Since the 18th century they have come to be regarded as completely separate disciplines. Astronomy, the study of objects and phenomena beyond the Earth's atmosphere, is accepted as a science and is a widely studied academic discipline. Astrology, which uses the apparent positions of celestial objects as the basis for psychology, prediction of future events, and other esoteric knowledge, is not widely regarded as science and is typically defined as a form of divination.
Historically, most cultures have not made a clear distinction between the two disciplines, lumping them both together as one. In ancient Babylonia, famed for its astrology, there were not separate roles for the astronomer as predictor of celestial phenomena, and the astrologer as their interpreter; both functions were performed by the same person. This overlap does not mean that astrology and astronomy were always regarded as one and the same. In ancient Greece, presocratic thinkers such as Anaximander, Xenophanes, Anaximenes, and Heraclides speculated about the nature and substance of the stars and planets. Astronomers such as Eudoxus (contemporary with Plato) observed planetary motions and cycles, and created a geocentric cosmological model that would be accepted by Aristotle -- this model generally lasted until Ptolemy, who added epicycles to explain certain motions. However, around 250 B.C., Aristarchus of Samos postulated a proto-heliocentric theory, which would not be reconsidered for nearly two millennia (Copernicus), as Aristotle's geocentric model was favored. The Platonic school promoted the study of astronomy as a part of philosophy because the motions of the heavens demonstrate an orderly and harmonious cosmos. In the third century B.C.E., Babylonian astrology began to make its presence felt in Greece. Astrology was criticized by Hellenistic philosophers such as the Academic Skeptic Carneades and Middle Stoic Panaetius. However, the notions of the Great Year (when all the planets complete a full cycle and return to their relative positions) and eternal recurrence were Stoic doctrines that made divination and fatalism possible.
While the Greek words astrologia and astronomia were often used interchangeably, they were conceptually not the same. Both words more often than not referred to astronomy. The words for astrology proper, were more typically apotelesma and katarkhê.
The earliest to differentiate between the terms astronomy and astrology was Isidore of Seville in the 7th century, while the earliest semantic distinction between astronomy and astrology was given by the Persian astronomer and astrologer Abu Rayhan al-Biruni circa 1000. Astrology was also refuted by al-Biruni and other medieval Muslim astronomers such as Al-Farabi (Alpharabius), Ibn al-Haytham (Alhazen), Abū Rayhān al-Bīrūnī, 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 astrology and divination.
Astrology was widely accepted in medieval Europe as astrological texts from Hellenistic and Arabic astrologers were translated into Latin. In the late Middle Ages, its acceptance or rejection often depended on its reception in the royal courts of Europe. Not until the time of Francis Bacon was astrology rejected as a part of scholastic metaphysics rather than empirical observation. A more definitive split between astrology and astronomy the West took place gradually in the seventeenth and eighteenth centuries, when astrology was increasingly thought of as an occult sciencel or superstition by the intellectual elite. Because of their lengthy shared history, it sometimes happens that the two are confused with one another even today. Many contemporary astrologers, however, do not claim that astrology is a science, but think of it as a form of divination like the I-Ching, an art, or a part of a spiritual belief structure (influenced by trends such as Neoplatonism, Neopaganism, Theosophy, and Hinduism).
Astrology and astronomy stayed together for a very long time - the funding from astrology supported some astronomical research, which was in turn used to make more accurate ephemerides for use in astrology. In Medieval Europe the word Astronomia was often used to encompass both disciplines as this included the study of astronomy and astrology jointly and without a real distinction; this was one of the original Seven Liberal Arts. Kings and other rulers generally employed court astrologers to aid them in the decision making in their kingdoms, thereby funding astronomical research. University medical students was taught astrology as it was generally used in medical practice.
The separation of astronomy from astrology occurred gradually from about the 17th century up to the beginning of the 19th. While Copernicus himself didn't practice astrology, the most important celestial mechanicans before Isaac Newton were astrologers by profession Tycho Brahe, Johannes Kepler, and the general father of modern physics, Galileo Galilei studied astrology as part of his education. Newton, however, allegedly rejected astrology. Since these fathers of celestial mechanism was paralleled by the emergence of Rationalism, Empiricism and the development of observational science, they may be regarded as founders of a mechanist world view, that explains the universe as a machine and gives no room for the mystical explanations commonly used to explain the ground for astrology.
Astrology and astronomy diverged paths the fastest during the rise of the and the scientific method in the Western World. This period is defined as the beginning of the scientific revolution, leading on into The Age of Enlightenment, sometimes referred to as The Age of Reason -- the two fields finally diverged completely in the West around the later part of the 18th century.