The foundational texts of Indian Astrology unanimously and explicitly define the zodiac based on solstices and equinoxes (i.e. tropical). On the other hand, they also unanimously define the nakshatra based on stars (i.e. sidereal). The current difference between the two - the solstices (ayana) and their marker stars - is described as "ayanamsha."
Problem: For many centuries Indian astrologers have applied ayanāṁśa to both the nakshatra and the zodiac - fuseing the two and divorcing the zodiac from its tropical anchors.
Solution: Reform... which requires patience, clarity, contemplation and humility from all parties.
bhacakra-nābhau viṣuvad dvitiyam samasūtragam
ayana dvitayan caiva catasraḥ prathitāstutāḥ
The two solstices and two equinoxes are the thread upon which the core of the zodiac exists.
tad antareṣu saṅkrānti dvitayam dvitayam punaḥ
Two borders are between each point.
bhānor makara saṅkrānteḥ ṣaṇmāsā uttarāyaṇam
karkādes tu tatha vasyāt ṣaṇmāsā dakṣināyaṇam
It takes the Sun six months to reach the northern solstice after entering Capricorn; and six months to reach the southern solstice after entering Cancer.
meṣādyo dvādaśaite māsāstair eva vatsaraḥ
The twelve divisions beginning from Aries are certainly the months which constitute a year.
Text 7 compares the zodiac to a wheel, and says the hub of that wheel is the intersection of lines drawn between two equinoxes and two solstices. A wheel and its hub move as a unit, the movement of the wheel determined by the rotation of the axle at it's hub. This beautifully depicts the tropical zodiac, anchored to solstices and equinoxes.
Text 10 states that the spatial divisions of the zodiac have a direct temporal effect as the divisions of the year into months. This refutes the claim that there is a different astronomy behind the calendar (tropical) and the zodiac (sidereal).
Division 5, Chapter 21, Text 3:
sa eṣa udagayana-dakṣiṇāyana-vaiṣuvata-saṁjñābhir māndya-śaighrya-samānābhir gatibhir
The sun moves at varying speed between the northern solstice, southern solstice, and equinotical points.
ārohaṇāvarohaṇa-samāna-sthāneṣu yathā-savanam abhipadyamāno
Its rise towards the northern solstice, its fall towards the southern solstice, and its crossing the middle point can be known by measuring the days.
makarādiṣu rāśiṣv aho-rātrāṇi dīrgha-hrasva-samānāni vidhatte.
The days and nights grow longer and shorter, revealing which of the signs – Capricorn, etc. – the Sun is in.
yadā meṣa-tulayor vartate tadāho-rātrāṇi samānāni bhavanti
When the Sun is in Aries and Libra, the days and nights are equal.
yadā vṛṣabhādiṣu pañcasu ca rāśiṣu carati
tadāhāny eva vardhante
hrasati ca māsi māsy ekaikā ghaṭikā rātriṣu.
When the Sun is in the five signs starting from Taurus, the days grow longer and nights which grow shorter, month by month.
yadā vṛścikādiṣu pañcasu vartate
tadāho-rātrāṇi viparyayāṇi bhavanti.
When the Sun is in the five signs starting from Scorpio, the days and nights change in the opposite way.
This establishes the technique for ascertaining the Sun's position in the Zodiac, which thereby establishes the Zodiac borders themselves. The technique is to measure the lengths of days and nights. This enables us to find equinoxes and solstices, the Sun's position at which establishes the borders of the cardinal signs.
This text from Bhāgavata Purāṇa appears almost verbatium in many other Purāṇa, Like Viṣṇu and Padma. No Purāṇa presents a different zodiac definition.
Some argue that Surya Siddhanta Defines a Sidereal Zodiac in Chapter 1, texts 27 and 28. They say 1.27 says to use the end of Revati as the reference point. 1.28 says to count a complete revolution to and from that point by dividing it into 12 “rāśi”, then dividing each of those into 30 “bhaga”, and each of those into 60 “kala”, and each of those into 60 “vikala.”
