The Precession of the Equinoxes
The Earth doesn’t so much shift on its axis as it wobbles. The Earth’s axis is tilted at an angle of approximately 23.5° to the plane of the ecliptic. This tilt is what produces the seasonal variations. The Earth is also not a perfect sphere; it bulges in the middle near the Equator. This unequal distribution of mass causes the Earth to wobble around its rotational axis like a gyroscope. What this means is that the Earth’s axis makes its own rotation, with the North and South Poles slowly describing a circle around the ecliptic pole (which is the pole exactly perpendicular to the plane of the ecliptic; the North and South poles, remember are tilted 23.5 degrees away from this plane). How slowly? Well, a complete cycle takes about 25,800 years. The precession can also be seen in terms of the North Star. Currently the North Pole of the Earth is aligned with the fixed star Polaris. This was not the case 3,000 years ago; and by the year 14,000 A.D., the North Star will be Vega, not Polaris.
This rotation of the Earth’s axis occurs at something like 1° every 71.5 years (about 5 seconds of arc per year). The wobble and the precession of the equinoxes were known to the Ancient Egyptians, although the first “official” discovery of it was made by an Ancient Greek astronomer, Hipparchus, who was born sometime around 190 B.C. It was noted because the Sun was in a slightly earlier position at the time of the Spring Equinox each year (as measured against the fixed stars). Because the movement slips backwards through the zodiac, it is called precession (as opposed to a forward-movement which would be called progression).
Now 1° every 71.5 years doesn’t sound like too much, but it certainly adds up over 2,000 years or so, and this is where we get into the different Zodiac systems.
The Tropical Zodiac and the Sidereal Zodiac
The ecliptic is a circle, and the thing about a circle is that it doesn’t have a beginning or an end. If you want to be able to measure something along a circle, you have to establish some sort of a reference point. The Zodiac as we know it today was first used by the Ancient Greeks over 2,000 years ago. Their year began with the Spring Equinox, and so it made sense to pick that point—that is, the point in the sky where the Sun appeared to be at the time of the Spring Equinox, as the reference point, and then divide the ecliptic into 12 equal segments from there. At the time, the Spring Equinox occurred when the Sun was in the band of the ecliptic that also included part of the Constellation of Aries. The first 30-degree division of the ecliptic was named “Aries,” and the remaining eleven segments were likewise named after the well-known and easily-recognized constellations that roughly corresponded in sequence. The Greeks never used the actual constellations to measure the positions of the planets, however, because the constellations did not divide the ecliptic into equal segments.
The type of astrology practiced at the time was entirely based on cycles. Each of the Signs of the Zodiac was associated with the type of qualities and energy that were experienced during the corresponding time of the year. The foundation of the interpretations of the Signs was seasonal. The Greeks were well aware of the precession of the equinoxes; however, as their system of astrology was based on the seasonal cycles, it did not concern them. Because this Zodiac begins with the Vernal Point, and the Spring Equinox, when the Sun is directly overhead at the Tropic of Cancer, this Zodiac is called the Tropical Zodiac, or the Seasonal Zodiac.
Although the vast majority of Western Astrologers uses the Tropical Zodiac, it is not the only Zodiac system. The Sidereal Zodiac (Sidereal = Star) does take the precession of the equinoxes into account, and rather than beginning its cycle at the point of the Spring Equinox each year, it begins when the Sun aligns with a Fixed Star in the Constellation of Aries. The Sidereal Zodiac is also known as the Fixed Zodiac.
While astrology was developing in the West, it was also developing in the East. Hindu astrology, called Vedic astrology or Jyotish astrology has always used the Sidereal Zodiac. Jyotish astrology has an entirely different set of techniques and interpretations for the signs and planets. The fundamentals may be the same as in Western Astrology, but the similarity ends there.
In the 1930’s, Cyril Fagan began to advocate using the Sidereal Zodiac in Western Astrology, rather than using the Tropical Zodiac. Although definitely in the minority, there are many astrologers who practice Western Sidereal Astrology, using basically the same interpretations for the signs and the planets, but an entirely different measurement system. Currently, the difference between the Tropical Zodiac and the Sidereal Zodiac is about 23°. What this means is that the Spring Equinox, which occurs at 0° of Aries (Tropical) actually occurs at about 7° of the Sidereal Sign of Pisces. Because no one can agree as to the exact location of the start of the Constellation of Aries, and therefore to the point where the Sidereal Zodiac would begin, the Sidereal Zodiac is calculated backwards from the Vernal Point, using one of many different ayanamsas.
Tropical Astrology and Western Sidereal Astrology have fundamentally different approaches to the symbolism and interpretation of the Signs. Tropical Astrology believes that the qualities associated with the signs are linked to the seasons, rather than to the fixed stars, and therefore the precession of the equinoxes and the growing difference between the Tropical Signs and the relative positions of their namesake constellations is of no consequence. Sidereal Astrologers (both Western and Eastern) believe that the qualities of the signs are not related to the seasons, but rather to the specific portions of the ecliptic as measured against the fixed stars.
With respect to the question of the accuracy of ancient charts and interpretations, we only need to remember what Zodiac system was used at the time, and keep things in context. The Western Astrological tradition, which includes the Greeks, the Europeans, the English (in the Middle Ages), and the Americans in more recent years, is based on the Tropical Zodiac. Therefore, all charts and interpretations from these times and places would be as accurate and valid today as they were then. Furthermore, the date, time and location information can be used to calculate a “modern” version of the ancient charts with no adjustments (except for the necessary conversions to translate the more ancient dates into the modern calendar).
Any charts from the Eastern tradition, however, as well as any Western Sidereal charts (post 1930’s) would require adjustments based on the precession of the equinoxes. The difference between the Tropical zodiac and the Sidereal Zodiac changes each year, and the degree of precession would have to be taken into account for the date of the chart. This would be rather nightmarish to try and calculate by hand; fortunately, most computer astrology programs that offer a Sidereal Zodiac option take this into account and can produce accurate Sidereal charts for any time or place.
A Digression: The Age of Aquarius
The precession of the equinoxes has to do with more than just the two different zodiac systems. As the equinoxes precess, they relate to the Great Ages of Man. These Ages mark different periods where significant evolutionary changes occurred. The Ages are defined by the Sidereal Sign that is the current location of the Vernal Point. Currently, the Spring Equinox (0° of Aries in the Tropical Zodiac) occurs at about 7° of the Sidereal Sign of Pisces, and we are currently very much in the Age of Pisces, where we will stay for another 150-300 years or so until the Spring Equinox precesses into the Sidereal Sign of Aquarius, which will mark the beginning of the Age of Aquarius. (Even agreeing on this definition of the “Ages” there is much dispute as to the actual year that the “Age of Aquarius” will begin. The reasons and reasoning for this aren’t terribly important to this discussion. Suffice it to say that it’s not terribly likely that any of us will still be here to witness it.)
Each Great Age is associated with a major evolutionary and cultural advancement of the species. In the Age of Gemini, language was developed. In the Age of Taurus, agriculture was discovered, and for the first time, towns, villages, and cities were formed because humans no longer needed to hunt and gather for their food and so were not required to be so nomadic. The Age of Aries ushered in wars and warfare, violence and conquest. The Age of Pisces has been dominated largely by religion, Christianity in particular, with its peculiar mixture of persecution and spiritual salvation. The general thoughts about the Age of Aquarius are that it will mark a period of enlightenment and freedom. But once again, even the most generous estimates put this off for another 100 years at least.