Welcome back to my intro to astrology series! Last time I gave a brief overview of the history of astrology and today I will be diving into how the zodiac is calculated. This series is here for people who are clueless about astrology and would like to develop a working knowledge of it. So please, do not use my series to administer astrological consultations to other people, as this will not prepare you to be able to ethically do that. This is to empower you to understand your own chart and understand what astrologers are talking about when you consult them.
Let’s dive in.
Tropical vs Sidereal Zodiac
In modern times, especially in the west, the zodiac is often taken for granted without realizing that there actually two to choose from: tropical or sidereal. In short, the tropical zodiac maps the zodiac by the Sun’s position relative to the Earth’s equator, while the sidereal zodiac maps the zodiac by the Sun’s position relative to the fixed stars that make up the 12 constellations of the zodiac. In the west, the tropical zodiac is the primary zodiac while in Vedic the sidereal zodiac is used. How did this happen? It helps to go back to our history lesson.
As you may recall, the 12-sign zodiac we are familiar with was created by the Babylonians around 500 BCE. They accomplished this by projecting the ecliptic, which is the path the Earth orbits around the Sun, outwards into space and observed the constellations that crossed it. If you observe the sky, there are actually 13 constellations that lie on the ecliptic and they do not all take up equal space. The 13th constellation, Ophiuchus, sits nearly directly atop of Scorpius but is discarded. The zodiacal wheel that we know of is a 360° circle that is divided into 12 equal portions of 30° each – this is a standardization that does not accurately reflect the true distribution of constellations across the ecliptic, but is convenient for the purpose of natal astrology and mathematical divisions. The image below shows the distribution of constellations along the ecliptic. You can observe that some constellations are wider, narrower, or only partially astride the ecliptic such as Scorpius. The 30° approximation of these signs allows for the Sun’s movements through each sign to measure to about the length of a month’s time.
How do we get from here to two separate zodiacs? The answer is a phenomenon known as the precession of the equinoxes. You probably know that Earth’s axis is tilted, not perpendicular to the ecliptic. Due to this tilt, the Earth “wobbles” and the direction towards which its north pole points progresses along a circular path over time. The north pole currently points at the star Polaris, known as the North Star – but it did not always. This phenomenon results in the slow drifting of the dates when our annual equinoxes and solstices occur, which means that the Sun’s position in the sky is not always in the same place when we experience a given equinox or solstice as time moves forward. Thousands of years ago, the astronomical change of seasons occurred at later dates than they do now, and they are steadily occurring earlier – but not at a pace that you will be able to perceive in your lifetime.
When the Babylonians created the zodiac, the Sun’s position in the sky during the equinoxes and solstices sat in the center of the cardinal signs: Aries, Cancer, Libra, and Capricorn. During approximately 134 BCE it was the Greek astronomer Hipparchus who observed that the Sun’s position in the zodiac during these seasonal events had drifted backwards through the signs. For example, during the time of the Babylonians, the Sun occupied around 15° Aries during the Spring equinox, but had drifted to about 5° Aries during Hipparchus’ discovery. Hipparchus is credited with having discovered the precession of the equinoxes in the west, though he was not necessarily the first in the world to have done so. Later on, Ptolemy observed some time during the 2nd century CE that the Sun’s position during the Spring equinox was exactly aligned with the start of Aries. He then asserted that the zodiac must be derived from tropical coordinates – using the Sun’s position relative to the Earth – and thus the tropical zodiac was born and put to use in the west.
What are tropical coordinates? We talked about the ecliptic, which is the path along which the Earth orbits the Sun. There is another circular plane that crosses the ecliptic known as the celestial equator, which is the Earth’s equator projected outwards. The celestial equator and the ecliptic cross at two points, and when the Sun’s position in the sky aligns with these points the equinoxes occur. When the Sun occupies its most northerly or southerly position relative to the celestial equator, the solstices occur. This is how we experience the change of astronomical seasons here on Earth. This was very relevant to agriculture during ancient times and that is why the equinoxes and solstices are paid so much attention by different cultural and spiritual practices, so it makes sense that the Babylonians would have divided the zodiacal wheel in such a way that these events occurred when the Sun was in the center of the cardinal signs. However, even though some evidence exists that suggests the Babylonians may have been aware of the precession of the equinoxes, it’s unclear how this factored into their calculation of the zodiac.
