by Vic Di Cara

The 12 Signs of The Zodiac Part I – Measuring the 12 Signs

This article is about the 12 signs of the zodiac. It’s not about, “If you are born between this and that date your personality is like this.” It is meant for people with a fairly serious interest in astrology, and is especially important for practicing astrologers. It has two main parts. The first will explain how the 12 divisions of the zodiac are calculated, and will address the question of Tropical vs. Sidereal zodiac definitions. The second part will explain the characteristics of the 12 signs, and how these result from the mixture of their ruler, element, mode, and natural order...


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The 12 Signs of The Zodiac
Part I – Measuring the 12 Signs

by Vic Di Cara


Editor's Note from Pat Lantz: In his previous series of articles Astrologer Vic Di Cara reconciled the two zodiacs, sharing with ATH readers some forgotten knowledge that will gave all astrologers, both Eastern and Western, much to ponder. This article continues to explore and make sense of Tropical vs Sidereal. If you missed the series you can catch up here: Reconciliation of the Tropical and Sidereal Zodiac, Part I, Part II, Part III, Part IV.

Note from Author: This article is about the 12 signs of the zodiac. It’s not about, “If you are born between this and that date your personality is like this.” It is meant for people with a fairly serious interest in astrology, and is especially important for practicing astrologers. It has two main parts. The first will explain how the 12 divisions of the zodiac are calculated, and will address the question of Tropical vs. Sidereal zodiac definitions. The second part will explain the characteristics of the 12 signs, and how these result from the mixture of their ruler, element, mode, and natural order.

The twelve signs 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.

Why twelve?

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 the hour hand move around the clock’s entire circle. This movement represents the Sun moving through the circle of space. A complete revolution of the hour hand around the clock is analogous to a year, because a year appears to be a complete circuit of the Sun through the circle of space.

Now put on the minute hand. Each time it goes around the clock is analogous to a month, because a month is defined as a complete cycle of the Moon through the circle of space; during which time we can see it change from full to new and back again to full.

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 Definitions

Sidereal and tropical zodiac systems both follow this definition. The only difference is where they start “marking the clock.”

The Tropical definition starts from the point where the circle of the Sun and planets (the “ecliptic”) is anchored to the middle of the sphere of space (the “celestial equator”). This point is called “equinox.”

The Sidereal definition starts in reference to a specific star. There are several different opinions about which stellar point should be used. The most popular is the Lahiri Sidereal zodiac, which starts at the point in space that is 180 degrees opposite the star named Spica.

The sidereal zodiac starts from a star, but it is not the stars themselves. Like the tropical zodiac, it is a system of twelve equal divisions expressing the lunisolar polyrhythm. The stars themselves form thirteen divisions on the circle of space, and each one is a different size – some being twice as big as others! Clearly, the zodiac signs cannot be identical with the constellations that bear their names. Neither sidereal nor tropical signs are stellar. The sidereal zodiac, however, uses one star to define the starting point of the divisions.

Which One is Correct?

People on either side of the issue tense up over this question, because the perception is that if ones definition of the zodiac is incorrect then ones astrology, and all of its history and heroes, must be completely bogus. However, it is a misconception that using the wrong zodiac would invalidate an entire astrological system. Many factors remain similar or identical regardless of whether the zodiac is defined from a tropical or sidereal starting point:

Aspects: The planets’ angular relationship to one anotheris absolutely unchanged. Therefore aspects by degree (called sphuta-dhṛṣṭi in India)are unaffected by ones choice of zodiac definition.

The angular relationship of the planets relative to the ascendant is absolutely unchanged. Thus planets in dynamic houses (called bhava-chalit in India) are absolutely unchanged, and planets in whole-sign houses also tend to sturdily resist being changed by the zodiac’s starting point. This is also true for the houses of subdivisional charts (candra-langa, navāmśa, etc).

Fixed Stars: Other systems of measuring space, like fixed stars (callednakṣatra in India), are absolutely unchanged. Techniques based on them are identical for either zodiac definition (e.g. nakṣatra daśā).

