Ptolemy

(c. 2nd century ad) Egyptian astronomer

Virtually nothing is known about the life of Ptolemy (full name Claudius Ptolemaeus). He was probably a Hellenized Egyptian working in the library at Alexandria. He produced four major works the Almagest, the Geography, the Tetrabiblos (Four Books), and the Optics. The first work – the culmination of five hundred years of Greek astronomical and cosmological thinking – was to dominate science for 13 centuries. Ptolemy naturally relied on his predecessors, especially Hipparchus. A work of such staggering intellectual power and complexity could never be created by one person alone. The basic problem he faced was to try to explain the movements of the heavens on the assumption that the universe is geocentric and all bodies revolve in perfectly circular orbits moving with uniform velocity. As the heavenly bodies move in elliptical orbits with variable velocity around a center other than the Earth, some quite sophisticated geometry is called for to preserve the basic fiction. Ptolemy used three complications of the original scheme: epicycles, eccentrics, and equants. These devices worked reasonably well except that they did not lead to particularly accurate predictions. Nor did they permit Ptolemy to develop a system of the universe as a whole. He could give a reasonable account of the orbit of Mars, and of Venus, and of Mercury, and so on, taken separately, but if they were put together into one scheme then the dimensions and the periods would start to conflict. Whatever its faults the system remained intact for 1300 years until it was overthrown by Copernicus in the 15th century.

In the Geography Ptolemy explains fully how lines of latitude and longitude can be mathematically determined. However no longitudes were astronomically determined and only a few latitudes had been so calculated. Positions of places were located on this dubious grid by reducing distances measured on land to degrees. Distances over seas were simply guessed at. As he had put the Canaries 7° east of their true position his whole grid was thrown out of alignment. The Geography had almost as great (and as enduring) an influence on the western world-view as theAlmagest. Columbus might never have sailed without Ptolemy's erroneous view that Asia was closer (westward) than it really is, a view endorsed by the map-makers contemporaneous with Columbus.

The only book of Ptolemy's that is readily available today and still widely read is the Tetrabiblos, which is a work on astrology. The work is long and comprehensive and is probably as well argued as the case for astrology can be. It is naturalistic in that he supposes that there might be some form of physical radiation from the heavens that affects mankind. Most of the concepts and arguments of modern astrology can be traced back to this Ptolemaic work.

The final major work of Ptolemy, the Optics, in which he sets out and demonstrates various elementary principles, is in many ways the most successful of all his works. Although he understood the principles of reflection reasonably well his understanding of refraction seems to be purely empirical. He gives tables he has worked out for the refraction of a ray of light passing from light into water for various angles of incidence.

His main work was known in Greek as the Syntaxis; it was the Arabs who named it the Almagest from the Arabic definite article ‘al’ and their own pronunciation of the Greek word for ‘great.’ Such was the tribute posterity has paid to Ptolemy.