This is not a definition of a sidereal zodiac. These videos translate and explain the relevant sections from Sūrya Siddhānta.
We don’t find any word that could be a reference to “the zodiac” in 1.27-28, or in any of the surrounding context. Compare this with Sūrya-Siddhānta’s tropical definition of the zodiac, which begins with the phrase bha-chakra – a well documented term for the zodiac (lit. “celestial wheel”).
What about "bhagaṇa"?
In this context, that word means “revolution”, “complete orbit.”
What about “60 seconds in 60 minutes in 30 degrees in 12 arcs in 1 revolution” - isn't this a reference to the zodiac?
It would be, if the zodiac were the only astronomical system to have this 60:60:30:12:1 structure. However, it is not. The entire first chapter of Sūrya Siddhānta is full of astronomical and chronological structures based on multiples of these numbers, with no reference at all to the zodiac.
In 1.28-27 we don’t find the names of any zodiac signs. In contrast, in Sūrya Siddhānta's tropical definition explicitly names three of the four cardinal signs.
What about "rāśi"?
“Zodiac sign” is not the primary denotation of the word rāśi. The word means (see Monier-Williams dictionary, for example):
“Rāśi” is primarily a geometry term, with connotations in astronomy and astrology that allow it to also be used contextually as a noun for a zodiac sign. Considering the lack of any reference to “Aries” etc. or any reference to “the zodic” in this chapter, it is less reasonable to understand the word rāśi as “zodiac sign” in this context, and more reasonable to understand it as a geometric “arc”, especially since the statement occurs when explaining how to measure orbits and arcs.
If we consider the context surrounding 1.27-28, we won’t mistake it as a definition of the zodiac.
Chapter One begins by explaining units of time on increasingly large scales. It begins with a biological scale of “breaths”, “semi-pulses”, “pulses”, and “days” (The ratio is 6:60:60:1). Next it describes a terrestrial scale of time: days, months, and years (30:12:1). Then it goes to an ecliptic scale of the same days, months and years (30:12:1). It then escalates to large scale amalgams of ecliptic years – yuga, manvantara and kalpa (the multiple used here is 12,000). Finally, it describes the ultimate universal days, months and years of Brahmā (30:12:1). Considering this, it does not seem surprising anymore to find the so-called “zodiac signature” (60:60:30:12:1) in 1.28.
After establishing this chrono-spatial matrix, the chapter then sets out to locate the reader correctly in the “cosmic clock” it just defined. Texts 27 begins the section where astronomy can be used to finely pinpoint our current place in cosmic time. It says:
If 1.27-28 defines a “zodiac,” then it is a “zodiac” with a very specific, exclusive use: to measure sidereal revolutions in comparison to a mean conjunction for the sake of establishing the amount of time passed since that conjunction. This “zodiac” is not a product of the four cardinal celestial directions, and hence it’s 12 divisions do not have symbolic meaning. In other words, it is a “zodiac” in geometry only.
If one inquires why the mean conjunction and the revolutions are defined in reference to a star (sidereally) rather than a cardinal celestial direction (tropically) the simple answer is that visual astronomy (dṛk-siddhānta) is much simpler in reference to visible things, stars.
This is not a definition of a sidereal zodiac.
It does show that Sūrya Siddhānta calculates orbital revolutions sidereally. However, to use those coordinates in reference to zodiac divisions, once must utilize ayanāṁśa.
Chapter 3, texts 2-5
Some argue that Bṛhat Parāśara Hora defines a sidereal zodiac in Chapter 3, texts 2-6.
śṛṇu vipra pavakṣyāmi, bha-grahānāṁ paristhitim
Listen, scholar, I will explain all about the stars and planets.