Where does that leave the sidereal zodiac? During the time of the Babylonians, and throughout the ancient world, while the equinoxes and solstices were very important, astronomical calculations were conducted relative to their sidereal positions. Of course, the zodiac started with the Babylonians and there were no two zodiacs until Hipparchus and Ptolemy emerged. The zodiac was being calculated through the sidereal method – that is how Hipparchus was able to observe that the positions of the Sun during equinoxes and solstices was shifting over time, by comparing his present calculations to the calculations of his predecessors. The sidereal method involves observing what are known as the fixed stars among the 12 constellations – 28 of these stars feature in the Indian lunar mansions from our history overview. Instead of observing the Sun’s position along the ecliptic relative to the Earth’s equator, the sidereal zodiac observes the Sun’s position relative to these fixed stars. This means that it is generally the same date year after year when the Sun’s position intersects with a given fixed star. The fixed stars have been tracked and used by the Indian tradition for thousands of years, even preceding the Babylonian zodiac, so it makes sense that we see the sidereal zodiac still in use prominently in Vedic astrology.
While tropical astrology accounts for (or doesn’t, depending on your perspective) the precession of the equinoxes by charting the zodiac according to the Sun’s movements relative to the Earth, sidereal astrologers has to make different calculations to account for precession with the sidereal zodiac. This is accomplished through what is known as an ayanamsa. An ayanamsa is the calculation of the precise degrees of separation between the start of the sidereal zodiac and the position of the Sun during the Spring equinox. There are different ayanamsas, leaving room for debate as to which one is best to use. However, the differences between them are generally incredibly marginal and relevant only to seriously meticulous astrologers.
Which Zodiac is Correct?
The debate on which zodiac is correct is neverending and frankly, insoluble. The answer largely comes down to preference and what arguments compel you the most. Tropical astrologers generally feel that the seasons are essential to the descriptions and significations associated with the zodiac signs, while sidereal astrologers feel that it is the power inherent in the stars and the mythic traditions they carry that are essential to the interpretations we’ve come to associate with the zodiac signs. While the debate will no doubt continue to carry on for eternity, you get to choose which zodiac you prefer.
Due to the precession of the equinoxes, the tropical zodiac has drifted around 24° behind the sidereal zodiac, which is why you sometimes see headlines going on about how your zodiac sign has changed. When the Sun is in tropical Aries up to about 24°, it is actually in sidereal Pisces – and will appear that way if you observe the sky. This means that interpretations could vary significantly whether you are seeing a tropical or a sidereal astrologer. It’s something to be aware of.
It should also be noted that there are Western astrologers who use the sidereal zodiac and there are Vedic astrologers who use the tropical zodiac. It really is about preference and what makes the most sense to you. I ended up converting to the sidereal zodiac because it didn’t make sense to me for the zodiac to not align to the stars in the sky. I like to be able to use my astronomical sky map to observe the position of the planets and know that the constellations I observe them in are in fact the zodiacal signs they are occupying at that moment. One of the downsides of the sidereal zodiac however is the lack of consensus around which ayanamsa should be used. Another criticism of the sidereal zodiac is that it does emphasize the seasonal rhythms we experience here on Earth. It cannot be denied how much emphasis was placed on the equinoxes and solstices in ancient times – they are universally celebrated around the world and played a huge role in the traditions that were adopted that eased people into living in accordance with the natural rhythm of the seasons. So, I understand where tropical astrologers are coming from. One major criticism of the tropical zodiac however is that much of its interpretations are relative to the way seasons are experienced in Greece – and the rest of the world is not like that. In fact, the seasons are opposite between the Southern and Northern hemispheres and must be accounted for depending on which hemisphere you are located in. This is not necessary if you use the sidereal zodiac.
So, again, it really comes down to whether you think stars are more important or seasons are more important. Compelling arguments can be made for either case. My goal here isn’t to tell you which zodiac is better or more correct – it’s to provide information so you can develop an informed opinion yourself. I meet people all the time that have no idea that there are two zodiacs, or that the tropical zodiac which is taken for granted is not anchored to the constellations in the sky. I hope that this overview has been helpful in that regard. Thank you for reading. I’ve included my sources below.
I am currently studying to become a professional astrologer and if you are interested in receiving a brief consultation please check out my rates and services page. I am grateful for opportunities to practice my skills and increase my experience.
Harrison, Y. (2021a, December 20). Tropical Vs. Sidereal Astrology. Tilt Astrology. https://www.tiltastrology.com/post/the-tropical-vs-sidereal-year
Harrison, Y. (2021b, December 30). The Trouble with Ptolemy. Tilt Astrology. https://www.tiltastrology.com/post/the-trouble-with-ptolemy
Koch, D. (2020, January 7). Ayanamshas in Sidereal Astrology. Astrodienst. https://www.astro.com/astrology/in_ayanamsha_e.htm
Perry, G. (2021, March 1). The Two-Zodiac Problem: Toward an Empathic Understandng. Glenn Perry. https://aaperry.com/two-zodiac-problem-toward-empathic-understandng/