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Sidereal Zodiac in Surya Siddhanta?

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Surya Siddhanta Defines the Sidereal Zodiac in Chapter 1, texts 27 and 28.

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.”

Thus Surya Siddhanta 1.27-28 defines a sidereal zodiac: a 12-fold division of the ecliptic based on the Aśvinī/Revatī border. 


Is this a definition of a sidereal zodiac? For argument’s sake, let’s begin by saying, “yes, it is.”

If this DOES define a sidereal zodiac…

Now what do we do with the tropical zodiac defined in later in the book? Ignoring it seems dishonest. But if we don’t ignore it, we are forced to ask why we have two conflicting definitions of one thing?

For argument’s sake, let’s say this is intentional. We are meant to have two different definitions of the zodiac – because  one is meant for certain things, and the other is meant for other things. 

Now we have to ask, “What is the sidereal zodiac meant for, and what is the tropical zodiac meant for, and why would either one fail at what the other succeeds at?”

Suggestion #1: Tropical for Calendars, Sidereal for Birth-charts

People have suggested that sidereal is used for charts and tropical for calendars. Ok, but if so, why? This question is particularly vexing once we realize that there is no difference between the zodiac and the calendar. Sūrya Siddhānta (14.10) explicitly says the calendar months and the zodiac signs are one and the same thing.

The calendar is how the zodiac relates to time. The zodiac is how the calendar relates to space. The calendar is formed by the 12 idealized lunar orbits within a solar ecliptic-orbit. The zodiac is formed by the sun’s idealized location during those 12 spans of time.

So, if a system correctly describes one thing, by what odd astronomical anomaly would it incorrectly describe the other?? If tropical correctly describes the timing of the twelve idealized lunar cycles (“months”), how could it not correctly describe the sun’s location during those cycles (“signs”)? Conversely, how can sidereal correctly describe the sun’s location during the 12 idealized lunar cycles (“signs”) if it does not correctly describe the cycles themselves (“months”)?

In short, if something is accurate in establishing a 12-month calendar, it must be accurate in establishing the zodiac, and if something is inaccurate in establishing that calendar, it cannot be accurate in establishing the zodiac.

Suggestion #2: Tropical for Latitude, Sidereal for Longitude

Some suggest that the tropical zodiac is meant for planetary latitudes (how far above or below the ecliptic a planet is) and the sidereal is meant for planetary longitudes (the planet’s degree in a sign).

Again, “If so, why?”

And again, the question is much more vexing when we understand astronomy. A planet’s latitude and longitude are inseparable. The longitude determines the latitude, and visa versa. So if a system correctly describes one, it is hard to imagine how it would incorrectly describe the other. If tropical correctly describes latitude, how could its longitude be inaccurate? Conversely, if sidereal correctly describes longitude, how could its latitude be inaccurate?

Suggestion #3: Something Else

Although the two suggestions given above are not viable, maybe there is a third suggestion we have not yet heard. To investigate this possibility, let’s look more closely at Sūrya Siddhānta 1.27-28 and its context to see if we find clues.

Maybe this is NOT a definition of a “Sidereal Zodiac”?

Point 1:

In 1.27-28, or in any of the surrounding context, we don’t find any word that could be a reference to “the zodiac.” Compare this with 14.7, 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”).


We might reply that 1.28 uses the word bhagaṇa, which could refer to the zodiac.

However, here the Sūrya Siddhanta uses the word to mean “revolution”, “complete orbit.” This is obvious from the context immediately before and after 1.27-28.

Another reply might be that “60 seconds in 60 minutes in 30 degrees in 12 arcs in 1 revolution” is obviously a reference to the zodiac.

This would be true if the zodiac were the only astronomical system to have this structure. However, it is not. The numbers 1, 3, 6, 12 are mathematically “uniquely clean” in how they can be subdivided and how they relate to each other. Hence the entire first chapter of Sūrya Siddhānta is full of astronomical and chronological structures based on multiples of these numbers. The author of Sūrya Siddhānta probably uses this 60/60/30/12/1 structure because of its mathematical convenience and aptitude, not because he or she is in fact talking about the zodiac in a veiled manner.

Point 2:

In 1.28-27 we don’t find any names like “Aries.” In contrast, in 14.9 we hear the name “Capricorn” (makara) and “Cancer” (karka). And in 14.10 we hear “Aries, etc.” (meṣa adi).


We might reply that 1.28 indeed uses the word rāśi – which we might recognize as “zodiac sign.”

However, “zodiac sign” is not the primary denotation of the word rāśi. Monier-Williams is a widely accredited Sanskrit-Engligh dictionary. According to it, the word rāśi means:

  1. A group or quantity of anything
  2. A sum of other numbers
  3. An arc, equal to a twelfth of a circle
  4. A sign of the zodiac (as a sum of degrees, minutes, and seconds)

The word “rāśi” is primarily a geometry term, with connotations in astronomy and astrology that allow it to also be used as a noun for a zodiac sign. Considering the lack of any reference to “Aries” etc. or any unambiguous reference to “the zodic”, it is less reasonable to understand the word rāśi as “zodiac sign” and more reasonable to understand it as a geometric “arc”.

Point 3:

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 – now it does not seem nearly as surprising to find the so-called “zodiac signature” 60:60:30:12:1 ratio in 1.28).

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:

  1. Assume an idealized mean conjunction of planets at Revati’s defining star occurring at a known point in history (the beginning of a yuga segment, for example)
  2. Compare that with currently observed positions of the planets.
  3. Knowing the mean speed that each planet orbits with, deduce how much time must have passed for the planets to move from their mean conjunction at Revati to their currently observed positions.

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.


It is very unlikely that 1.27-28 defines the zodiac. If it does, it has two flaws: (1) it is not explicit, and (2) it is incompatible with the explicit definition of the zodiac given elsewhere in the same book.

If we try to wrestle with the incompatibility, we can resolve it by considering 1.27-28 to be a “zodiac” in geometry only – whose specific purpose is to measure sidereal revolutions and locations – something very important and useful in visual astronomy, and therefore a potential technology “under the hood” in constructing zodiac charts.

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