ākāśe yāni dṛśyante, jyotir-bimbāny anekaśaḥ
In the sky we see many dots of light.
teṣu nakṣatra-saṁjñāni graha-saṁjñāni kānicit
Some of them are known as “stars” (nakṣatra), others as “planets” (graha).
Rāśī are not mentioned here. This establishes that the author does not consider them to be "dots of light in the sky" (stars). Hence, it is already evident that the author does not intend to define the zodiac as a stellar (sidereal) entity.
tani nakṣatra-nāmāni sthira-sthānāni yāni vai
"Star" (nakṣatra) is the name for those that stay fixed in their place.
gacchanto bhāni gṛhlanti, satataṁ ye tu te grahāḥ
But those that always move independently (gṛhl), are “planets” (graha).
The word tu ("but") conveys that planets are different from stars, although they have something in common. What they have in common is that they are both "dots of light" in the sky (jyoti-bimba), but what distinguishes them is that stars don't move independently like planets do.
bha-cakrasya nagāśvy aṁśa aśviny ādi-samāhvayāḥ
The lights move as a unit, like a wheel turned by a horse.
Its segments are known as "the horsemen" (aśvinī), etc.
This line clarifies that "planets" move independently ("gṛhlanti"), but stars move together - keeping the same distances from one another, exactly like spokes on a wheel. This particular "wheel" has 27 spokes, named Aśvinī, and so on.
tad dvādaśa-vibhāgas tu tulya meṣādi-saṁjñakāḥ
But something similar to that can be divided into twelve,
known as Aries and so on.
"But" (tu) is always used to indicate something different that what was previously discussed. Thus, here the author says, "The stars and the signs (nakṣatra & rāśī) are similar but not identical." They are similar in that they both demarcate sections of the sky. They are also similar in both being unchanging, fixed things that move as a unit, and which the planets move in relation to. However, they are not identical because nakṣatra are based on stars (jyoti-bimba), and rāśi are not.
This reinforces and clarifies that the stars and the signs (nakṣatra & rāśī) are similar (tulya), not identical.
The author describes the signs as "vibhāga" - "differentiated divisions".
In contrast, he described the stars (nakṣatra) as "aṁśa" - "integral parts".
This reinforces his concept that signs and stars are similar but not identical. The nakshatra are integral do the stars (aṁśa) the rāśi are distinct from the stars (vibhāga).
The author opened this section by saying that planets and stars (not zodiac signs) can be seen in the sky.
First, we have the Vedas themselves, which used astrology to determine the correct time to perform important and sacred endeavors ("yajña"). Mostly they relied on 27 sidereal nakshatras, but whenever they utilized a 12-fold division, they did so in relation to solstice and equinox (i.e. they utilized only the tropical 12-fold division), as evidenced in Śatapatha Brāhmana.
Next, we have the astronomical sections of the Purāṇas, along with the Indian astronomical and astrological classics like Sūrya Siddhānta and Bṛhat Parāśara Hora Śāstra. As shown above, these unequivocally and uniformly define the zodiac as a solstice/equinox phenomenon - i.e. "tropical." (see above for exact quotes)
Classical astrologers such as Varāha Mihira and Aryabhata spoke in favor of a tropical zodiac. Aryabhata did so in the first statement of the fourth chapter of his book, Aryabhatiya. Varāha Mihira did so in Bṛhatsaṁhita (3.1-3) and Pañcasiddhāntikā (3.21 & 32) where he noted that the solstices are the markers for the beginning of Cancer and Capricorn, and these drift through the nakshatras over time, but the tradition developing around him was ignoring this in favor of just keeping things the same.
In the late 19th century, S.B. Dikshita, in his book, History of Indian Astronomy, Part II, concluded that the Indian calendar should be based on tropical zodiac signs, not sidereal – because of the importance of the solstices and equinoxes in Vedic religion. Dikshita recognized the unlikeliness of conservative Indian culture adopting such a radical change and suggested that a partial reform would be to standardize a singular sidereal zodiac on the citrapakṣa ayanāṁśa.
