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The differences between Sidereal and Tropical astrology are perhaps not so great to the astrologer, but they can be vast to the layperson and the implications to the field of astrology are far-reaching.
I am by no means the expert and certainly not as experienced as a great many other astrologers, but studying the history and the background behind the Sidereal and Tropical systems has profoundly impacted my use of astrology and the direction of my studies.
The divergence between the two systems dates to the time of Ptolemy, who is credited with creating the Tropical system. Prior to that, astrology used the Sidereal zodiac of the Babylonians.
Now, when Ptolemy created the Tropical system, its purpose was primarily agricultural in nature: creating a calendar system that could be used to plan planting and harvesting consistently from year to year, and it is still well-suited to this purpose today.
It is important to note that at the time the Tropical zodiac was created, 0° Aries was perfectly aligned with the vernal point. There was no discrepancy. They were, in essence, the same system. And, because the procession creates a discrepancy of somewhat more than 1° per century, it likely wasn’t given all that much thought in Ptolemy’s time.
By precession the equinox has, in the interim, drifted from that point and is now approximately 24° precessed. In 2015, for example, the vernal point is at 8°46’ Pisces.
Now that you have a bit of the history and the scientific explanation of the difference, I’ll share what I’ve come to understand about the two systems. I make no claims to “right” or “wrong”, because I think that only serves to inflame the “us vs. them” paradigm and that ultimately doesn’t serve anyone.  I will say that there are a number of astrologers who use both systems.
tropical chart 2015 vernal point
March 20, 2015 Tropical chart  
Sidereal chart 2015 vernal point
March 20, 2015 Sidereal chart  


This comparison is from the standpoint of a Western style chart. Bear in mind that there are more similarities than differences.

  • Planetary aspects and aspects to midpoints, angles, or other parts of the chart are no different.
A square is a square is a square, a square between Saturn and Uranus in one system is still a square between Saturn and Uranus in the other.
  • The houses are no different between systems.
In fact, houses have no bearing on the discussion whatsoever. Planet in house interpretations are therefore unaffected, and you can see that Saturn square Uranus from the 1st House in one system is still Saturn square Uranus from the 1st House in the other, provided you are using the same house system in your comparisons.
  • Transits and progressions are essentially unaffected.
There are still 360° in the zodiac, and the celestial bodies all still move at the same relative rate of travel.
Where the difference becomes apparent is in the placement of the signs:
  • The signs of Sun and the Ascendant, and other celestial bodies and chart points as well.
  • The signs on the house cusps.
So the main differences are four fold, three of which affect interpretation:
1.)   The Sun sign and ascendant—the main expressions of self—for those folks born with the Sun or the Ascendant in the first 24° of a Tropical sign.
2.)   The planetary (or other) interpretations, for folks with planets (or chart points) in the first 24° of a Tropical sign.
3.)   The interpretations for signs on house cusps (house rulers).
4.)   With a Sidereal zodiac, there is no need for a separate method of calculating charts for southern latitudes.*

So you can see, the dynamics are essentially the same, but howthose dynamics are expressed is the main difference. So there is really no impact on any predictive aspect in regards to timing. The themes are what change, in the way we interpret the energies to be expressed according to sign. And for the time being, there’s still 6° of overlap even there.

To give an example of how this has played out in my own life, I went the first 30+ years of my life thinking I had an Aquarius Sun and Cancer ascendant. I have a favorite astrologer whose interpretations I’ve followed pretty consistently for the past 6 or 8 years or so.

My experience played out the opposite of what you might expect from reading the above explanation: in my most recent readings the essential themes of her writing seemed pretty true to life, but the larger events and circumstances never panned out the way she predicted–for my husband, yes–but not for me. Of course, his Aries Sun doesn’t shift signs from one zodiac to the other.
I was pretty confused, needless to say. Until I dug deeper into Sidereal astrology and double checked my birth calculations, and suddenly things clicked into place. I diligently retraced my steps and re-read past interpretations for Capricorn and Gemini, and my “aha” moment was so big I experienced an unexpected moment of grief. . . here I had been looking right when approaching every intersection, when I ought have been looking left. No wonder I felt blindsided so much of the time! Those themes were not only reflective of the larger themes that had been playing out in my life, the description of circumstances and events fit to a “T”.
Now—as to why the “wrong” themes seemed to fit before my paradigm shift—a simple progression of my Sidereal natal chart put my natal Sun transiting Aquarius and my natal ascendant transiting Cancer.
Personal experiences aside, I have a few observations to make about the consequences of the dichotomy between the Sidereal and Tropical zodiacs.
One, it is highly unlikely that astrology as a science will gain any degree of credibility or serious inquiry within traditional scientific circles as long as the Tropical zodiac remains the most widespread and commercial variant in use in the West. The Sidereal/Tropical dichotomy is the most widely cited argument used to discredit astrology, its use, and study; i.e. “it’s not based on astronomical fact”.
Another observation I’ve made since beginning a deeper study of astrological archetypes: on more than one occasion I’ve come across writings by modern astrologers—primarily online—where characteristics more accurately descriptive of one particular sign were attributed to the neighboring sign. This is a natural consequence of a system which is progressively diverging from its foundational basis.
And finally—perhaps most obviously—the degree of discrepancy will only continue to grow over the course of the next 24,000 or so years, until they match up again. I imagine that eventually it will grow large enough that the broader astrological community will have difficulty ignoring it. It is an issue with generational implications similar in nature to that of climate change, unquestionably man-made, and just as polarizing.






 * I doubt Ptolemy had ever been to Australia. He probably didn’t care that spring in Greece was autumn down under. 


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