About 25% of the time, even the signs of the planets and houses will not change.

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 working more than a dozen centuries ago, for example, faced almost 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.

The Indian Opinion

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.

Sūrya Siddhānta

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.” A tropical definition of the signs might be evident in several early places of the book:

1.13 says that the signs are identical to the tropical months:

A solar month occurs when the Sun enters a new zodiac sign. There are twelve months in a year.

1.28 states that the signs are mathematical divisions, not constellations:

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).

3.9-10 gives the principle on which ayanāṁśa is based:  that the stars and signs are different; evidenced by their changing positions in relation to one another: [1]

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. It says that 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

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 Western Opinion

Before addressing the perplexing question regarding how India became so devoutly sidereal, first let’s examine how fundamental western texts define the zodiac.

Mul.Apin of Babylon

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 the twelve divisions to the equinoxes and solstices, and created stars to serve as reference points for the occurrence of solstices and equinoxes. 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. [2]

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.

Tetrabiblos of Greece

In Ptolemy’s Tetrabiblos (the “bible” of Western Astrology), 1.10 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.

Where does the Sidereal Idea Come From?

The stars of sidereal space are a valid and important astrological entity.  India, China, and the Arab world have systems for measuring and dividing sidereal space. However, sidereal space is measured with the Moon, not the Sun as the focal point. Therefore it has close to 30 divisions, not 12 –there are about that many days during one circuit of the Moon through space.

Where does the idea of twelve sidereal signs come from? It is a side-effect of the valid need for measuring the discrepancy between tropical and sidereal space. This measurement is important for defining very long periods of time (“ages”) and for knowing how and when to keep solar and lunar calendar systems synchronized. To know it we have to measure the distance between a sidereal reference point and the equinox’s current heliacal location.

Babylonians measured the synchronicity of their autumnal equinox date with the heliacal rising of the stars they called The Scales. The Greeks measured the discrepancy of tropical Aries against a stellar counterpart bearing the same name. Indians did the same by noting the heliacal position of the equinox in reference to their fixed stars and projecting the current position of the signs into the 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.

These cultures gave the stars the same names as the tropical divisions that they stood as sidereal references for, or visa versa. This seems to have opened a door for people to think of the stars as the signs themselves. 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.


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, uniting Sun, Moon and Earth.

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 the early centuries AD.

But if we indeed value tradition we must never lose sight of its original authorities; in this case Sūrya Siddhānta, Śrīmad Bhāgavatam, Mul.Apin and Tetrabiblos. They all 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?


[1] The reason the stars and signs move in relation to one another is not easy to explain. The equinox occurs when the Sun moves into a specific angle to the Earth’s axis and equator. But the Earth’s axis slowly rotates in the same direction as the Earth’s revolution around the Sun. So the axis will align with the Sun for an equinox slightly sooner than the earth actually completes an exact orbit. Gradually the equinox occurs at an earlier and earlier point in the Earth’s orbit, and thus the apparent position of Sun in relation to the stars on that day slowly changes. They very gradually drift backwards through the circle of stars.

[2] According to: Hermann Hunger and David Pingree, "MUL.APIN. An Astronomical Compendium in Cuneiform", p. 43



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About the Author

Victor Damien DiCara practices astrology based on very ancient principles ("Vedic") but does it with a very modern frame of mind.

Initiated into Vedic spiritual culture in 1992 (in India), Vic was awarded the symbolic thread of the brahmin in 1994. This award signified aptitude in the role of a councilor, philosopher and guide. While living as a celibate yogi in an ashram for 8 years, he studied Vedic philosophy and the culture of Vedic astrology and began offering astrological consultation to the public in 2007. (Vic's astrological autobiography)

Vic has written and published in over a dozen magazines, has published textbooks for his courses on astrology and is currently putting the finishing touches on a full length book.

To find out more about Victor DiCara, visit his Website, Authentic Astrology, Blog or join him on 
Reading with Victor DiCara




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