Tropical vs. Sidereal Debbie Asks: “I found on-line a scientist that stated the charts that were devised in ancient times are invalid today, but they are still used. He says that the Earth shifts on its axis, and that makes the constellations go into the Zodiac at the wrong dates. Like March 23 should be Aries, but in his mind the Sun is in Pisces. Can you explain what, if any adjustments for all of this have been made over time? This is the only thing negative that I ever hear concerning the accuracy of astrology, and I would like to understand what’s going on.” Kevin Answers: Debbie, Thank you for an excellent question. This question, in fact, is so fundamental to the understanding of astrology and is the source of so many misunderstandings and misconceptions, that I’ve also included this response as part of the FAQs (frequently asked questions) part of my web site. Answering this question, however, is going to require some astronomy as well as some astrology, and the clarification of some terms. Let’s start with the definitions; your terminology in your question was a bit confused, and I know that you’re not alone in this. Definitions and Glossary of Terms Celestial Sphere. The ancient understanding of the universe was quite different than our modern understanding of it. First of all, the ancients believed that the Earth was the center of the universe, and that everything revolved around the Earth. They also believed that a vast black sphere that contained all of the stars, called the Celestial Sphere, surrounded the Earth. Even though this is obviously not the way things really are, projecting an imaginary Celestial Sphere onto the night sky makes it possible for astrologers and astronomers to measure, track and calculate the relative positions of the stars and planets as they appear from the Earth. Geo-Centric. Geo-Centric literally means “the Earth in the Center” and this is the approach that is used by most astrologers, and also by astronomers when measuring and observing the stars and planets. On a practical level, it simply means that the sky is being observed from the Earth, and that measurements are based on spherical geometry and the use of the Celestial Sphere. Great Circle. A Great Circle is any circle that divides a sphere (or in particular the Celestial Sphere) into two equal halves. The equator is a Great Circle, dividing the sphere of the Earth into two halves. All lines of Longitude are also Great Circles (connecting the North and South poles). Lines of Latitude (except for the equator), however, are not Great Circles. Fixed Stars. When the ancients observed the night sky, they noticed that some of the stars seemed to move or wander from night to night. These, of course, were the planets. The way that they were able to determine that the planets moved, however, was because they noticed that the rest of the stars in the sky stayed in the same positions night after night. These are the fixed stars, and they are used as reference points in order to measure the relative positions and movement of the planets. Constellations. Constellations are groups of fixed stars that have become associated with a figure, and often with a myth. The Big Dipper, the Little Dipper, and Orion are probably the best-known and most easily recognized constellations in the night sky (at least in North America). Different constellations are visible at different times from different locations on the Earth. There are literally hundreds of constellations. Among these are the constellations of Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. (As we will discover soon, these constellations are not the same as the Signs of the Zodiac that share these names.) Ecliptic. The Ecliptic is the Great Circle that describes the apparent path of the Sun around the Earth (but which is really the orbit of the Earth around the Sun. The Ecliptic extends approximately 8-9° of arc above and below (North and South of) the actual path of the Earth/Sun. The other planets in the solar system are always visible within this band of sky. The longitudinal (East-West) position of celestial bodies (i.e. planets, asteroids, etc.) is measured along the ecliptic. Signs. The Signs are units of measurement each equal to 30 degrees of arc along the ecliptic. Zodiac. The Zodiac refers to the different names for the Signs dividing the ecliptic. The Signs of the Zodiac are named after twelve of the Constellations that intersect the ecliptic. Vernal Point. The point measured along the ecliptic that represents the apparent position of the Sun at the moment of the Vernal (Spring) Equinox. The Precession of the Equinoxes When you mentioned that the scientist stated that “the Earth shifts on its axis and that makes the constellations go into the Zodiac at the wrong dates,” what is being described is the precession of the equinoxes. Just to clarify, though, the constellations don’t move—remember, they are made up of fixed stars. The Sun is what appears to enter the different Signs of the Zodiac. As to the part about the dates and the Zodiac, we’ll get to that shortly. 