Sidereal and tropical astrology

Sidereal and tropical are terms used to describe two different definitions of a year. In astrology, they refer to two different systems of ecliptic coordinates used to divide the ecliptic into twelve "signs". Each sign is divided into 30 degrees, making a total of 360 degrees.

While sidereal systems of astrology define the signs relative to the apparent backwards movement of fixed stars of about 1 degree every 72 years from the perspective of the Earth, tropical systems fix the vernal point or vernal equinox (also known as the March equinox in the Northern hemisphere) to 0 degrees of Aries and define the rest of the zodiac from this point. Sidereal astrology uses corrective systems known as ayanamsas (Sanskrit: 'ayana' "movement" + 'aṃśa' "component") to account for the precession of equinoxes, whereas tropical astrology does not use such measures. This has caused the two systems, which were identical around 2,000 years ago, to drift apart over the centuries.

Ayanamsa systems used in Hindu astrology (also known as Vedic astrology) include the Lahiri ayanamsa and the Raman ayanamsa. The Fagan-Bradley ayanamsa is an example of an ayanamsa system used in Western sidereal astrology. As of 2020, sun signs calculated using the Sri Yukteshwar ayanamsa were around 23 or degrees behind tropical sun signs. Per these calculations, persons born between 12th March - 12th April, for instance, would have the sun sign of Pisces. By contrast, persons born between March 21st - April 19th would have the sun sign of Aries per tropical calculations.

Cyril Fagan, a Western sidereal astrologer, 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.

Precession of equinoxes, the changing position of the vernal equinox over the course of about 25,800 years. The yellow line is a section of the ecliptic, the apparent path the Sun appears to follow over the course of an Earth year. The purple line is the celestial equator, the projection of Earth's equator onto the celestial sphere. The point (red) where these two lines cross is the vernal equinox. In 1500 BCE, it was near the end of Aries; in 500 BCE, it was near the beginning of Aries; and in 1000 to 2500 CE Pisces.

Astronomic zodiac

Main articles: Ecliptic and Zodiac

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.

Stephen Schmidt in 1970 introduced Astrology 14, a system with additional signs based on the constellations of Ophiuchus and Cetus.

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 Cyril Fagan	Based on IAU boundaries Walter Berg
	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

See also

• Great year
• Astrology and science
• Synoptical astrology

References

• "The Real Constellations of the Zodiac." Dr. Lee T. Shapiro, Planetarian, Vol 6, #1, Spring (1977). [1]
• "The Real, Real Constellations of the Zodiac." John Mosley, Planetarian, Vol. 28, # 4, December (1999).[2]
• "The Primer of Sidereal Astrology," Cyril Fagin and Brigadier R. C. Firebrace, American Federation of Astrologers, Inc., (1971) ISBN 0-86690-427-1
• A History of Western Astrology, by S. Jim Tester, 1987, republished by Boydell Press (January 1999),ISBN 0-85115-255-4, ISBN 978-0-85115-255-4
• Raymond, Andrew (1995). Secrets of the Sphinx Mysteries of the Ages Revealed. Hawaii: U N I Productions. ISBN 0-9646954-6-4.

External links

• Vedic astrology -- critically examined by Dieter Koch, with an extended discussion of sidereal and tropical astrology.
• Sidereal Right Ascensions by Juan Antonio Revilla