What is Zodiac ? Tropical & Sidereal Zodiacs
- Nikita
- Jun 27
- 14 min read
Updated: Dec 1
Introduction
I am sure you have heard that there are differences in the systems of Western Astrology and Vedic Astrology. The major difference lies in coordinate systems - tropical and sidereal zodiac. This post is a complete guidance to understand the difference of this two coordinate systems. We will explore the origin of zodiac, nakshatras ( lunar mansions) and astronomical principles of zodiac, which underline any astrology system.
At the end you will have all the necessary information to understand the "back-end" of astrology.
Coordinate Systems
The so called "Tropical" and "Sidereal" Zodiac are two different reference system used in Astrology.
The Tropical Zodiac is used in Western Astrology and "pop astrology" - it is where you read predictions based on "your zodiac sign"
The Siderial Zodiac is used in Jyotish (Vedic ) Astrology.
This is a one major difference between this two systems.
So what are the difference between them ? To answer this questions completely - we need to define some terms first.
Reference system - coordinate system.
To plot the position of stars, planets - we need to create a coordinate system with reference to some object or position, like "0" position when you draw your graphs during your math class back in school.
So what object we use as "zero point" in the astronomical calculations ?
Our Earth is the most convenient object, which can serve as reference point, because we as the observers are located on Earth.
However, Earth is not static, as "0" on the intersection of two planes of X and Y.
The Earth motions occur in various dimensions of 3D space and it must be considered in our coordinate system if we want to plot and track movement of celestial bodies correctly.
This coordinate system is called a celestial sphere - it's a conceptual model astronomers use to help map and understand the sky from Earth.
Why Do We Build the Celestial Sphere?
1. To Create a Universal Reference System
The celestial sphere allows us to locate stars, planets, and other celestial objects in the sky, no matter where you are on Earth.
It's like wrapping the universe around the Earth and projecting all stars onto this giant dome.
We assume:
The Earth is at the center and all celestial bodies lie on a huge, imaginary sphere around us.
2. To Use Coordinates Like Latitude/Longitude
Just like we use latitude and longitude on Earth, we use:
Right Ascension (RA) = like longitude
Declination (Dec) = like latitude on the celestial sphere to pinpoint the position of any star or planet.
How do we build a Celestial Sphere ?
Celestial Sphere is a coordinate system using a Geocentric space model.
Geocentric System (Earth-Centered) :
A model where Earth is at the center of the universe. All celestial bodies - including the Sun, Moon, stars, and planets - revolve around the Earth. Developed in ancient times, notably by Ptolemy (~2nd century CE).
Key charecteristics:
Earth is stationary.
Matches the naked-eye observation of the sky (everything seems to move around us).
Heliocentric System (Sun-Centered) :
A model where the Sun is at the center of the solar system. The Earth and other planets revolve around the Sun. First suggested by Aristarchus of Samos (~3rd century BCE). Formalized and popularized by Nicolaus Copernicus in the 16th century. Later confirmed by Galileo, Kepler, and Newton.
Key Characteristics:
Earth is a planet that rotates on its axis and revolves around the Sun.
Matches modern astronomical observations and physics.
Again, I want to emphasis that Sun rotates around the Earth. The Geocentric system is just a conceptual model that is used to simplify the mapping of cosmos for practical use. It is a useful coordinate system, where we "tie up" all our calculation to a "zero point" or reference point which is Earth.
Geocentric system - is just a conceptual model useful for astronomical calculations.
The Use of Geocentric System
Geocentric system is used in Celestial Navigation - it is when you need to determine your position on Earth, using a celestial objects as a reference objects.
And it is used in Astrology for observation of Sun and planets moving through Zodiac.
As we assume that Earth is the center of our coordinate system, when we need to plot the orbit of the Sun. ( It is actually the orbit of the Earth, but so as the observer is located on Earth, we plot the apparent movement of the Sun around the Earth, which is called Ecliptic )
The Ecliptic - is the path the sun appears to take across the sky as the Earth orbits it. It's a line on the celestial sphere, essentially the Earth's orbital plane projected onto the sky.
The Zodiac - is a region of the sky, about 8 degrees wide on either side of the ecliptic, where you can find the Sun, Moon, and planets. It's traditionally divided into 12 constellations, which are also known as astrological signs.
Another use of celestial sphere - navigation and mapping of celestial objects.
