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Sidereal and tropical are terms used to describe two different definitions of a year, applied in sidereal solar calendars or tropical solar calendars. 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 define 0 degrees of Aries to coincide with the vernal point or vernal equinox (also known as the March equinox in the Northern hemisphere), and define the rest of the zodiac from this point. Sidereal astrology maintains the alignment between signs and constellations via corrective systems known as ayanamsas (Sanskrit: 'ayana' "movement" + 'aṃśa' "component"), to allow for the observed precession of equinoxes, whereas tropical astrology ignores precession. This has caused the two systems, which were aligned 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 Yukteswar ayanamsa were around 23 degrees behind tropical sun signs. Per these calculations, persons born between March 12 - April 12, for instance, would have the sun sign of Pisces. By contrast, persons born between March 21 - April 19 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.
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, as the first sign rather than of Aries. The IAU defined the constellation boundaries without consideration of astrological purposes.
The dates the Sun passes through the 12 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.
|Symbol||Constellation||Tropical date||Sidereal Date
Cyril Fagan
|Based on IAU boundaries|
|Aries||March 21 – April 19||April 15 – May 15||April 18 – May 13|
|Taurus||April 20 – May 20||May 16 – June 15||May 13 – June 21|
|Gemini||May 21 – June 20||June 16 – July 16||June 21 – July 20|
|Cancer||June 21 – July 22||July 17 – August 16||July 20 – August 10|
|Leo||July 23 – August 22||August 17 – September 16||August 10 – September 16|
|Virgo||August 23 – September 22||September 17 – October 17||September 16 – October 30|
|Libra||September 23 – October 22||October 18 – November 16||October 30 – November 23|
|Scorpio||October 23 – November 21||November 17 – December 16||November 23 – November 29|
|Sagittarius||November 22 – December 21||December 17 – January 15||December 17 – January 20|
|Capricorn||December 22 – January 19||January 16 – February 14||January 20 – February 16|
|Aquarius||January 20 – February 19||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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