Astrology consists of a number of belief systems which hold that there is a relationship between astronomical phenomena and events in the human world. In the West, astrology most often consists of a system of horoscopes that claim to predict aspects of an individual's personality or life history based on the positions of the sun, moon, and other planetary objects at the time of their birth. Many cultures have attached importance to astronomical events, and the Indians, Chinese, and Mayans developed elaborate systems for predicting terrestrial events from celestial observations.
Among Indo-European peoples, astrology has been dated to the third millennium BCE, with roots in calendrical systems used to predict seasonal shifts and to interpret celestial cycles as signs of divine communications. Through most of its history, astrology was considered a scholarly tradition. It was accepted in political and academic contexts, and its concepts were built into other studies, such as astronomy, alchemy, meteorology, and medicine. At the end of the 17th century, new scientific concepts in astronomy (such as heliocentrism) began to damage the credibility of astrology, and subsequent controlled studies failed to confirm its predictive value. Astrology thus lost its academic and theoretical standing. Astrology saw a popular revival in the 19th and 20th centuries as part of a general revival of spiritualism and later New Age philosophy, and through the influence of mass media such as newspaper horoscopes.
While astrology may bear a superficial resemblance to science, it is a pseudoscience because it makes little attempt to develop solutions to its problems, shows no concern for the evaluation of competing theories, and is selective in considering confirmations and dis-confirmations
A central principle of astrology is integration within the cosmos. The individual, Earth, and its environment are viewed as a single organism, all parts of which are correlated with each other. Cycles of change that are observed in the heavens are therefore said to be reflective (not causative) of similar cycles of change observed on earth and within the individual. This relationship is expressed in the Hermetic maxim "as above, so below; as below, so above", which postulates symmetry between the individual as a microcosm and the celestial environment as a macrocosm. Accordingly, the natal horoscope depicts a stylized map of the universe at the time of birth, specifically focussed on the individual at its centre, with the Sun, Moon, and celestial bodies considered to be that individual’s personal planets or stars, which are uniquely relevant to that individual alone.
At the heart of astrology is the metaphysical principle that mathematical relationships express qualities or ‘tones' of energy which manifest in numbers, visual angles, shapes and sounds – all connected within a pattern of proportion. Pythagoras first identified that the pitch of a musical note is in proportion to the length of the string that produces it, and that intervals between harmonious sound frequencies form simple numerical ratios. In a theory known as the Harmony of the Spheres, Pythagoras proposed that the Sun, Moon and planets all emit their own unique hum based on their orbital revolution, and that the quality of life on Earth reflects the tenor of celestial sounds which are physically imperceptible to the human ear. Subsequently, Plato described astronomy and music as "twinned" studies of sensual recognition: astronomy for the eyes, music for the ears, and both requiring knowledge of numerical proportions.
Later philosophers retained the close association between astronomy, optics, music and astrology, including Ptolemy, who wrote influential texts on all these topics.Ptolemy recognised the unreliability of astrological predictions in comparison to the predictive power of astronomy, noting in Tetrabiblios that the "second and less sufficient method [of prediction] in a proper philosophical way, so that one whose aim is the truth might never compare its perceptions with the sureness of the first, unvarying science [astronomy]". Alkindi, in the 9th century, developed Ptolemy's ideas in De Aspectibus which explores many points of relevance to astrology and the use of planetary aspects.