|Tropical and sidereal|
One technical difference often cited as a major divide between traditional western and Indian astrology is the choice of the tropical and sidereal zodiac, respectively. The zodiac is a belt extending some 9° to either side of the circle of the ecliptic and divided into twelve equal parts. These twelve parts or signs get their names from twelve actual constellations found along the zodiac, although the signs are not identical with the constellations (which are unequal in size).
In the earliest definitions of the twelve-sign zodiac, found in Mesopotamia in the 5th century BCE, the zodiac is fixed in relation to the constellations. In other words it is sidereal, or ‘derived from the stars’. In this zodiac, the position of the Sun at the time of the vernal equinox (around 21 March each year) was defined in one system as 10° Aries (the first sign), and in another as 8° Aries. Although the Babylonians did not know it, in a fixed or sidereal zodiac, the solstice and equinox points – which mark the seasons of the year – regress slowly through the twelve signs, completing a full circle in some 26,000 years (a phenomenon known as precession).
The early Greek-speaking astrologers continued to use a fixed zodiac, and after Greek astrology was transmitted to India in the first centuries of the Common Era, so did Indian astrologers – a practice which continues to the present day. In the 2nd century CE, however, Claudius Ptolemy as the first astrological author (following the astronomer Hipparchus some three centuries earlier) argued that the Sun’s position at the vernal equinox should be used to define the beginning of the zodiac, or 0° Aries. This definition constitutes the tropical or moving zodiac, which is fixed in relation to the seasons but has the constellations slowly progressing through the signs.
Ptolemy’s reform was part of his striving to make astrology conform to the science of his time by anchoring it firmly to seasonal changes and the fourfold qualities of hot, cold, moist and dry. In so doing, he formulated definitions which are in fact applicable only to the northern hemisphere and thereby make astrology less universal. The Sun’s passage through Cancer and Leo may mark the hottest season of the year around the Mediterranean, but this is simultaneously the coldest season in New Zealand; and in areas close to the equator, there are no seasons at all in Ptolemy’s sense.
The difference was of little practical consequence in Ptolemy’s day, when the equinox point was in fact approaching the beginning of Aries in the fixed zodiac; but today the two definitions have grown around 25° apart (depending on which star or stars are used exactly to define the sidereal zodiac), so that the tropical signs are now almost wholly out of alignment with the constellations after which they were originally named.
The tropical zodiac was not an immediate success. For several centuries after Ptolemy, the fixed or sidereal zodiac continued in use; and there is even evidence of Arabic authors in the 8th–9th centuries using tropical and sidereal tables side by side to calculate planetary positions – for one and the same chart! The simple fact is that authors of earlier times were not always aware of precession, and often defined the zodiacal signs simultaneously in relation to the fixed constellations and in relation to the seasons. Eventually, however, Ptolemy’s tables, calculated in the tropical zodiac, became the standard outside of India; and by the time Arabic astrology was adopted by the Latin west in the 11th–12th centuries, the memory of the sidereal zodiac had faded away – although perhaps not completely.
As late as 1573, Francesco Giuntini (known in Latin as Junctinus) in his Speculum Astrologiae advocated the use of sidereal positions in annual revolutions, ascribing the idea, ironically enough, to Ptolemy:
My personal preference is for a fixed or sidereal zodiac, which (like Giuntini) I find both more theoretically valid and more useful in practice. If employing such an ‘Indian’ zodiac with astrological techniques known today as ‘western’ seems strangely eclectic or untraditional to some, it would not have appeared so in Baghdad or Saurashtra a millennium ago, nor in Alexandria a millennium before that.