« ... Claudius Ptolemaeus of Alexandria (known in English as "Ptolemy"), probably lived from about A.D. 100 to 170 (second century A.D.). What is known about his life can be deduced, within reasonable limits, from a series of chronological data he left us. The earliest news on his works dates back to A.D. 127, the 11^th year of Adrianus, the Spanish (49), one of the most learned emperors that happened to reign over Rome. Ptolemy’s catalogue of stars is dated A.D. 137, the 1^st year of the reign of Antoninus, while the last dating, known to us, goes back to A.D. 150. He, therefore, lived in a period that was culturally much advanced. 

His native city, Alexandria, was a powerful and lively centre, heart of the Hellenistic culture (50). Here knowledge was quite specialized and, as much as possible, emancipated, free from religious ties and, consequently, also from dogmas. Attitude, however, differed very much from that of our modern science. It essentially developed the theoretical aspects, but avoided all the practical approaches. 

Ptolemy inherited the greatness of Alexandria and of its past. He is the author of many works. Plenty of them reached us. His most important and known treatise is the "Mathematical Treatise - Mathematiké Syntaxis", a summa of the ancient astronomical wisdom, known as the "Almagest". This name is derived from the Arabic language, due to the excellent consideration he enjoyed among the Arabian people. (51) 

Other works are: the "Planetary Hypotheses", the "Geography", the "Optics", the "Armonics", "On judgment and guiding reasons", and the Tetrabiblos (Four Books), in Latin "Quadripartitum", a treatise of great success, that followed the Almagest, completing it with a number of astrological notions. During the Middle Ages and the Renaissance it became a subject of great study, as it codified the astrological beliefs and the foretelling opportunities in a clear and systematic way. 

Proclus commented the "Planetary Hypotheses", a work that reports numerical values referable to the contents of the Syntaxis. A further few data can also be found in a dedication made to Ptolemy I Soter, the Saviour, dating back to the 10^th year of Antoninus. 

Ptolemy was the last and the most famous scholar of the Hellenistic period and the most competent personage that ancient wisdom expressed. His Almagest takes into consideration Aristotle’s criteria. Starting from a subdivision of the sciences: creative, practical and theoretical, he analyses the importance and the priority that must be credited to each science. 

According to Ptolemy, theoretical sciences, including theology, physics and mathematics, must be considered of greater importance compared to others, even if they showed some weak points. Theology’s object is totally separated from all sensitive surroundings, while physics only take into consideration entities that change continuously. Ptolemy, in fact, states that the first two subdivisions of the theoretical science [theology and physics] have a conjectural character, more than a scientific awareness – theology for its relation with invisibility and with inconceivability; physics for its instability and mysteriousness. Therefore, there is no hope philosophers may agree on these two forms of knowledge. Only mathematics, when used logically and meticulously, can offer a secure and solid scientific platform, as both the arithmetical and the geometrical demonstrations are based on indisputable proceedings. Ptolemy’s studies are, therefore, mainly based "on what is concerned with celestial and divine matters", immutable and perpetually lasting from an ontological point of view. Henceforth offering an essential help to all other sciences. 

Ptolemy’s interest and his motivations go far beyond. Astronomy must become a reason of study as it represents an ethical answer to man’s demands. The universe gives us an example of simplicity, order and perfection, to which every human being must aim. Concerning the nobility of actions and character, the science that studies the universe, more than any other, shall teach us the similitude, the order, the symmetry and the lack of vanity which are to be found in the divine. It will also pursue the person, following such tendencies, to become a lover of the divine beauty, so that with time, a spiritual intention, near and similar to such a beauty, will naturally arise. 

