Sidereal and tropical astrology

Sidereal and tropical are astrological terms used to describe two different definitions of a year. They are also used as terms for two systems of ecliptic coordinates used in astrology. Both divide the ecliptic into a number of "signs" named after constellations, but while the sidereal system defines the signs based on the fixed stars, the tropical system defines it based on the position of vernal equinox in the northern hemisphere (i.e., the intersection of the ecliptic with the celestial equator). Because of the precession of the equinoxes, the two systems do not remain fixed relative to each other but drift apart by about 1.4 arc degrees per century. The tropical system was adopted during the Hellenistic period and remains prevalent in Western astrology. A Sidereal system is used in Hindu astrology, and in some 20th century systems of Western astrology.

While classical tropical astrology is based on the orientation of the Earth relative to the Sun and planets of the solar system, sidereal astrology deals with the position of the Earth relative to both of these as well as the stars of the celestial sphere. The actual positions of certain fixed stars as well as their constellations is an additional consideration in the horoscope.

The classical zodiac was introduced in the neo-Babylonian period (around the seventh to the sixth century BCE). At the time, the precession of the equinoxes had not been discovered. Classical Hellenistic astrology consequently developed without consideration of the effects of precession. The discovery of the precession of the equinoxes is attributed to Hipparchus, a Greek astronomer active in the later Hellenistic period (ca. 130 BCE).

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