Exposing PseudoAstronomy

June 20, 2010

Astrology vs. Astronomy: What Coordinate System to Use?


As probably anyone who reads my blog knows, I have recently done a few blog posts related to astrology, specifically criticizing the astrologer Terry Nazon, both for her bad astronomy and for her threats and harassment towards me and a fellow blogger. My compatriot and much more famous and well known anti-astrologer (among other things) Phil Plait was even kind enough to mention me on his blog, though I’ll admit that I did point out my problems with Ms. Nazon.

This post is not about Ms. Nazon. This post is about astrology in general, and specifically whether my criticisms have been fair given that astrologers – some Western astrologers, anyway – use a different coordinate system and different definitions than the world’s astronomers.

What Is Precession?

Also, those who have been reading my blog know that I’m teaching an introductory astronomy class this month for college undergraduates. The second and third day of class were spent going over the motions of the the sky — the stars, sun, planets, moon, etc., over the course of one day, several days, a year, and then thousands of years.

One might think that going to thousands of years is a bit much — shouldn’t the sky look the same year after year? Actually, no. Earth spins on its rotational axis once every 24 hours. It goes around the sun once every 365.25 days (approximately). And during that time around the sun, the rotational axis is pointed at the same spot in the sky (hence why we have seasons).

That’s the year-after-year motion. However, this rotational axis direction does change. Just like a top spins on its axis, but then that axis wobbles in a circle, so does Earth’s axis. Over the course of approximately 26,000 years, Earth’s rotational axis makes one complete “wobble,” returning to the same spot it was before in the sky. This is the mechanism behind what you may have heard about the ancient Egyptians — they had a different North Star than we do today.

You also may have heard that many thousands of years ago, Vega was our North Pole Star, and that it will be again in many more thousands of years. This is all due to precession.

On a short timescale, say, a person’s lifetime, precession will not be noticeable in any meaningful way. But, astrology was not invented one person’s lifetime ago. Western astrology was codified nearly 2000 years ago by Ptolemy, but its origin really dates back more towards ancient Babylon, around 2500 years ago. 2500 is nearly 1/10th of 26,000, and that is a significant fraction.

Practical Effect of Precession

A practical effect, besides the north pole star (and south pole star) changing, is that the sun will appear to be in a different part of the sky on the same date from one year to the next.

For example, this year on the vernal (Spring) equinox, the sun will be in the constellation Pisces. But, 2500 years ago, when Western astrology was being developed, the sun on the vernal equinox was in the constellation Aries. And in roughly 700 years, the sun on the vernal equinox will be in the constellation Aquarius.

The bottom-line: The stars have moved!

To Precess or Not to Precess?

Pretty much all the criticisms of my posts about Ms. Nazon’s astrology focused on the basic fact that many western astrologers do NOT take into account precession. Astronomers, on the other hand, do. And so do many other western astrologers. So is it fair of me to criticize when we’re not, as the saying goes, comparing apples to apples?

In different terminology, astrologers use what is called the “tropical” zodiac instead of the “sidereal” zodiac. (At least, based upon my understanding these terms are interchangeable — tropical zodiac is non-precessed, sidereal is precessed.)

Definition of a Constellation

Another criticism I received is that astrologers do not use the same constellation system that astronomers do. As far as the Zodiac is concerned, astrologers have divided it into 12 equal parts, with each constellation exactly 30° (360° in a circle / 12 = 30°). Astronomers, on the other hand, follow the definition of the International Astronomical Union, as codified in 1930. The constellations do not have fixed 30° intervals, and they of course move with precession.

Here is an area will I will definitely admit that my criticism should at least have been tempered, somewhat. After all, constellations are arbitrary boundaries around pareidolia-ed points of light in the sky as seen from an arbitrary vantage point. Who’s to say that the astrologic system of dividing the Zodiac is any better or worse than the IAU’s?

However, I will unequivocally state that my criticism was not specifically about her boundaries, but more that she was off by about 10% of the sky (precession), rather than a degree or two due to different constellation boundaries.

