Sidereal and tropical astrology
Two zodiac systems divided by axial precession.
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Sidereal and tropical astrology are two systems of ecliptic coordinates that divide the zodiac into twelve signs of 30 degrees each. They differ in their treatment of Earth's axial precession: tropical astrology ties the start of Aries to the March equinox, while sidereal astrology adjusts for precession to align signs with the observable sky. This divergence, which began when the two systems were aligned about 2,000 years ago, has caused the zodiac signs to drift apart over the centuries.
Quick Facts
- Degrees per zodiac sign
- 30
- Total degrees in zodiac
- 360
- Precession rate
- about 1 degree every 72 years
Facts from the source article.
Astronomic zodiac
Sidereal astrology defines signs based on the actual width of individual constellations, unlike tropical astrology's equal division. It also includes constellations that touch the ecliptic but are omitted from the traditional zodiac.
The constellation boundaries used for this purpose are those set by the International Astronomical Union in 1930, which were defined without astrological considerations. For instance, the Sun enters the IAU boundary of Aries on April 19 at a position closer to the body of Pisces than to Aries. The dates the Sun passes through the 12 astronomical constellations are listed with corresponding tropical and sidereal dates, accurate to 2011, and progress by one day every 70.5 years.
More in Spring Equinox
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Sidereal and tropical astrology (CC BY-SA 4.0).
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