The Precession of the equinoxes is a process whereby the position of the stars and constellations gradually move in relation the seasons. This means that the twelve astrology signs that once roughly aligned with the constellations of the same name, no longer do.
The word Zodiac comes from the Greek zõdiakos (animals) kuklo (circle). It refers to the constellations of stars around the ecliptic. The ecliptic is the path that the Sun, Moon and planets take through the stars as viewed from Earth. Astronomers still refer to the zodiac as this path through the stars and constellations.
Astrologers refer to the zodiac as the 12 equal Sun signs of celestial longitude. Once upon a time, astronomy and astrology were one, and the shamen, druids, or priests simply divided up this ring of stars into varying numbers of constellations depending upon the culture.
For example, in ancient Babylon the circle was divided up into 17 or 18 constellations of varying length. In ancient India, this ring of stars was divided up into 27 or 28 lunar mansions of varying length based on bright stars.
Astrology is the study of the effects that celestial objects like planets and stars have on us down here on Earth. It was always based on the spatial relationships between the planets and between the planets and the stars. That is why the original zodiacs were star based. Because some cultures evolved from a nomadic to a farming way of life, a new zodiac was developed based on seasons.
It was a calender really and was eventually divided up into 12 equal segments which is the zodiac we use today. It was developed around the 7th century BC and as an ecliptic coordinate system, it “was a convenient celestial measuring device, unrelated to astrological activity…it was being used at that time for the recording of astronomical data, yet observation of the effects of the planets for astrological purposes continued to be related to the visible constellations.” 
The 12 equal segments in this calendrical measuring device were named after the twelve major constellations around the ecliptic, Aries, Taurus etc because the sort of roughly fitted in at the time. It was a very rough fit though, constellation Virgo is 46 degrees in length while constellation Cancer is only 4 degrees long across the ecliptic. Gradually the astrological meaning of the constellations was transferred to this new measuring device and the thing was given interpretive meaning.
The divisions are called sun signs, named after the constellations but now not at all related to them, purely symbolic. What we know as horoscopic astrology today is the result, and most astrology, Western and Vedic, traditional and modern, comes from this. All the rules such as elements, sign rulers, detriment etc are based on these zodiac signs, or Sun signs.
These abstract ideas were cemented in a book called Tetrabiblos by Claudius Ptolemy (Ptolemy) in the 2nd century AD. “he adopted an astronomical calculation concerning the starting point of the zodiac which resulted in a ficticious moving (tropical) zodiac being popularised” which “became the basis of the birth of modern Western astrology” .
Now astrology shows little resemblance to it’s origins. The signs of the zodiac we use today do not relate to the constellations from which they derive their name and meaning. The idea of having 12 equal segments reflecting the influence of the stars is a nonsense. Even stars within a single constellation can have very different influences, and of course there are many more constellations to consider that are not on the ecliptic. To make things even more ridiculous, precession of the equinoxes in the last 2,000 years means the constellations are a whole sign away from the sun signs bearing their name.
Precession is the movement of the stars in relations to the ecliptic. They move forward through our tropical zodiac by about 5 minutes every 6 years, so in one lifetime, or 72 years a star will move one degree forward through the zodiac. This is because the Earth is tilted and moves around this axis every 26,000 years.
It a bit hard to work it out or visualize, but this calendrical zodiac we have is based on the path of the Sun through the ecliptic (red line). The equator of the Earth’s rotational axis is the white line. Where they intersect is the equinoctial point and it is always zero degrees Aries and Libra in the tropical zodiac, but the stars on those points keep changing.
Vedic astrologers get around this problem by using a sidereal zodiac, it keeps moving with precession. It’s a patch up job which tries to resolve the problem and keep the Sun signs matched up with the constellations of the same name, but this system is still based on the incorrect assumption that these 12 equal divisions reflect the influence of the irregularly shaped constellations and the rules of Vedic astrology are based on this. When the tropical zodiac was developed, zero degrees of Aries matched up with the first star in constellation Aries. Mesarthim, “the First Star in Aries, as at one time nearest the equinoctial point (392 BC)” . That star is now at 3 degrees Taurus.
Ptolemy and the Hellenistic astrologers that followed knew about precession but they chose to ignore it. “There was, perhaps, a more pertinent reason why the astrologers of the early Classical period chose to ignore this matter, one that suggests that by covering up this ‘inherent flaw’ of the zodiac, they were also obscuring a philosophical problem that had wider implications…It was tempting indeed to turn a blind eye, to let some other astrologer deal with it in some future generation when it really mattered.”  It really matters now. Astronomers and skeptics can easily make astrology a laughing stock because of precession and they continually do so.
Marina and I use the zodiac at it was originally intended. We prefer to use the tropical zodiac to describe the position of the stars and planets. The degrees of the tropical zodiac are a universal way for astrologers to communicate where a planet or star is, that’s all. The interpretation comes from the relationship between the planets – the aspects, and from the influence of the fixed stars.
What we are doing is nothing new. “The great astrologer Johannes Kepler had a solution to this problem – he basically abandoned the zodiac and built his astrological system around the aspects between the planets. He thought that the zodiac itself was merely a human geometry exercise and primarily served to aid the memory of astrologers as they computed aspects in their heads” .
We also correct for precession to work out transits. Robert Hand, one of the most respected astrologers in the world says“it is possible to treat the tropical zodiac as if it were moving. In other words, one should treat the natal positions of the planets as if they were fixed stars…My own experience has been that in timing an event the corrected positions are more accurate than the uncorrected ones” . 
Nearly all astrologers continue to use the Sun signs for interpretation but it has to be understood that these interpretations are purely symbolic, not based on our relationship to the cosmos. It is similar in a way to the use of the cards by a Tarot reader, it aids the intuition to help in a reading.
Taking the meaning of the signs a step further by using rulerships and detriments, even saying someone is lacking in an element such as fire is fundamentally flawed. Astrology is based on the principle that the stars and planets have an effect on us down here, summed up by the words of Hermes Trismegtistus, “As above, so below”. It is the planets moving across the backdrop of the stars which influence our lives, not the planets moving through imagined 30 degree divisions on a piece of paper.
13th Zodiac Sign Ophiuchus - Our astrology techniques
 Heavenly Imprints, Deborah Houlding.
 Cosmologers Blog – Ptolemy’s mistake.
 Star Names, Their Lore and Meaning, Richard Hinckley Allen, 1889, p.82.
 The Zodiac: Sidereal Vs. Tropical, Bruce Scofield.
 Planets in Transit, Robert Hand, 2001, p.29-30.