 1300, after the Byzantine scholar Maximus Planudes rediscovered the text, and the place lists were probably revised over the centuries after Ptolemy.

Ptolemy treated astrology as the companion to astronomy. His Tetrabiblos tried to explain celestial influences in terms of the natural philosophy of the day and became a standard reference for astrologers; a collection of aphorisms long credited to him, the Centiloquium, is now thought to be a much later work. His Harmonics used experiments with a single-stringed instrument to test musical ratios, and later inspired Johannes Kepler. His Optics, surviving only in Latin, discussed vision, reflection and refraction and influenced the 11th-century optics of Ibn al-Haytham.

Our 100 and 200 maps show the Roman Empire in Ptolemy's lifetime, with Egypt and Alexandria as one of its richest provinces.