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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.
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
Cyril Fagan
|Based on IAU boundaries|
Walter Berg[verification needed]
|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|
- "The Tropical, Sidereal & Constellational Zodiacs: the Power of World View". www.renaissanceastrology.com. Retrieved 2020-07-29.
- Subramanian, Vijaya. "Sidereal and Tropical Zodiac – Vijaya Jyoti". Retrieved 2020-07-29.
- galacticcenter (2016-10-10). "What is Vedic Astrology?". Galactic Center with Joni Patry. Retrieved 2020-07-29.
- Taya. "Understanding Sidereal Charts". astro-charts.com. Retrieved 2020-07-29.
- Vallée, Guylaine (2020-03-01). "What's Your Vedic Astrological Sign?". Birla Vedic Center. Retrieved 2020-07-29.
- "Ayanamshas in Sidereal Astrology". www.astro.com. Retrieved 2020-07-29.
- "Zodiac Sign Dates: What Are The Dates for Every Star Sign?". Astrostyle: Astrology and Daily, Weekly, Monthly Horoscopes by The AstroTwins. Retrieved 2020-07-29.
- New astrological sign: Professor finds horoscopes may be a little off kilter a January 14, 2011 article from the Los Angeles Times
- "The Real Constellations of the Zodiac." Dr. Lee T. Shapiro, Planetarian, Vol 6, #1, Spring (1977). 
- "The Real, Real Constellations of the Zodiac." John Mosley, Planetarian, Vol. 28, # 4, December (1999).
- "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.