The Celestial Sphere
A useful construct for describing locations of objects in the sky is the celestial sphere, which is considered to have an infinite radius.
The center of the Earth is the center of the celestial sphere, and the sphere's pole and equatorial plane are coincident with those of the Earth.
We can specify precise location of objects on the celestial sphere by giving the celestial equivalent of their latitudes and longitudes.
The point on the celestial sphere directly overhead for an observer is the zenith. An imaginary arc passing through the celestial poles and through the zenith is called the observer's meridian.
The nadir is the direction opposite the zenith: for example, straight down from a spacecraft to the center of the planet.
Declination and Right Ascension
Declination (DEC) is the celestial sphere's equivalent of latitude and it is expressed in degrees, as is latitude. For DEC, + and - refer to north and south, respectively. The celestial equator is 0° DEC, and the poles are +90° and -90°.
Right ascension (RA) is the celestial equivalent of longitude. RA can be expressed in degrees, but it is more common to specify it in hours, minutes, and seconds of time: the sky appears to turn 360° in 24 hours, or 15° in one hour. So an hour of RA equals 15° of sky rotation.
Another important feature intersecting the celestial sphere is the ecliptic plane. This is the plane in which the Earth orbits the Sun, 23.4° from the celestial equator.
The Equinoxes
The equinoxes are times at which the center of the Sun is directly above the equator, marking the beginning of spring and autumn. The day and night would be of equal length at that time, if the Sun were a point and not a disc, and if there were no atmospheric refraction.
Above definitions taken from:
You can read this article if you want to explore the mapping of the celestial objects further.
Sidereal and Tropical Zodiac
Now we can explore the concept of Sidereal and Tropical zodiac.
Tropical Zodiac
The Tropical Zodiac is a system of astrology that divides the sky into 12 equal segments of 30 degrees each, based on the seasons and the Sun’s movement relative to Earth. It is most commonly used in Western astrology.
Tropical Zodiac is aligned with the Earth's seasons, not the actual positions of constellations.
Why does this happen?
The starting point of the zodiac (0° Aries) is fixed to the Spring Equinox (around March 21), when day and night are equal in length. But here is the thing - the point of Spring Equinox is not the fixed point - its slowly moving ( I will describe down below why does this happen ) and as a result, the Tropical signs and actual star constellations are no longer aligned. For example, the Sun may be in the astronomical constellation of Pisces, but still be considered in Aries in the tropical system.
How to check this ?
If you know the constellations - you can just observe the sky and position of the planets.
But today with technological advancements - you can use and app called "Stellarium" or other similar. It is free - you download it to your phone - point to the sky - and it will show you the position of Sun, Moon and planets against the actual astronomical constellations.
And its a great way to learn the constellations.
Example:
If say - you were born on April 1st, the Sun is in Tropical Aries, but astronomically the Sun may be in Pisces.
Sidereal Zodiac
Tropical Zodiac is good for a seasons, but for us interested in Dharma ( Purpose ) the actual position of planets, which is astronomically correct, hence scientific and thus logical is important.
Sidereal Zodiac is based on astronomical positions of the planets and not tied to the seasons of the Earth.
Why Did the Rishis Use Sidereal Instead of Tropical?
The Rishis were sky-watchers, yogis, and seers.
Their system was built on direct observation of the stars.
They weren’t trying to link astrology to seasons - but to cosmic rhythms and karma.
The Nakshatra system, which is foundational to Jyotish, can only be sidereal - it’s purely star-based.
What Makes It Sidereal?
The sidereal zodiac is based on actual constellations - not where the Sun appears to be from Earth at equinox (like the tropical zodiac).
In Jyotish:
Aries (Mesha) begins where the constellation Ashwini Nakshatra begins.
The zodiac signs are anchored to these star positions, not seasonal dates.
Sidereal Coordinate System
Sidereal Zodiac is also an Earth-centred (Geocentric ) coordinate system:
The Vedic system assumes that Earth is at the center of the celestial sphere.
All planetary positions are observed and calculated from Earth’s perspective, just as we see them in the sky.
Ecliptic-based:
The Vedic zodiac follows the ecliptic — the apparent path the Sun traces across the sky.
Planetary positions are measured in degrees along the ecliptic from a fixed point (0° Aries) defined by stars (not the equinox).
