Astrology
Background
Worship of heavenly bodies History of astrology Astrology and astronomy Planets Behenian Classical Traditions, types, and systems Astrology and science Astrologers Astrological organizations
Traditions
Babylonian Chinese Hellenistic Hindu Islamic Jewish Tibetan Western
Branches
Natal Electional Horary Medical Financial Locational Psychological Meteorological
Astrological signs
Aries Taurus Gemini Cancer Leo Virgo Libra Scorpio Sagittarius Capricorn Aquarius Pisces
Symbols
Planet Metals
v t e

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]

Astronomic 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.

Symbol	Constellation	Tropical zodiac dates[12]	Sidereal zodiac dates[13][14][15] (Lahiri ayanamsa)	Based on IAU boundaries[16]
	Aries	March 21 – April 19	April 14 – May 14	April 18 – May 13
	Taurus	April 20 – May 20	May 15 – 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 15	November 23 – November 29
	Ophiuchus[a]	—	—	November 29 – December 17
	Sagittarius	November 22– December 21	December 16 – January 14	December 17 – January 20
	Capricorn	December 22 – January 19	January 15 – February 12	January 20 – February 16
	Aquarius	January 20 – February 18	February 13 – March 14	February 16 – March 11
	Pisces	February 19– March 20	March 15 – April 13	March 11 – April 18