Dikshita’s opinion was echoed exactly by the official Calendar Reform Committee, led by N.C. Lahiri and commissioned by the Indian Government in 1950. The first and second resolution in the committee’s official report was that the start of the year (and thus the zodiac) should be marked by northerly equinox. But they conceded that an ayanamsha can be applied “for religious purposes… as a concession to the prevailing customs.”
Again on page 7 of the report, when elaborating on standardization of the religious calendar, the committee wrote,
“This recommendation [citrapakṣa ayanaṁśa] is to be regarded only as a measure of compromise, so that we avoid a violent break with the established custom. But it does not make our present seasons in the various months as they were in the days of Varahamihira or Kalidasa. It is hoped that at not a distant date, further reforms for locating the lunar and solar festivals in the seasons in which they were originally observed will be adopted.”
It is clear that the seven extremely highly-esteemed scientists and scholars commissioned in this committee agreed that the tropical zodiac was correct, but a sidereal zodiac would be required to accommodate the prevailing customs. Thus, they viewed the sidereal zodiac as a compromise with tradition. I encourage all readers to read the committee's full report. Many esteemed native opinions in favor of tropical orientation can be found there.
In 1930 G.V. Krishnaswami published a paper entitled, Reform of the Indian Calendars, in which he says, “The method of placing the ayanas and the months on a sidereal basis in the Indian calendars is defective and hence there is a difference of about 23 days in the calculations.”
Among contemporary people at least three scholars – Krishen Kaul, M.K.A. Patrakam, and T.V. Sivaraman speak very openly against the sidereal zodiac. Several important religious leaders also. These include: Sri Swarupananda Saraswathi Maharaj of Dwaraka Mutt, Sri Jayendra Saraswathi Swamigal of Kanchi Mutt and Sri Gangadharendra Saraswathi Swamiji of Shri Sonda Swarnavalli Mutt.
The 12 signs of the zodiac is an “eternal” concept. No one invented it, and it didn't evolve out of more primative ideas - contrary to many popular versions of history. As soon as you have a living being watching the world, trying to figure out when to do what, they easily realize the cycle of roughly 12 full moons within one solar circuit - which is the fundamental basis of the 12 sign zodiac system. But the ability to USE those 12 zones of the Sun's path as part of a skymap did not come until human mathematics and observational tools evolved sufficiently (c.2-3k years ago).
To help the maths evolve, the pioneers of using the zodiac correlated the 12 signs to various observable stars - allowing them to visually fact-check what their calculations proposed. Soon enough, people (like Hipparchus, who is famous for being the first to make a statement about it which we have a record of) noticed that the correlation between the marker stars and the zodiac markers (solstices/equinoxes) wasn’t truly a constant. The stellar markers slowly drifted from the zodiac points they were supposed to mark.
To keep track of this drift, people then came up with what we call “ayanaamsha” in Indian astrology (lit. “solstice segment”).
Most people kept the zodiac anchored to solstice/equinox and used ayanaamsha as a variable to keep their stellar cross-checking accurate, but India wanted to anchor the zodiac to their anchor stars, their indigenous system of lunar constellations: the nakshatras. Thus they created what we call today, “sidereal signs.”
Over the centuries various important people - like Varahamihira, S.B. Diksita, and N.C. Lahiri - have questioned this and hoped for a reform: because this directly subverts the fundamental definition of what the 12 signs were in the first place (divisions of the sun’s path between its solstices and equinoxes). Finally, in the 21st century, we seem to be making some headway on that.
They are a way to measure space.
Space is like as a sphere all around us. The Sun and planets move through a narrow band anchored around the middle of that sphere. The twelve signs divide this narrow band into twelve discrete sections.
The easiest way to explain is to use an analog clock. The clock’s round face represents space, the circular band through which the Sun and planets move. The hour hand represents the Sun. The minute hand represents the Moon.
