Sidereal and Tropical Astrology

Sidereal and Tropical Astrology

In astrology, sidereal and tropical are terms that 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.[1] The terms sidereal and tropical may also refer to two different definitions of a year, applied in sidereal solar calendars or tropical solar calendars.

While sidereal systems of astrology calculate twelve zodiac signs based on the observable sky and thus account for the apparent backwards movement of fixed stars of about 1 degree every 72 years from the perspective of the Earth due to the Earth’s axial precession, tropical systems consider 0 degrees of Aries as always coinciding with the March equinox (known as the spring equinox in the Northern Hemisphere) and define twelve zodiac signs from this starting point, basing their definitions upon the seasons and not upon the observable sky wherein the March equinox currently falls in Pisces due to the Earth’s axial precession.[2][3][4] These differences have caused sidereal and tropical zodiac systems, which were aligned around 2,000 years ago when the March equinox coincided with Aries in the observable sky, to drift apart over the centuries.[5][6][7]

Sidereal astrology accounts for the Earth’s axial precession and maintains the alignment between signs and constellations via corrective systems known as ayanamsas (Sanskrit: ‘ayana’ “movement” + ‘aṃśa’ “component”), whereas tropical astrology, to reiterate, is based upon the seasonal cycle of the Northern hemisphere and does not take axial precession into consideration. Though tropical astrology typically considers the zodiac of the Northern Hemisphere to be applicable without change to the Southern hemisphere, a small number of tropical astrologers modify the zodiac to reflect seasons in the Southern hemisphere, taking Libra as the sign that coincides with the spring equinox instead of Aries.[3][8][9]

Ayanamsa systems used in Hindu astrology (also known as Vedic astrology) include the Lahiri ayanamsa and the Raman ayanamsa, of which the Lahiri ayanamsa is the most widely used.[10] The Fagan-Bradley ayanamsa is an example of an ayanamsa system used in Western sidereal astrology.[10] As of 2020, sun signs calculated using the Sri Yukteswar ayanamsa were around 23 degrees behind tropical sun signs.[8] Per these calculations, persons born between March 12 – April 12, for instance, would have the sun sign of Pisces.[8] Per tropical calculations, in contrast, persons born between March 21 – April 19 would have the sun sign of Aries.[11]

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[edit]

Main articles: Ecliptic and Zodiac

A small number of sidereal astrologers[who?] 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.

SymbolConstellationTropical zodiac dates[12]Sidereal zodiac dates[13][14][15]
(Lahiri ayanamsa)
Based on IAU boundaries[16]
AriesMarch 21 – April 19April 14 – May 14April 18 – May 13
TaurusApril 20 – May 20May 15 – June 15May 13 – June 21
GeminiMay 21 – June 20June 16 – July 16June 21 – July 20
CancerJune 21 – July 22July 17 – August 16July 20 – August 10
LeoJuly 23 – August 22August 17 – September 16August 10 – September 16
VirgoAugust 23 – September 22September 17 – October 17September 16 – October 30
LibraSeptember 23 – October 22October 18 – November 16October 30 – November 23
ScorpioOctober 23 – November 21November 17 – December 15November 23 – November 29
Ophiuchus[a]November 29 – December 17
SagittariusNovember 22– December 21December 16 – January 14December 17 – January 20
CapricornDecember 22 – January 19January 15 – February 12January 20 – February 16
AquariusJanuary 20 – February 18February 13 – March 14February 16 – March 11
PiscesFebruary 19– March 20March 15 – April 13March 11 – April 18
  1. ^ Ophiuchus is not a sign in traditional forms of tropical and sidereal astrology, and only appears in a few forms of astrology which use the IAU-defined constellations.
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Writer and Journalist living in Canada since 1987. Tamil activist.

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