The word astrology comes from the Latin astrologia, deriving from the Greek noun for â€˜star, celestial bodyâ€™ and logia, â€˜study of, theory, discourse (about)â€™.
Historically, the word star has had a loose definition, by which it can refer to planets or any luminous celestial object. The word planet (based on the Greek verb for â€˜to wander/strayâ€™), was introduced by the Greeks as a reference to how seven notable â€˜starsâ€™ were seen to â€˜wanderâ€™ through others which remained static in their relationship to each other, with the distinction noted by the terms asteres aplaneis â€˜fixed starsâ€™, and asteres planetai, â€˜wandering starsâ€™. Initially, texts such as Ptolemyâ€™s Tetrabiblos referred to the planets as â€˜the star of Saturnâ€™, â€˜the star of Jupiterâ€™, etc., rather than simply â€˜Saturnâ€™ or â€˜Jupiterâ€™, but the names became simplified as the word planet assumed astronomical formality over time.
The seven Classical planets therefore comprise the Sun and Moon along with the solar-system planets that are visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. This remained the standard definition of the word â€˜planetâ€™ until the discovery of Uranus in 1781 created a need for revision. Although the modern IAU definition of planet does not include the Sun and the Moon, astrology retains historical convention in its description of those astronomical bodies, and also generally maintains reference to Pluto as being an astrological planet.
Although most cultural systems of astrology share common roots in ancient philosophies that influenced each other, many have unique methodologies which differ from those developed in the west. Outside of western astrology, the two most significant of these are Hindu astrology (also known as â€œIndian astrologyâ€ and in modern times referred to as â€œVedic astrologyâ€) and Chinese astrology. Both of these systems have yielded great influence upon the worldâ€™s cultural history.
Western astrology is largely horoscopic, that is, it is a form of divination based on the construction of a horoscope for an exact moment, such as a personâ€™s birth, in which various cosmic bodies are said to have an influence. Astrology in western popular culture is often reduced to sun sign astrology, which considers only the individualâ€™s date of birth (i.e. the â€œposition of the Sunâ€ at that date).
Western astrology is founded on the movements and relative positions of celestial bodies such as the Sun, Moon, and planets, which are analyzed by their aspects (angles) relative to one another. These are usually considered by their placement in houses (spatial divisions of the sky), and their movement through signs of the zodiac (spatial divisions of the ecliptic). Western astrology is largely horoscopic, that is, it is a form of divination based on the construction of a horoscope for an exact moment, such as a personâ€™s birth, in which various cosmic bodies are said to have an influence.