Let’s take a clock before it has any hands or markings on its face. Put the hour hand in place and watch it move around the clock’s entire circle. This movement represents the Sun moving through the circle of space (a "year").
Now put on the minute hand. It's movement represents the Moon moving through the entire circle of space (a "month").
Now let’s set both hands to the 12 o’clock position, get out a pen, and let the clock start running. The minute and hour hands both move, but the hour hand moves much slower. Whenever the minute hand comes back to the 12 o’clock position, mark the clock face right where the hour hand is pointing. You’ll wind up with twelve marks.
What you’ve just done is exactly analogous to creating the twelve divisions of the zodiac. Each mark on the clock is the border of a new sign. There are twelve of them because the Moon makes twelve cycles through space in the time it takes the sun to make one. So, each sign contains the amount of space travelled by the Sun during the time it takes the Moon to make one complete circuit.
Take a protractor and measure the distance between each of the twelve marks on your clock. Each one is the same: 30 degrees. That’s why each sign consists of 30° of the complete circle of space.
Sidereal and tropical zodiac systems both follow this definition. The only difference is where they start“marking the clock.”
Tropical signs starts from where path of Sun and planets intersects the celestial equator. This point is called “equinox.”
Sidereal signs start reference to a specific star. There are several different opinions about which star should be used, and how. The most popular is the Lahiri Zodiac, which says the signs begin at a point 180 degrees opposite the star named Spica.
Sidereal signs start from a stellar reference point, but they are not literally the stars themselves, which form thirteen divisions of the circle of space, each one a different size (some twice the size of others). Obviously these constellations are not identical to the sidereal signs, which like the tropical signs, are twelve in number and each one equal in size.
People on either side of the issue tense up over this question, because the perception is that if one defines the zodiac incorrectly then ones astrology, and all of its history and heroes, must be completely bogus. This is a misconception. Using the wrong zodiac does not invalidate an entire astrological system. Many factors are unaffected by how we define the zodiac signs. For example:
It’s also very important to note that the further back we go in history towards the early AD centuries, the smaller the difference between tropical and sidereal starting points becomes. Astrologers 1700 years ago, for example, faced no difference at all between the sidereal and tropical definitions.
One definition of the zodiac might be “correct” and the other “incorrect,” but this does not mean that using the wrong one renders your astrological system useless, or discounts its rich history. If we define the zodiac correctly, however, our astrological systems will become clearer and simpler to use. This will only become truer as we move into the future and the discrepancy between the sidereal and tropical starting points continues to increase.
Modern India is a bastion of the sidereal zodiac. Thus it comes as a great shock that India’s own classical literature defines the zodiac tropically! Probably no one is more shocked by this than Indians themselves, who faithfully assumed they had been “following the ancients” by using a sidereal zodiac. Let’s look at a sample of what Indian classical texts actually say.
The most authoritative, fundamental Indian text on astronomical astrology is Sūrya Siddhānta, a title which declares the book to be “Perfect Conclusions of the Sun.” Several early places of the book seem to disfavor sidereal and favor the tropical definition of the signs:
A solar month begins when the Sun enters a new zodiac sign. There are twelve months in a year.
60 seconds (vikāla) make a minute. 60 minutes (kāla) make a degree. 30 degrees (bhaga) make a sign. 12 signs (rāśi) complete the circle (bhagaṇa).
In one age (yuga) the circle of stars lags behind 600 revolutions towards the east. Use a formula [math omitted for brevity] to find the current location (ayana) of the equinox relative to the stars.
The above references could support the tropical zodiac, but texts 7-10 of the Fourteenth Chapter explicitly and unequivocally put it in black and white:
It is well-known that the circle of signs is split by two diameters. One is the line from equinox to equinox. The other is the line from solstice to solstice. Between each solstice and equinox are two other markers. Each solstice /equinox and the two following markers represent the three strides of Vishnu.
