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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.
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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).
At the same time, Hindu astrology had been introduced based on the Hellenistic system via Indo-Greek cultural contact. The Hindu system opted for defining the zodiac based on the fixed stars, i.e. directly tied to the eponymous zodiacal constellations.
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.
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.
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 2002. 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.
| Constellation | Sidereal Date Cyril Fagan |
Tropical date | IAU Definition Walter Berg |
|
|---|---|---|---|---|
| Aries | April 15 - May 15 | March 21 - April 20 | April 19 - May 13 May 17 - May 18 | |
| Taurus | May 16 - June 15 | April 21 - May 20 | May 14 - May 16 May 19 - June 19 | |
| Gemini | June 16 - July 15 | May 21 - June 20 | June 20 - July 20 | |
| Cancer | July 16 - August 15 | June 21 - July 21 | July 21 - August 9 | |
| Leo | August 16 - September 15 | July 22 - August 22 | August 11 - September 15 | |
| Virgo | September 16 - October 15 | August 23 - September 22 | September 16 - October 30 | |
| Libra | October 16 - November 15 | September 23 - October 22 | October 31 - November 22 | |
| Scorpius | November 16 - December 15 | October 23 - November 21 | November 23 - November 28 | |
| / | Ophiuchus | N/A | November 29 - December 17 | |
| Sagittarius | December 16 - January 14 | November 22 - December 21 | December 18 - January 17 | |
| Capricorn | January 15 - February 14 | December 22 - January 20 | January 18 - February 15 | |
| Aquarius | February 15 - March 14 | January 21 - February 19 | February 16 - March 11 | |
| Pisces | March 15 - April 14 | February 20 - March 20 | March 12 - April 18 | |