The Tetrabiblos

Ptolemy published another work relevant to cosmology, the Tetrabiblos, which deals with the influence of the celestial bodies on events on Earth. Astrology as a subject emerged in the later part of Greek antiquity, during the Roman Imperial period in the second century b.c. Earlier Greek thinkers such as Aristotle evinced no interest at all in astronomical prognostication. The popularity of astrology in the time of Ptolemy was connected to the emergence of Stoic philosophy and the Stoic concept of cosmic sympathy and belief in the interconnectedness of everything in the universe. The Tetrabiblos consists of the recitation of conventional beliefs rather than an exposition of reasoned doctrine or the derivation of theses. A prominent topic is what is known as judicial astrology, also known as horoscopic astrology, involving astronomical prognostication for an individual based on the position of the planetary bodies at the time of birth. Although it is difficult to gauge the level of Ptolemy's interest in astrology, the demands of a subject that enjoyed widespread popularity were a strong motive for the compilation of planetary tables. Just as divination from the entrails of animals contributed to the knowledge of physiology, so the interpretation of personality and the forecasting of events using celestial signs contributed to the study of astronomy.

There is one respect in which Ptolemy's conception of the physical influence of celestial bodies on the Earth is connected to his cosmology. According to Ptolemy, the different planets have a physical effect on Earth according to their positions in the planetary ordering, Moon-Mercury-Venus-Sun-Mars-Jupiter-Saturn. The Sun is the primary source of warmth, and the Moon is the primary humidifying agent. Mars and Venus both impart warmth to the Earth because of their closeness to the Sun, while Saturn has a cooling effect because of its distance from the Sun. The Moon serves to humidify, and Mercury also does so because of its proximity to the Moon.

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