Claudius Ptolemy
Claude Ptolemy born around 100 and died around 168 in Canopus, is a Greek astronomer, astrologer, mathematician and geographer who lived in Alexandria (Egypt). He is also one of the precursors of geography. His name (nomen) is Ptolemy (from Latin: Ptolemais), his first name (praenomen) is Claudius (from Latin: Claudius).
The name of Ptolemy is hardly known today except as it designates a system: the astronomical system which placed the immobile Earth at the center of the world and whose questioning, from Copernicus to Newton, commanded the scientific revolution.
The Masterful Work of Claudius Ptolemy
The masterful work of Claudius Ptolemy is the Mathematical Composition, known since the Arabs as Almagest (from al=the and magistos=very great).
In this treatise, Ptolemy gives a mathematical description of the movement of the planets. He places himself in continuity with Aristotle, adopting geocentrism (the earth is immobile at the center of the world), and he perfects Hipparchus' theory of epicycles: the planets describe a circular movement around a point which itself describes a circular movement. The improvements proposed by Ptolemy make it possible to better take into account the variation in the apparent speeds of the planets. This theory is very sophisticated and, although it may be false, it works very well, making it possible to predict, for example, eclipses with great precision.
The Almagest marks the apogee of the development of Greek trigonometry and its applications to astronomy. However, there is no overall presentation of trigonometry, the theorems being demonstrated as and when the resolution of particular astronomy problems requires it. For example, Ptolemy established a table giving the lengths of the chords which subtend the arcs of a circle, which is equivalent to drawing up a table of sines. It also gives an excellent approximation of the number π.
Finally Ptolemy is also known for his mathematical work and his theorem (in a convex quadrilateral inscribed in a circle, the product of the diagonals is equal to the sum of the products of the opposite sides) as well as for his studies on the properties of light (color , reflection, refraction and vision theory).
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