Western astrology emerged in Mesopotamia in 3000 BCE. Priests were responsible for watching the skies so that they could warn the ruler of any ill omen. Through constant observation, they eventually established the cycle of eclipses and identified several planets, placing each under the protection of a deity. This research into the positions of the heavenly bodies was possible only thanks to calculations developed by the Babylonians, particularly in the study of angles.
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Astrology in the ancient world
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Following Alexander's conquests, the Greeks of the Hellenistic period took up the observation of the heavens, though they preferred to honour the gods of Olympus. Whereas Babylonian predictions concerned the community or the king who personified it, the Greeks gave pride of place to individual horoscopes.
In the second century CE, Claudius Ptolemy wrote an astrological work, Tetrabiblos (known in French as the Tétrabible), bringing together all the knowledge—and above all the beliefs—of his time. He distinguished natural astrology, now called astronomy, which studies the motion of the heavenly bodies, from judicial astrology, which determines people's futures. At a time when the laws of physics were unknown, the only alternative to attributing an influence over our lives to the heavenly bodies was to hold the gods responsible. Yet the calculations involved seemed so complex that astrologers would long be known as mathematici.