The basic assumptions of his work are summarized in the beginning of the first book of the Almagest: 
 

1)	The sky is a sphere revolving about a fixed axis, deducible from the observation of the circular motion of the fixed stars and from the rising and the setting of the other stars that always takes place in the same identical position. The sky is ethereal and un-perishable.  z
2)	The Earth has a spherical shape. The Sun, the Moon and the stars do not rise and set at the same identical time for all the inhabitants of the Earth living in different areas. It happens at an earlier time, for those who live Eastward and later, for those who live Westward. Moreover, navigators proceeding towards a line of a mountains, from whatever direction they may come from, see the same, alternatively becoming bigger and bigger, as if they emerged from the sea.  z
3)	The Earth is in the centre of the firmament. It is in the middle of the universe, otherwise a part of the sky would appear nearer to us than the other and the stars, nearer to us, would appear bigger. If the Earth was positioned onto the celestial axis, nearer to a pole than to the other, the horizon would not slice the equator in two equal parts, but would cut one of the circles parallel to the equator; if the Earth was instead positioned outside the axis, we would have an ecliptic unequally divided by the horizon. Furthermore, if the Earth was not in the centre, the whole order relevant to the rising and setting of the night and day, would become upset. The lunar eclipses would not take place at opposition with the Sun with respect to all the other positions in the sky, as Earth’s interposition would often occur with these two bodies in opposite positions, but separated by intervals smaller than a semicircle.  z
4)	The Earth is similar to a point, in comparison to the sphere of the fixed stars, as these always appear to have the same dimensions and the same distance from wherever they may be observed from Earth.  z
5)	The Earth is motionless. It has no motion of translation, because all the heavy celestial bodies fall into the centre of the sky, which is also the Earth’s centre. Furthermore, a standpoint must exist to which all motions of the other concerns may be referred to. Besides, if the Earth moved, its motion would be proportioned to its mass, therefore, the objects and the animals would find themselves suspended or casted into the air. Such an assertion denies the theories of those who assume that the Earth revolves about its axis, which would make it quite easy to explain the evidence that arises from the observations.  z

The thesis displayed are the foundation of the geocentric system. Their importance lasted up to Copernicus.  

The Sun, the Moon and the five planets revolved with an uniform circular motion, as per Hipparcus’ theories, adopted by Ptolemy, even though he incurred in an error of some significance: he did not recalculate the Sun’s position, that, three hundred years after the values registered by Hipparcus (52), possibly showed an error 100’. 

He did not show the same attitude for the Lunar theory and improved it very much. Same he did for the planetary theory, defining the planets as five wandering stars because of their irregular motions. 

He assumed the motion of the fixed stars to be provoked by the uniform motion of rotation of the concentric ethereal sphere of the fixed stars, while the basic assumption, appropriate with the nature of the divine, explaining the astral motions as uniform circular motions, was never changed. Such motions were the motions of the eccentric orbits, i.e. those having a centre not coincident with that of the Earth’s and the motions of the epicyclic orbits, i.e. the orbits that revolve about a centre situated on a circle that is also rotating. 

Much probably, the use of circles was considered only a suitable medium to calculate the position of the planets, just a technical expedient to give a mathematical explanation to the phenomena. 

The Ptolemaic system also gave us tables, in which the celestial motions were represented with such a precision, that learned people still consider them as a monument to mathematical wisdom. 

The problem of motors, a sequel of tradition, was still not solved. Therefore, he asserted that the planets moved because of a natural, innate vital force . He attributed the motions of the single celestial bodies to an internal cause, an assumption that anticipated the concept of gravitational force. 

Geo-centricity places man into the centre of the universe: in a position of supremacy both worth-wise and tangible. As such, Ptolemy followed astronomy’s ancient rules that, notwithstanding the attempts of Aristarchus of Samo, always placed the Earth in a central position. As so, Ptolemy did not contravene religious and philosophical dogmas. 

Ptolemy’s ability is to be found in the equilibrium he was able to exercise between ancient times and his present. Even though he edified a geometrical Universe, perfectly structured with the use of mathematical arguments, just before Graecism died out, he revived the reminiscences of his ancient credo, that astral influence dominated man and that Fate dominated and ruled over everything. His genius ended an era lasted about a thousand years. 

Other scholars, such as, Apollonius of Perga (53) and Hipparcus (54), will again take up and carry forward the epyciclic theories with original contributions. The latter are mentioned only because they are part of a strict technical astronomical context. 

To appreciate the thought of another great astronomer, we must await for Copernicus and for his revolutionary contribution ... »