Sidereal and tropical astrology

Sidereal and tropical are the 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.

A tropical system was introduced by Ptolemy in the 2nd century 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. (Over very long astronomical time scales, these fixed stars are of course themselves far from stationary.)

Within modern astrology, there has been some debate between proponents of each system as to which is preferable or "correct.

Some sidereal astrologers denounce tropical astrologers for failing to relate to the "actual heavens," seeing in this a fundamental degeneration of the subject .

Tropical astrologers, on the other hand, may counter with the fact that despite claims to the contrary, the sidereal zodiac does not actually match up with the constellations of the same name because the signs of the sidereal zodiac are of equal 30 degree arc, whereas the constellations are not, with the Sun being in Virgo, for example, for just two weeks, whilst being in Leo for just over a month (see table below). In addition, they point out that tropical astrologers elect the precise calculation of their zodiac from the intersection of the celestial equator with the ecliptic and so their zodiac is precisely mathematically aligned with the 'actual heavens' whereas sidereal astrologers use a number of competing Ayanamshas to determine theirs.

Tropical astrology is based on the idea that astrology is based on the configuration of the solar system relative to the Sun, not to the remote fixed stars. For tropical astrologers therefore it is irrelevant that the solsticial points (tropics) have drifted from one constellation to another over the millennia, due to the precession of the equinoxes. The names of the zodiacal constellations that became the star signs are supposed to suggest the characteristics of (the sun in) each segment of the year. Thus, Aries (House 1), representing the sun just returning to the northern hemisphere at the vernal equinox, Leo (House 5), representing sun of mid-summer, Sagittarius (House 9), representing the retreating sun close to the winter solstice, etc. In its emphasis on the symbolic or metaphorical meaning of the star signs tropical astrology differs from sidereal astrology which claims intrinsic meaning for the star signs and wishes to preserve those meanings by laying out horoscopes against fixed stars.

History

In India Hindu astrology, Sidereal astronomy was well known since the Vedic period. The Rig Veda is known to contain verses indicating the sidereal year. The Hindu system opted for defining the zodiac based on the fixed stars, i.e. directly tied to the eponymous zodiacal constellations.

Classical Hellenistic astrology consequently developed without consideration of the effects of precession. 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 in the west. The Hellenistic system is known to have been influenced by Hindu astrology via Indo-Greek cultural contact.

The discovery of the precession of the equinoxes in the west 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).