Sidereal and tropical astrology
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New millennium astrological chart
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 twelve "signs" that are divided into 30 degrees each (making a total of 360 degrees) and named after constellations, but while the sidereal system defines the signs relative to the apparent backwards movement of fixed stars of about 1 degree every 72 years as per our perspective from Earth, the tropical zodiac fixes the vernal point (also known as vernal equinox or March equinox in the Northern hemisphere) to 0 degrees of Aries, without taking the precession of equinoxes into account, and defines the rest of the zodiac from this point.
Because of the sidereal zodiac using a correction called ayanamsa to account for the precession of equinoxes, the two systems do not remain fixed relative to each other and drift apart by about 1 degree per 72 years. The current difference stands at about 23 degrees and 50 minutes. Ayanamsa systems are used in both Vedic sidereal astrology and in Western sidereal astrology. The tropical system is thought to have been adopted during the Hellenistic period and remains prevalent in Western astrology.
The tropical zodiac is thought to have been used by Western astrologers since about 100 B.C. and does not correspond to the visible zodiac. Instead, it is defined based on seasons and their relationship to the cycles of the Sun. Seasons are defined as seen from the Northern hemisphere and the March equinox has a fixed relationship to 0 degrees of Aries, regardless of the astronomical position of the Sun with respect to constellations as seen from the Earth. This means that during the March equinox, the Sun is always seen as being "in" Aries. Some schools of thought have emerged to account for seasons as seen from the Southern hemisphere, where they are more or less in reverse. For instance, in the Northern hemisphere, the March equinox occurs during Spring, whereas in the Southern hemisphere, it occurs in Autumn.
Traditional Hindu astrology, also known as Vedic astrology, uses a sidereal system of zodiac and accounts for the precession of the equinoxes using a system of correction called ayanamsa, which allows it to be more accurately reflective of current astronomical trends than the tropical zodiac. There are various types of ayanamsa that are in use, such as Lahiri Ayanamsa and Raman Ayanamsa.
In contrast to the tropical zodiac which has stuck with 0 degrees of Aries as the vernal point (the location of the Sun during the March equinox), the sidereal zodiac currently uses approximately 6 degrees of Pisces as the vernal point as per its corrective calculations, putting it almost a full sign (or about 24 degrees) behind the tropical zodiac. This also puts sidereal calculations of the position of the sun with respect to zodiacal constellations and equinoxes more accurately in keeping with current astronomical calculations of the sun's position with respect to the same. Walter Berg's astronomical zodiac, like the sidereal zodiac, lags nearly a full sign behind the tropical zodiac.
Sidereal western astrology
Western sidereal astrology also uses ayanamsa, with the Fagan/Bradley ayanamsa being the most prevalent.
Cyril Fagan assumed the origin of the zodiac to be based on a major conjunction that occurred in 786 BC, when the vernal equinox lay somewhere in mid-Aries corresponding to a difference of some 39 degrees or days.
A small number of sidereal astrologers do not take the astrological signs as an equal division of the ecliptic, but define their signs based on the actual width of the individual constellations. They also include constellations that are disregarded by the traditional zodiac, but are still in contact with the ecliptic.
In 1995, Walter Berg introduced his 13-sign zodiac, which has the additional sign of Ophiuchus. Berg's system was well received in Japan after his book was translated by radio host Mizui Kumi (水井久美) in 1996.
For the purpose of determining the constellations in contact with the ecliptic, the constellation boundaries as defined by the International Astronomical Union in 1930 are used. For example, the Sun enters the IAU boundary of Aries on April 19 at the lower right corner, a position that is still rather closer to the "body" of Pisces than of Aries. The IAU defined the constellation boundaries without consideration of astrological purposes.
The dates the Sun passes through the 13 astronomical constellations of the ecliptic are listed below, accurate to the year 2011. The dates will progress by an increment of one day every 70.5 years. The corresponding tropical and sidereal dates are given as well.
|Constellation||Tropical date||Sidereal Date
|Based on IAU boundaries|
|Aries||March 21 – April 20||April 15 – May 15||April 18 – May 13|
|Taurus||April 21 – May 20||May 16 – June 15||May 13 – June 21|
|Gemini||May 21 – June 21||June 16 – July 16||June 21 – July 20|
|Cancer||June 22 – July 22||July 17 – August 16||July 20 – August 10|
|Leo||July 23 – August 23||August 17 – September 16||August 10 – September 16|
|Virgo||August 24 – September 22||September 17 – October 17||September 16 – October 30|
|Libra||September 23 – October 23||October 18 – November 16||October 30 – November 23|
|Scorpio||October 24 – November 22||November 17 – December 16||November 23 – November 29|
|Ophiuchus||N/A||November 29 – December 17|
|Sagittarius||November 23 – December 22||December 17 – January 15||December 17 – January 21|
|Capricorn||December 23 – January 20||January 16 – February 14||January 20 – February 16|
|Aquarius||January 21 – February 18||February 15 – March 15||February 16 – March 11|
|Pisces||February 19 – March 20||March 16 – April 14||March 11 – April 18|
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- Vedic astrology -- critically examined by Dieter Koch, with an extended discussion of sidereal and tropical astrology.
- Sidereal Right Ascensions by Juan Antonio Revilla
- The Art of Vedic Astrology: a simple explanation of the relevance of the precession of equinoxes
- The Story of Vedic Astrology Zodiac by Sam Geppi: an explanation of the sidereal system