What Does Astrology Claim to Be?

Don’t worry, I’ll get back to the discussion of precession and the criticism I received …

Believe it or not, I had difficult finding this — what the real root belief is about astrology — so I am going off of what I have read on many astrologers’ websites and my own understanding. So, please correct me if I’m wrong here:

The basic premise of astrology is that the apparent arrangement of stars, planets, moons, the sun, and other objects in the sky, as observed from Earth, all will have an effect on a person (or nation, or company, or whatever) based upon where they were relative to each other on the minute the person/nation/company/thing was born/created, and then where they are now (“they” meaning the astronomical objects).

Okay, even with my rather loose understanding of an exact mechanism (and no two astrologers actually agree on a precise mechanism — but that’s a different post), I think most would agree that that’s the basic idea of how astrology is supposed to work.

So, you need to know, say, where the planet Venus is in the sky. And what constellation it’s in, so you know what personality traits will be taken on. Among other things. Right?

Returning: To Precess or Not to Precess?

Told’ja I’d get back to this. So we need to know where in the sky certain objects will be. From Earth. After all, most of us were probably born on Earth (that’s another blog post, too …).

So shouldn’t you take into account precession? If I look up at the sky, right now, I see Mars in the western sky deep within the constellation Leo. As in, the planet Mars is, in Earth’s sky, physically surrounded by stars in Leo.

Contrast that with a non-precessed sky. This time of year, around 1 B.C., Virgo is squarely in the patch of sky that is currently occupied by Leo. So instead of Mars being surrounded by Leo, it’s surrounded by the stars of Virgo. Now, I don’t pretend to know what that would signify, but I would think that the God of War being surrounded by the Lion is different from being surrounded by the Virgin. Wouldn’t that change your readings?

Besides just this, and I apologize if this may seem to be a silly question, but doesn’t it actually matter what you can see outside versus what you imagine based on what the sky looked like over 2000 years ago? Virgo’s not where it was 2000 years ago … doesn’t this change things?

Final Thoughts

Alright, I realize that this post may seem condescending to some, ignorant to others, or stupid to a few more. But I actually am in all honesty, legitimately curious here. Especially because I know that western astrologers themselves are divided over whether to take into account precession.

Maybe it’s just the way I was trained in science — I think that if stuff moves, you move with it. It doesn’t make sense to me to use something as it was 2500 years ago when everything else has changed. To me, it seems the same as, say, starting to write on a piece of paper 15 minutes ago, and you still writing away in the same spot even though someone has moved the piece of paper far away from you. You’re just writing in air now.

And that’s why I feel justified in criticizing Ms. Nazon, and by proxy, all other astrologers who do not account for precession. She’s writing on air or her desk, not the paper that was there 15 minutes ago. The stars haven’t been where she says they are for thousands of years. Shouldn’t that matter?



  1. waow… the great post today. thank you.

    Comment by ankarajeoteknik.com — June 20, 2010 @ 11:31 pm | Reply

  2. I had no idea. I had astrology sitting nicely on the “Silly Wingnut” shelf, but now I find they belong in the “Babbling Braindead” category. I always assumed that they were amateur astronomers, only possessing a seriously daffy delusion. It’s startling to learn they’re just drawing random circles with purple crayon, as you say, in the air.

    Comment by Blaze — June 21, 2010 @ 7:52 am | Reply

  3. Astronomers regularly get into a knot over the ‘precessed’ zodiac compared to the zodiac used by most western astrologers. Every few years a lengthy article will appear somewhere in a newspaper or magazine complaining that astrologers have not taken into account precession and consequently the zodiacal signs used by astrologers do not coincide with the constellations. Originally all astrologers used the zodiacal constellations but somewhere in the second half of the 1st millennium BC the Greeks replaced the haphazard zodiacal constellations with the sidereal zodiac (based on the zodiacal constellations but ‘tidied’ up so that each of the 12 sidereal signs are exactly 30 degrees each).