Fixed to stars
While Western (Tropical) astrology ties the zodiac to the seasons (equinox = 0° Aries), Vedic astrology ties it to fixed stars, making it sidereal.
This means the coordinate system is Earth-centered, but not season-based. It reflects the actual position of stars and planets in the sky.
Ayanamsha and Precession of Equinoxes
The axis of rotation of our Earth shifts cyclically over time.
Now the axis of the Earth is aiming onto the Polaris. So Currently, Polaris is close to the north celestial pole, making it our current North Star. However, The Earth’s axis will be directed toward Vega in approximately 12,000 years from now. And this is called a precession of Equinoxes.
The precession is very slow, at about 1 degree every 72 years. Over the course of about 25,772 years, the Earth completes one full cycle of precession, a period known as the Great Year or Platonic Year.
The precession of the equinoxes is a slow, gradual shift in the orientation of Earth's rotational axis, which causes the positions of the equinoxes to move westward along the ecliptic over time.
Because of this precession - the vernal equinox point (where the Sun crosses the celestial equator moving northward) slowly shifts westward along the zodiac.
As you remember - the vernal equinox is the starting point of Tropical Zodiac ( 0° Aries ) but because it is also moving - the Tropical Zodiac is no longer aligned with Astronomical Constellations.
Age of Aquarius
The Vernal equinox point used to align with the constellation Aries about 2,000 years ago. Now, due to precession, it has drifted into Pisces. In a few hundred years, it will drift into Aquarius — this is what people mean by the Age of Aquarius.
Ayanamsha
Interesting that Vedic Rishis were well aware about the precession of equinoxes long before the modern science. By understanding concept of ayanamsha will know how.
Ayanamsha (Sanskrit: अयनांश) literally means:
"Ayan" (अयन) = movement, path (often of the solstices or equinoxes)
"Amsha" (अंश) = portion, degree
So Ayanamsha means "the portion of movement" - specifically:
The angular difference between the Tropical Zodiac and the Sidereal Zodiac.
Due to the precession of the equinoxes, the Tropical Zodiac (used in Western astrology, based on seasons) slowly drifts apart from the Sidereal Zodiac (used in Jyotish, based on constellations).
Around 285 CE, both zodiacs were roughly aligned.
Since then, the Tropical zodiac has shifted westward - about 1° every 72 years.
As of today (2025), the ayanamsha is around 24° (depending on the system).
Example:
If your Sun is at 20° Aries Tropical:
With an Ayanamsha of 24°, your Sidereal Sun would be at 26° Pisces.
Surya Siddhanta - a classical Sanskrit astronomy text which formalizes the use of the sidereal zodiac. This text defines the sidereal zodiac signs (Rāśis) and Nakshatras very precisely, but it also contains astronomical constants and calculations, like ayanamsha still accurate today.
The original Surya Siddhanta does not use the word “ayanamsha,” but it provides the necessary math for calculating precession. The Surya Siddhanta calls precession - “ayana-chalana” and gives the rate at which the equinox shifts. Based on its verses, scholars have interpreted the implied ayanamsha value.
Surya Siddhanta contains precession data which modern scholars use to derive a Surya Siddhanta-based ayanamsha. The Lahiri Ayanamsha, though not identical, is influenced by Surya Siddhanta principles, especially in using Chitra (Spica) as a reference. However, there are other systems for ayanamsha calculation.
You can choose different ayanamsha setting in your atrology software.
Ayanamsha Name | Founder | Used by |
Lahiri | N.C. Lahiri (1955) | Govt of India, most astrologers |
Raman | B.V. Raman | Popular in South India |
Krishnamurti (KP) | K.S. Krishnamurti | Used in KP astrology |
Fagan/Bradley | Western sidereal system | Some Western astrologers |
Nakshatras
Nakshatras is a profound and unique concept of Vedic Astrology and what makes it purely sidereal ( star-fixed). Rishis ( the acnient sages who compiled Vedas somewhere 1500 BCE ) where yogis and observers. They observed the world inside and outside them, and thus they observed the sky and noted the patterns of the universe.
Nakshatras are the first division of the sky into the constellations.
Here is the time-line of how nakshatras appeared, evolved and combined later with 12 zodiac signs.
The Vedas (especially the Rig Veda and Yajur Veda) contain poetic references to constellations and seasonal markers like the Krittika (Pleiades).
