After the occupation by Alexander the Great in 332BC, Egypt came under Greek rule and influence, and it was in Alexandrian Egypt where horoscopic astrology first appeared. The endeavour to trace the horoscope of the individual from the position of the planets and stars at the time of birth represents the most significant contribution of the Greeks to astrology. This system can be labeled as "horoscopic astrology" because it employed the use of the ascendant, otherwise known as the horoskopos in Greek. Although developed under Hellenistic rule, it was in large measure derived from the teachings of the Babylonians and the Egyptians.
The system was carried to such a degree of perfection that later ages made but few additions of an essential character to the genethlialogy or drawing up of the individual horoscope by the Greek astrologers. Particularly important in the development of horoscopic astrology was the astrologer and astronomer Ptolemy , whose work, the Tetrabiblos laid the basis of the Western astrological tradition. Under the Greeks and Ptolemy in particular, the planets, Houses, and Signs of the zodiac were rationalized and their function set down in a way that has changed little to the present day. Ptolemy's work on astronomy was also the basis of Western teachings on the subject for the next 1,300 years.
To the Greek astronomer Hipparchus belongs the credit of the discovery (c. 130 B.C.) of the theory of the precession of the equinoxes, for a knowledge of which among the Babylonians we find no definite proof; but such a single advancement in pure science did not prevent the Greeks from developing in a most elaborate manner the theory of the influence of the planets upon the fate of the individual.
Babylonia or Chaldea was so identified with astrology that "Chaldaean wisdom" became among Greeks and Romans the synonym of divination through the planets and stars, and it is perhaps not surprising that in the course of time to be known as a "Chaldaean" carried with it frequently the suspicion of charlatanry and of more or less willful deception.
Astrology in Egypt developed under the Ptolemies after the conquest of Egypt by Alexander the Great.
Astrology and the sciences
Partly in further development of views unfolded in Babylonia, but chiefly under Greek influences, the scope of astrology was enlarged until it was brought into connection with practically all of the known sciences: botany, chemistry, zoology, mineralogy, anatomy and medicine. Colours, metals, stones, plants, drugs and animal life of all kinds were each associated with one or another of the planets and placed under their rulership.
By this curious process of combination, the entire realm of the natural sciences was translated into the language of astrology with the single avowed purpose of seeing in all phenomena signs indicative of what the future had in store.
The fate of the individual, as that feature of the future which had a supreme interest, led to the association of the planets with different parts of the body and so with medicine . Here, too, we find various systems devised, in part representing the views of different schools, in part reflecting advancing conceptions regarding the functions of the organs in man and animals.
From the planets the same association of ideas was applied to the constellations of the zodiac . The zodiac came to be regarded as the prototype of the human body, the different parts of which all had their corresponding section in the zodiac itself. The head was placed in the first sign of the zodiac, Aries, the Ram; and the feet in the last sign, Pisces, the Fishes. Between these two extremes the other parts and organs of the body were distributed among the remaining signs of the zodiac. In later phases of astrology the signs of the zodiac are sometimes placed on a par with the planets themselves, so far as their importance for the individual horoscope is concerned.
With human anatomy thus connected with the planets, with constellations, and with single stars, medicine became an integral part of astrology. Diseases and disturbances of the ordinary functions of the organs were attributed to the influences of planets and explained as due to conditions observed in a constellation or in the position of a star.
Arab and Persian Astrology
Main articles: Arab and Persian astrology and Islamic astrology
The system was taken up almost in its entirety by the Arab astrologers. From their great centres of learning in Damascus and Baghdad they revived the learning of the ancient Greeks in astronomy, astrology, mathematics and medicine which Europe had forgotten and developed it immensely. Their knowledge was then imported into Europe, during and after the Latin translations of the 12th century, helping to start the Renaissance. Albumasur was the greatest of the Arab astrologers, whose work 'Introductorium in Astronomiam' was later highly influential in Europe. Also important was Al Khwarizmi , the Persian mathematician, astronomer, astrologer and geographer, who is considered to be the father of algebra and the algorithm. The Arabs greatly increased the knowledge of astronomy, naming many of the stars for the first time, such as Aldebaran, Altair, Betelgeuse, Rigel and Vega. In astrology they discovered a system still known as Arabic parts , which accorded a significance to the difference or "part" between the ascendant and each planet. The Arabs were also the first to speak of favourable and unfavourable indications in astrology, instead of categorical events fated to happen.
The first semantic distinction between astrology and astronomy was given by the Persian Muslim astronomer Abu Rayhan al-Biruni in the 11th century, and he later refuted astrology in another treatise. The study of astrology was also refuted by other medieval Muslim astronomers such as Al-Farabi (Alpharabius), Ibn al-Haytham (Alhazen), Avicenna and Averroes. Their reasons for refuting astrology were often due to both scientific (the methods used by astrologers being conjectural rather than empirical) and religious (conflicts with orthodox Islamic scholars) reasons.
Ibn Qayyim Al-Jawziyya (1292-1350), in his Miftah Dar al-SaCadah, used empirical arguments in astronomy in order to refute the practice of astrology and divination. He recognized that the stars are much larger than the planets, and thus argued:
"And if you astrologers answer that it is precisely because of this distance and smallness that their influences are negligible, then why is it that you claim a great influence for the smallest heavenly body, Mercury? Why is it that you have given an influence to al-Ra's and al-Dhanab, which are two imaginary points [ascending and descending nodes]?"
Al-Jawziyya also recognized the Milky Way galaxy as "a myriad of tiny stars packed together in the sphere of the fixed stars" and thus argued that "it is certainly impossible to have knowledge of their influences."