    However when Hipparchus ‘discovered’ precession around 125 BC, he also created the new tropical zodiac which is NOT based on the constellation but upon the two equinoxes and solstice points. Tropical Aries commences with the Vernal Equinox, Cancer with the summer solstice, Libra with the autumn equinox and Capricorn with the winter solstice. The other signs are calculated from these four points, with two of these points remaining on the modern atlas as the Tropic of Cancer and Tropic of Capricorn. In the 2nd century AD Ptolemy publicized Hipparchus’ new tropical zodiac which was picked up some centuries later by the Islamic scholars who finally past it on to Europe after the European Dark Ages. Most western astrologers to this day continue to use the tropical zodiac because it WORKS. There are a small band of western astrologers who use the sidereal zodiac instead.

    The situation in India is different. In ancient times they not only had been using the zodiacal constellations, but also the Nakshatras which is the same ecliptic but divided into 27 sections (28 in China) based on the average movement of the moon each day. Perhaps because of this extra level of involvement with a sidereal framework, when Hellenistic astrology arrived in India in the early 1st century AD the vacillated between the two zodiacs – the tropical and sidereal zodiac but ended up preferring the sidereal zodiac which they continue to use to this day. The reason they use the sidereal zodiac is because it WORKS for them.

    The sidereal and tropical zodiacs are not mutually exclusive. Both western astrology, based on the tropical zodiac, and Vedic (Indian) astrology based on the sidereal zodiac work in the same way that French and English both work in communication – but the words, grammar etc are almost totally different. Some astrologers use both systems simultaneously. From memory, 15% of the members of the International Society of Astrological Researchers refer to both systems and approaches. An analogy it is like having two cameras – one takes the front on view while the other takes a silhouette view.

    So despite the fact most astronomers go into apoplexy about western astrologers not taking into account precession, even astrologers fight about this difference amongst themselves. Many Vedic astrologers consider western astrologers use the wrong system and vice versa. However those astrologers that make this claim only use one system. It’s a bit like an English person claiming a Frenchman cannot speak intelligibly and vice versa when the language they use is no reflection upon intelligence.


    Comment by Terry MacKinnell — July 31, 2010 @ 10:42 pm | Reply

  4. […] There is really very little astronomy in this forecast other than the common astrological nonsense about houses and having all the planets in the wrong place in the sky. […]

    Pingback by Picking Apart a Sham: 2011 Astrology Predictions from Terry Nazon « Exposing PseudoAstronomy — January 12, 2011 @ 1:21 am | Reply

  5. […] affect Earth in any way, shape, nor form (unless you’re an astrologer, but then I’ve talked about that before). And of course, a coming leap year would also have absolutely nothing to do with the sun rising […]

    Pingback by Planet X and 2012: Sun Returns to Greenland … Two Days Early!? And a Major Fail by Time Magazine « Exposing PseudoAstronomy — January 21, 2011 @ 12:37 pm | Reply

  6. To precess or not to precess? To precess. Accordingly, shall we please rename the tropics “Tropic of Gemini” and “Tropic of Sagittarius” to accurately reflect the solstices? Who’s with me?

    Comment by TheGideonLion — March 16, 2011 @ 7:10 am | Reply

  7. Actually the signs are nowhere near 30 degress each and vary widely. I’ve yet to see a sidereal system take this into account. If not taken into account, it renders the logic above invalid. The Babylonians of course new this but the evidence is 100% that they ignored it, tidying the zodiac into 12 equal signs. Also a fact is that they based the meanings of their signs on the seasons – certain signs were associated with the fertile flooding of rivers, others with the heat of summer, etc. So seasonality cannot be excluded. Third, if you are born at a high or low latitude, the sun and starts do not project onto any of the 12 signs of the zodiac, but one of the constellations south or north of these. Astrology is based on tradition and the accretion of cultural wisdom. Those who try to increase their own standing by appealing to “accuracy” are doomed to fail.