The Rishis were already observing fixed stars and lunar mansions (Nakshatras).
Sidereal orientation is clear: stars, not seasons, were their calendar.
2. Vedāṅga Jyotiṣa
This is the earliest known astronomical text in India.
It describes a 27-Nakshatra lunar zodiac.
It uses sidereal reckoning - the positions of the Moon and stars were tracked against actual constellations.
The year was divided based on stellar risings, not solar seasons.
3. Surya Siddhanta
This classical Sanskrit astronomy text formalizes the use of the sidereal zodiac.
Defines the sidereal signs (Rāśis) and Nakshatras very precisely.
What Are Nakshatras?
Nakshatras (Sanskrit: नक्षत्र) are 27 divisions of the sky along the ecliptic, the path the Moon travels each month.
They are often called "lunar mansions" or star constellations.
The Moon takes ~27.3 days to complete its orbit, spending about one day in each nakshatra.
Each nakshatra spans 13° 20' (13 degrees and 20 minutes) of the zodiac (360° ÷ 27 = 13.33°).
Origins and Symbolism
Nakshatras were first described in the Rig Veda, possibly as early as 4000 BCE.
Each nakshatra is associated with:
A symbol
A deity
A planetary ruler
One of the three Gunas (Sattva, Rajas, Tamas)
Varna ( ( Brahmana, Kshatriya, Vaishya, Shudra, Butcher )
Totem Animal
Nature
Species ( Deva, Rakshasa, Manushya )
Meaning in Natal Chart
As it was said - Nakshatras are also called the "lunar mansions" - the places where the Moon "stops" while travels her orbit. The Moon is not actually stops, she is constantly moving (like every other celestial object), but she spends about one day in every nakshatra, which is why we are calling them stops.
As you can see from the drawing below - Nakshatras are the division of the sky into 27 equal parts ( same like division of the sky into 12 equal zodiac sign - the same principle applies here) .
When graha ( planet ) is placed in a particular nakshatra - she merges with energy of this nakshatra ( again same methodology, as if planet is placed in a particular zodiac sign ).
But in context of nakshatras - the placement of the Moon is the most important.
At the time of a person's birth, the Moon occupies a particular nakshatra, determined by the exact time and place of birth. This nakshatra heavily determines our subconscious behavior, social behavior patterns. Nakshatra of the Moon is one the main factors determining our sexual preferences and sexual compatibility.
List of 27 Nakshatras
# | Nakshatra | Symbol | Deity | Ruling Planet |
1 | Ashwini | Horse’s head | Ashwini Kumaras | Ketu |
2 | Bharani | Yoni | Yama (god of death) | Venus |
3 | Krittika | Razor/flame | Agni (fire god) | Sun |
4 | Rohini | Chariot | Brahma (creator) | Moon |
5 | Mrigashira | Deer’s head | Soma (nectar god) | Mars |
6 | Ardra | Teardrop | Rudra (storm god) | Rahu |
7 | Punarvasu | Quiver of arrows | Aditi (universal mother) | Jupiter |
8 | Pushya | Cow’s udder | Brihaspati (guru) | Saturn |
9 | Ashlesha | Serpent | Naga (serpent gods) | Mercury |
10 | Magha | Throne | Pitris (ancestors) | Ketu |
11 | Purva Phalguni | Hammock | Bhaga (pleasure) | Venus |
12 | Uttara Phalguni | Bed | Aryaman (companion god) | Sun |
13 | Hasta | Hand | Savitar (sun god) | Moon |
14 | Chitra | Pearl/Jewel | Tvashtar (celestial architect) | Mars |
15 | Swati | Coral/Plant | Vayu (wind) | Rahu |
16 | Vishakha | Triumphal arch | Indra & Agni | Jupiter |
17 | Anuradha | Lotus | Mitra (friendship god) | Saturn |
18 | Jyeshtha | Umbrella | Indra (king of gods) | Mercury |
19 | Mula | Roots | Nirriti (goddess of destruction) | Ketu |
20 | Purva Ashadha | Fan | Apas (water god) | Venus |
21 | Uttara Ashadha | Elephant tusk | Vishvadevas (universal gods) | Sun |
22 | Shravana | Ear | Vishnu | Moon |
23 | Dhanishta | Drum | Vasus (elemental gods) | Mars |
24 | Shatabhisha | Hundred healers | Varuna (cosmic order) | Rahu |
25 | Purva Bhadrapada | Sword, fire | Aja Ekapada (serpent deity) | Jupiter |
26 | Uttara Bhadrapada | Twins/Couch | Ahir Budhnya (water serpent) | Saturn |
27 | Revati | Fish | Pushan (protector of journeys) | Mercury |
Origin of 12 Zodiac Signs
Origins: Mesopotamia (c. 1800–500 BCE)
Early Star Lists (c. 1800 BCE):
Ancient Sumerians and Babylonians catalogued fixed stars and constellations.