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. Myths, Misconceptions and Misinformation The fact that the Signs of the Zodiac share the same names as 12 of the constellations, and were, in fact, named after the constellations, has resulted in the popular misconception that the signs are the same thing as the constellations. This fallacy has given rise to all sorts of pseudo-scientific attacks on the validity of astrology, all of which come from individuals who do not understand astrology in the first place. Some have even come from a small faction of Western Sidereal astrologers who attempt to discredit Tropical astrology. I’ll list some of the most popular examples below, and then, since they all can be explained or refuted by the same information, tackle them all at once. “The 13th Sign of the Zodiac.” This one pops up in the media from time to time. Sometimes it takes the angle that a new sign of the zodiac has been “discovered.” Other times, it’s used as an argument by skeptics attempting to discredit astrology. What it reefers to is the Constellation of Ophicuchus, which also intersects the ecliptic, and which actually occupies more space along the ecliptic than the Constellation of Scorpio. “The Sidereal Zodiac is the only ‘real’ zodiac because it uses the constellations and not imaginary divisions of the ecliptic.” “How can you say that ‘Jupiter is in Libra’ when I can look up in the sky and see it clearly in Virgo?” “The Zodiac has all of the dates wrong because of the Precession of the Equinoxes.” Part of this is addressed above when the difference between the Tropical and the Sidereal Zodiacs is covered. The rest will be addressed below. The data in the following table was published by Dr. Lee T. Shapiro, Director Morehead Planetarium, CB #3480 Morehead Building, University of North Carolina, Chapel Hill, NC 27599-3480. The dates and days refer to the time that the Sun appears to spend in each of the constellations. I took the days (based on a 365 day year) and converted them to the corresponding arcs that each constellation occupies along the ecliptic. I also included the approximate dates that the Sun enters each of the Signs, both in the Tropical Zodiac and also in the Sidereal Zodiac. Constellation Sun Enters/Leaves # of Days # of Degrees Tropical Dates Sidereal Dates Aries Apr 19 - May 13 25 24.66 Mar 21 - Apr 20 Apr 14 - May 14 Taurus May 14 - Jun 19 37 36.49 Apr 21 - May 21 May 15 - Jun 14 Gemini Jun 20 - Jul 20 31 20.58 May 22 - Jun 21 Jun 15 - Jul 15 Cancer Jul 21 - Aug 9 20 19.73 Jun 22 - Jul 22 Jul 16 - Aug 16 Leo Aug 10 - Sep 15 37 36.49 Jul 23 - Aug 22 Aug 17 - Sep 16 Virgo Sep 16 - Oct 30 45 44.38 Aug 23 - Sep 23 Sep 17 - Oct 16 Libra Oct 31 - Nov 22 23 22.69 Sep 24 - Oct 23 Oct 17 - Nov 15 Scorpio Nov 23 - Nov 29 7 6.9 Oct 24 - Nov 22 Nov 16 - Dec 15 Ophichchus Nov 30 - Dec 17 18 17.75 N/A N/A Sagittarius Dec 18 - Jan 18 32 31.56 Nov 23 - Dec 21 Dec 16 - Jan 13 Capricorn Jan 19 - Feb 15 28 27.62 Dec 22 - Jan 20 Jan 14 - Feb 12 Aquarius Feb 16 - Mar 11 24 23.67 Jan 21 - Feb 19 Feb 13 - Mar 12 Pisces Mar 12 - Apr 18 38 37.48 Feb 20 - Mar 20 Mar 13 - Apr 13 The table should illustrate clearly the difference between the signs and the constellations. The signs, you will remember, are units of measurement, each consisting of 30 degrees of arc. The constellations may take up an average of about 30 degrees of arc each, but they certainly aren’t very useful as units of measurement. This is also the answer to the “13th Sign” myth. While there are most certainly 13 constellations that cross the ecliptic, the signs are not the same thing as the constellations. Why the Ancients chose to name the 8th Sign after Scorpio (which barely takes up 7° of arc) rather than Ophicuchus (which covers a more respectable 17.75°) will probably remain a mystery. It should also be obvious from looking at this table that the Sidereal Zodiac does not rely on the constellations any more than the Tropical Zodiac does. While there is certainly a greater correlation between the Sidereal Signs and the constellations along the ecliptic, again, the constellations do not divide the ecliptic into equal segments and therefore they are not used as the basis for the Sidereal Zodiac. In fact, there even appears to be a discrepancy between when Dr. Shapiro notes that the Sun Enters the Constellation of Aries and when the Sidereal Sign of Aries is thought to begin (for the year 1997). The discrepancy between where astrologers place a planet in the night sky, and where astronomers place that same planet is also related to the difference between the Constellations, the Tropical Zodiac and the Sidereal Zodiac. Based on the dates in the table, on October 24, the Sun would be found in the Constellation of Virgo, the Tropical Sign of Scorpio, and the Sidereal Sign of Libra. And finally, the argument that astrology can’t work because the precession of the equinoxes make it invalid, or at least wildly inaccurate (which is essentially what the “scientist” who prompted Debbie’s question was referring to), simply brings up the difference between the Tropical and the Sidereal Zodiacs. Once again, although the Tropical and the Sidereal Zodiacs are very different, they each represent an entirely valid system of astrology.