The Sun has entered Capricorn when it begins moving north for six months. It has entered Cancer when it begins moving south for six months. Seasons last for two signs each, beginning from Capricorn with the frozen season. The twelve signs named Aries, etc. are the months which altogether comprise the year.
Here, Sūrya Siddhānta plainly says that solstices and equinoxes define the 12 signs of the zodiac. Capricorn is defined by the Sun beginning to move north at the winter solstice, and that Cancer is defined by the Sun beginning to move south at the summer solstice.
Śrīmad Bhāgavatam is held by most Indians to be penultimate Pūraṇa representing the full maturity of Indian thought. Its fifth division concerns astronomy. Texts 2-6 of the 21st chapter of that division define the twelve zodiac signs, unambiguously, as tropical:
Outer space is measured by relation of heaven and earth. The Sun is the king of all the planets, in the center of everything, keeping everything together. It moves to the north, crosses the equator, and moves to the south. When it goes north of the equator days get longer. When it crosses the equator days and nights are equal. When it goes south of the equator days get shorter. On this basis the Sun moves through the twelve divisions called Capricorn and so forth.
The Sun is at Aries and Libra when the days and nights are equal. Passing through Taurus, etc. the days become longer and then decrease until again equal with the night. Passing through Scorpio, etc. the night becomes longer and then decrease to again become equal with the days.
Thus it is impossible to deny that Śrīmad Bhāgavatam presents a tropical definition of the 12 zodiac signs.
The stars of sidereal space are a valid and important astrological entity. India (and almost all cultures) has a valid system of dividing sidereal space with the Moon as the focal point, not the Sun as in tropical space. This system has 27 divisions, not 12, because there are that many sunrises during the time it takes the Moon to complete a full circuit of the heavens.
The idea of 12 sidereal signs comes from wrongly assuming that the correlation between the stars and signs is permanent. In fact, there is no permanent correlation between the two, the signs eternally and very gradually drift backwards through the stars - a phenomena commonly known as "precession of the equinox."
There is a valid need to correlate the signs and stars because their relationship is important for defining very long periods of time (“ages”) and for knowing how and when to keep solar and lunar calendar systems synchronized.
Babylonians measured the correlation of their autumnal equinox with the heliacal rising of stars they called The Scales. Greeks measured the discrepancy of tropical Aries against a stellar counterpart bearing the same name. Ancient and Classical Indians measured the heliacal position of the equinox in reference to their fixed stars. For example, the Ṛg Veda notes Kṛttikā as the “first” star and the beginning of the celestial circle, because in Ṛg Vedic times, four to five thousand years ago, Kṛttikā heliacally rose with the Vernal Equinox. Later Indian works from nearly two thousand years ago note Aśvinī as the first star, because at that time Aśvinī was the star heliacally rising with the equinox.
Projecting the 12 zodiac divisions into space, based on the then-current position of the vernal equinox, was useful for mathematics and essential for long-term timekeeping, but opened a door for people to think of the signs as stellar entities. It is an easy mistake to make considering that for centuries there was almost no significant difference between the signs and their homonymic sidereal namesakes.
Now let’s examine how fundamental western texts define the zodiac.
Mul.Apin is one of the oldest existing documents of astronomical astrology. It reveals that the ancient Babylonians used a lunisolar calendar of twelve 30-day months per year. They anchored these twelve divisions to the equinoxes and solstices, and created stars to serve as reference points for the occurrence of solstices and equinoxes.
Mul.Apin 1.3.1-12 says:
On the 15th of Tashritu the Scales, the Mad Dog, EN.TE.NA.BAR.HUM and the Dog become visible; 3 minas is a daytime watch, 3 minas is a nighttime watch. 