They used the "Three Paths" (Enlil, Anu, and Ea) to describe the sky, based on latitude zones.
These early texts do not yet show a structured 12-sign zodiac.
Development of Zodiacal Thinking (c. 1000–600 BCE):
Babylonians began dividing the ecliptic into uneven zodiacal constellations (e.g., "The Lion" for Leo).
The Mul.Apin tablets (c. 7th century BCE) list star risings, with seasonal associations.
Birth of the 12-Sign Zodiac (c. 500 BCE):
Around 5th century BCE, Babylonian astronomers formalized a 12-part division of the ecliptic:
Each sign = 30 degrees.
Aligned with 12 lunar months.
Based on sidereal constellations, not the equinox.
This was the first mathematical zodiac—not just symbolic constellations, but geometrically equal signs.
Greek Influence: Hellenistic Astrology (c. 2nd century BCE)
The Greeks (especially Ptolemy) adapted Babylonian methods and added planetary rulerships, aspects, and tropical coordinates (starting the zodiac at the vernal equinox).
Tropical zodiac became dominant in Western astrology.
Why do we use Tropical Coordinates ?
The Tropical Zodiac is tied to the seasons - not the stars:
0° Aries in the Tropical system always starts at the March Equinox (when day = night).
This makes it a stable seasonal reference point for:
Observing solar cycles
Agricultural calendars
Climate studies
Celestial navigation
Consistency in Coordinate Systems
Astronomy uses Earth-centered coordinates, and the Vernal Equinox is the zero-point for many systems:
Right Ascension (RA) is measured from the Vernal Equinox point.
This point slowly precesses — but the system tracks it, and keeps updating.
So in practical terms, it's easier to use the precessing equinox point as a reference, which aligns with the Tropical system.
3. Historical Convention
Western science and navigation systems were developed in a framework influenced by Greek astronomy, which evolved with the Tropical zodiac tied to equinoxes and solstices.
When astronomers want to describe where a star or planet really is against the backdrop of constellations, they use actual coordinates (RA & DEC) or the Sidereal zodiac.
India and the Sidereal Zodiac
Indian sages like Lagadha (Vedāṅga Jyotiṣa )already used Nakshatras (27 lunar mansions).
By the time of Surya Siddhānta , India used a sidereal 12-sign zodiac, aligned with fixed stars (not the equinox).
Some historians suggest the 12-sign system was independently refined in India, others argue for Babylonian influence via Persian or Hellenistic intermediaries.
Stellar Rising - Sidereal Year
In ancient times, especially in Vedic India, Egypt, and Babylon, the calendar year and seasonal events were often tracked by observing which stars or constellations rose or set at specific times — not by how high the Sun was in the sky (which is what modern Tropical solar seasons do).
What is a "stellar rising"?
A heliacal rising is when a star or constellation first becomes visible just before sunrise, after having been hidden in the Sun's glare.
Example:
The heliacal rising of Sirius (Sothis) marked the Nile flood in Egypt.
In Vedic tradition, the heliacal rising of certain nakshatras (like Krittika, Ashvini, or Rohini) marked the beginning of agricultural, ritual, or astronomical cycles.
Conclusion:
As you can see from the informaiton above - the tropical zodiac is aligned with seasons, it is useful for calendars and celestial navigation. Tropical zodiac is useful for this practical things, because it's coordinates are easier to standardize.
Sidereal zodiac is aligned with fixed stars, and not with the seasons. When we speak about our natal chart - the moment of our birth which is fixed by the planets against the backdrop of the constellations - we want to know actual position of this planets against the constellations. This is why we use sidereal zodiac.
Rishis were well aware of precession before the modern science and they used sidereal system for a reason.
I hope now you can clearly understand the difference between this two systems. And if you still have a questions - just drop a comment below and we will continue exploring .