Sidereal and tropical astrology

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Sidereal and tropical are astronomical terms used to describe two different definitions of a "year". They are also used as terms for two systems of ecliptic coordinates used in astrology.

Both divide the ecliptic into a number of "signs" named after constellations, but while the sidereal system defines the signs based on the fixed stars, the tropical system defines it based on the position of vernal equinox (i.e. the intersection of the ecliptic with the celestial equator). Because of the precession of the equinoxes, the two systems do not remain fixed relative to each other but drift apart by about 1.4 arc degrees per century.

The tropical system was adopted during the Hellenistic period and remains prevalent in western astrology. A sidereal system is used in Hindu astrology, and in some 20th-century systems of western astrology.

While classical tropical astrology is based on the orientation of the Earth relative to the Sun and planets of the solar system, sidereal astrology deals with the position of the Earth relative to both of these as well as the stars of the celestial sphere. The actual positions of certain fixed stars as well as their constellations is an additional consideration in the horoscope.



Changing position of the vernal equinox. The red line is a section of the apparent path traced by the Sun through the Earth's year. The red/green line is a projection of the Earth's equator onto the celestial sphere. The crossing point of these two lines is the spring equinox. In 1500 BC it was near the end of the constellation of Aries, in 500 BC it was near the beginning of the constellation of Aries, and in AD 150 (the time of Ptolemy) it was in the center of the constellation of Pisces.

The classical zodiac was introduced in the neo-Babylonian period (ca. 7th to 6th century BC). At the time, the precession of the equinoxes had not been discovered. Classical Hellenistic astrology consequently developed without consideration of the effects of precession. The discovery of the precession of the equinoxes is attributed to Hipparchus, a Greek astronomer active in the later Hellenistic period (ca. 130 BCE).

Ptolemy writing some 250 years after Hipparchus was thus aware of the effects of precession. He opted for a definition of the zodiac based on the point of vernal equinox, i.e. the tropical system. While Ptolemy noted that Ophiuchus is in contact with the ecliptic, he was aware that the twelve signs were just conventional names for 30 degrees segments (especially since the Aries sign had ceased to be in contact with the Aries constellation already in his time).

The Hindu Jyotisha system opted for defining the zodiac based on the fixed stars, i.e. directly tied to the eponymous zodiacal constellations unlike Western astrological systems.

Sidereal traditions

Hindu astrology

Traditional Hindu astrology is based on the sidereal or visible zodiac, accounting for the shift of the equinoxes by a correction called ayanamsa. The difference between the Vedic and the Western zodiacs is currently around 24 degrees. This corresponds to a separation of c. 1700 years, when the vernal equinox was approximately at the center of the constellation Pisces and the tropical zodiac coincided with the sidereal one (around 290 AD, or at 23.86° as of 2000 according to N. C. Lahiri, renowned author of Lahiri's Ephemeris published from kolkata, India. The separation is believed to have taken place in the centuries following Ptolemy (2nd century AD), apparently going back to Indo-Greek transmission of the system. But earlier Greek astronomers like Eudoxus spoke of vernal equinox at 15° in Aries, while later Greeks spoke of vernal equinox at 8° and then 0° in Aries (cf. p. 16, S. Jim Tester in ref.), which suggests use of sidereal zodiac in Greece before Ptolemy and Hipparchus.

Sidereal western astrology

Some western astrologists have shown interest in the sidereal system during the 20th century.

Cyril Fagan assumes the origin of the zodiac in 786 BC, when the vernal equinox lay somewhere in mid-Aries, based on a major conjunction that occurred that year,[1] corresponding to a difference of some 39 degrees or days.

Most sidereal astrologers simply divide the ecliptic into 12 equal signs of 30 degrees but approximately aligned to the 12 zodiac constellations. Assuming an origin of the system in 786 BCE, this results in an identical system as that of the classical tropical zodiac, shifted by 25.5 days, i.e., if in tropical astrology, Aries is taken to begin at March 21, sidereal Aries will begin on April 15.

But 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. 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 has been well received in Japan after his book was translated by radio host Mizui Kumi (水井久美) in 1996.

Modern constellations along 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 than of Aries. Needless to say, 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 increment by one day every 70½ years, and already several have changed. The corresponding tropical and sidereal dates are given as well.

ConstellationSidereal Date
Cyril Fagan
Tropical dateIAU Definition
Walter Berg[2]
Aries.svgAriesApril 15 - May 15March 21 - April 20April 18 - May 13
Taurus.svgTaurusMay 16 - June 15April 21 - May 20May 13 - June 21
Gemini.svgGeminiJune 16 - July 15May 21 - June 20June 21 - July 20
Cancer.svgCancerJuly 16 - August 15June 21 - July 21July 20 - August 10
Leo.svgLeoAugust 16 - September 15July 22 - August 22August 10 - September 16
Virgo.svgVirgoSeptember 16 - October 15August 23 - September 22September 16 - October 30
Libra.svgLibraOctober 16 - November 15September 23 - October 22October 30 - November 23
Scorpio.svgScorpiusNovember 16 - December 15October 23 - November 21November 23 - November 29
Ophiuchus zodiac.svgOphiuchusN/ANovember 29 - December 17
Sagittarius.svgSagittariusDecember 16 - January 14November 22 - December 21December 17 - January 20
Capricorn.svgCapricornJanuary 15 - February 14December 22 - January 20January 20 - February 16
Aquarius.svgAquariusFebruary 15 - March 14January 21 - February 19February 16 - March 11
Pisces.svgPiscesMarch 15 - April 14February 20 - March 20March 11 - April 18

See also


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