Ptolemy (Ptolemaios in his native Greek and Claudius Ptolemaeus in Latin), was the most important author working in the mathematical and physical sciences during the Roman Empire. Among Ptolemy’s several works on astronomy, occupying a central place is the Almagest – the medieval nickname derived from the Greek megistos (“greatest”) by way of Arabic and Latin; Ptolemy entitled it Mathematical Composition (Suntaxis Mathematike). In it Ptolemy attempts to use mathematics to establish models for the motions of the Sun, Moon, planets, and stars.

The translation of Ptolemy’s Almagest from Arabic into Latin made in 1175 in Toledo, Spain, was the greatest astronomical event in Western Christendom and was first published in Venice in 1515. A second translation directly from Greek into Latin was made by the Greek scholar George of Trebizond in Rome in 1451 and first published in Venice in 1528 by Luca Gaurico (1476-1558). The Greek original text was published ten years later in Basel. Copernicus, who was both a scientist and a humanist, read all three editions as a basis for the scientific revision of the Ptolemaic model carried over in his masterpiece De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres).

The present image illustrates book 5 chapter 15, on the Geometrical determination of the sun's distance from the Earth, showing the Sun, Earth and the Moon in a straight-line configuration. Circle ABG with center D represents the sun, circle MCL with center N the moon, X the center of the earth and OPR on the circle of the shadow in which the moon is immersed at its greatest distance from the earth. Ptolemy has previously calculated the radius of the earth and the distance between the earth and the moon at its greatest distance. By geometrical means the author demonstrates that where the earth’s radius is 1 and the mean distance between the earth and the moon is 59, the distance of the earth from the sun is 1210.

**Sources**

- Paul T Keyser and Georgia Irby-Massie (eds), The encyclopedia of ancient natural scientists : the Greek tradition and its many heirs, Milton Park, Abingdon, Oxon ; New York, NY : Routledge, 2008
- George Sarton, Introduction to the history of science, Baltimore: Williams & Wilkins Co., 1927-1948, vol. 1, pp. 273-275.
- Ludovico Geymonat (ed.) Storia del pensiero filosofico e scientifico. Milano, Garzanti, 1970-1972, vol. 1, pp. 250-253, vol. 2, p. 78.
- Note: “Pheludiensis” is a translational mistake made by the first translator from Arabic into Latin, Gerard of Cremona (1175) – it is a misreading of “Claudius”, Ptolemy’s first name. See Nicolai, Elena, La tradizione greco - latina e arabo - latina del I libro dell'Almagesto. saggio di analisi e traduzione. [Ph.D. thesis], Padova, 2010, p. 104.
- http://paduaresearch.cab.unipd.it/2521/