This says that when the days and nights are of equal length (an equinox) the ecliptic star called “The Scales” (Modern-day Libra) rises heliacally (just before the Sun). This occurs on the 15th day of their month named Tashritu (an Autumn month). Similarly, Mul.Apin also says that when the nights are shortest (summer solstice) the ecliptic stars of modern Cancer rise. And when the nights are longest (winter solstice) the ecliptic stars of modern Capricorn rise.
What we see in Mul.Apin is that the solstices and equinoxes are fundamental to the Babylonian’s twelvefold division of space, and that reference stars are used to measure the discrepancy between tropical and sidereal time. The Babylonian’s would insert leap months to re-sync the stars with the equinoxes periodically. This explains why they gave stars the same names as the twelve signs.
One cannot convincingly argue that the Babylonian’s had a sidereal twelvefold zodiac, because the Mul.Apin explicitly describes eighteen sidereal divisions of the ecliptic.
Ptolemy’s Tetrabiblos is thought of as the “bible” of Western Astrology. 1.10 of that text defines the zodiac with a tropical starting point:
The zodiac is a circle, so there is no clear “beginning” to it. But the sign that begins with the Vernal Equinox, Aries, acts as the beginning.
The next section, 1.11, makes it perfectly clear that the signs are defined in relation to the solstices and equinoxes.
There are two signs at solstices: Cancer is the 30° interval beginning at the Summer Solstice. Capricorn is the same from the Winter Solstice. There are two signs at equinoxes: Aries begins from the Vernal Equinox and Libra from the Autumnal Equinox.
Siderealists suggest that Ptolemy “changed” the Greek system to a tropical one, but I don’t consider this a valid opinion. Ptolemy’s forerunner, Hipparchus, discovered the precession of the equinoxes. Greek authors prior to Hipparchus therefore could not differentiate tropical from sidereal. The first significant post-Hipparchus author, Ptolemy, explicitly states that the “beginning” of beginningless space is judged from the equinoxes and solstices and must follow them, not the stars that temporarily and approximately represent them.
First I explained that the twelve signs are not stars; but are mathematical divisions of space derived from the interplay between the Sun and Moon. So there is no compelling reason to assert that the beginning of the twelve signs should be anchored to a star. It is easier to see the rationale of using the equinox to mark the beginning of the divisions – for this binds the twelve lunisolar polyrhythms firmly to the center of the Earth’s local space.
Siderealists, and Indians in particular, will have a difficult time digesting or accepting this point, because it throws the expertise and validity of their rich astrological history into doubt. “So many great astrologers have been successfully doing it this way in India for so many centuries, how can you say they are wrong?!”
I am sympathetic to this reaction. I went through the same dilemma myself – being of Indian astrological, philosophical and religious background. This is why I felt it important to explain that the zodiac is only one of several fundamental components of astrology, and thus an error in calculating the zodiac does not totally invalidate an astrologer or an astrological system. And I wanted to note that the further back in history we look towards our revered founders, the more insulated they are against the effects of error in this regard, for the discrepancy between the sidereal and tropical measurements grows smaller as we move back towards 300 AD.
But if we indeed value tradition we must never lose sight of its original authorities. Sūrya Siddhānta, Śrīmad Bhāgavatam, Mul.Apin and Tetrabiblos all clearly state that the twelve signs of the zodiac are entirely based on solstices and equinoxes, not stars. If we ignore this, what exactly are we loyal to?
Today the discrepancy between the sidereal and tropical reckoning of the twelve signs is too big to overlook. We cannot postpone taking this issue seriously. Once we admit that stars are not signs, there is no compelling reason to use a star to define the zodiac’s beginning. Beside force of habit, injured pride, the paralysis of shock, or fear of change – is there anything that would stop us all from embracing the unequivocal tropical definitions of the zodiac found in all the ancient and classical literature of the world?
- Vic DiCara
 According to: Hermann Hunger and David Pingree, "MUL.APIN. An Astronomical Compendium in Cuneiform", p. 43