    Comment by W — October 4, 2011 @ 12:04 am | Reply

  8. You can use tropical zodiac if you want 0 degrees at the vernal equinox. Either way, the first of what is called “Aries” is astrology is zero. Forget about the constellations (some of the signs of the zodiac are not even exactly the same as names of constellations anyways). “Cancer” means 90 degrees and “Capricorn” means 270 degrees (of ecliptic longitude, or sometimes of equatorial right ascension). No need to rename the tropics; just remember that these names refer to angular measurement rather than constellations in this context.

    If you said “…if you are born at a high or low latitude, the sun and starts do not project onto any of the 12 signs of the zodiac, but one of the constellations south or north of these…” note that the “signs” are just angular measurement system. Use degrees directly if you prefer (I do use degrees directly). Measure declination if you want to. It doesn’t really matter because the sun and stars don’t affect your life anyways; but if you want to know where the sun is and stuff, you need to have a system to measure it.

    You can take precession into account if you want to and use sidereal zodiac; really it depends what you are trying to measure.

    Houses do not consists of complete nonsense either. There are different systems to split the houses, but in general 1 to 6 are below the horizon, 7 to 12 are above the horizon, so the Sun rises at 1st cusp, is solar noon at 10th cusp, and sets at 7th cusp. If you set Astrolog to hours/minutes notation and equatorial right ascension, and tell it to plot the “Midheaven” (a.k.a. Zenith, or 10th cusp), that is the Local Sidereal Time. I have even used Astrolog to determine the dates of Chinese New Year.

    Comment by Anonymous — November 5, 2011 @ 8:05 pm | Reply

    • I think you’re missing the point of my post. The point is – as I understand it – that astrology claims to predict things on where stars are in the sky, and where planets and other celestial objects are relative to those stars/constellations. (If this is not what the basic claim is, please correct me.) Based on this, you need to take into account precession. If you do not, you are not determining an astrological reading based on where stuff actually is, you’re basing it on where it would be if the year were around 500 B.C. That does not make any sense; it’s called “magic” if it actually worked. My contention is that since you can get just as “accurate” a reading from using precession or not, that astrology is just a basic magician’s cold reading, and in some cases even a hot reading because the sitter will tell the astrologer about themselves.

      Comment by Stuart Robbins — November 5, 2011 @ 8:35 pm | Reply

  9. “The point is – as I understand it – that astrology claims to predict things on where stars are in the sky, and where planets and other celestial objects are relative to those stars/constellations. (If this is not what the basic claim is, please correct me.)”

    Even if you totally disbelieve 100% of all claims re astrology, you will not understand it unless you have studied it – even as an academic trying to understand the methodology. So to make claims without the study is useless and ignorant. Your definition of astrology does slightly overlap with Hindu astrology, as Hindu astrology uses 12 sidereal zodiacal signs superimposed over the zodiacal constellation. However in both western and hindu astrology, the focus in both is not on the stars at all. There is a field within astrology whereby the relationship of planets etc to the fixed stars is applied, but very few astrologers do this. For the most part, both western and Hindu astrologers are solely concerned with the relationships between the sun, moon and planets, and the astrological signs – fixed stars rarley enter the sphere of investigation. so you are right, your statement of the basic claim of astrology is extremely wide of the mark.

    There is abosultely no need to take into account precession in western astrology as it uses the tropical zodiac based on the seasons, and has nothing to do with the stars or constellations. However in Hindu astrology, precession must be taken into account as they use a sidereal (star-based) zodiac. If you were really willing to disprove astrology you would first have to study it. Most academic archeo-astronomers have excellent knowledge of the techniques of astrology (modern and ancient) without believing in it, otherwise they would be ignorant of their subject matter because nearly all ancient astronomy was for the purposes of astrology.

    Comment by Terry MacKinnell — November 7, 2011 @ 7:49 pm | Reply

    • Your statement, “For the most part, both western and Hindu astrologers are solely concerned with the relationships between the sun, moon and planets, and the astrological signs …” does not make sense to me in light of astrology not caring about where stars are. “Astrological signs” are based – at least originally – on constellations, right? So when western astrology was codified, it cared about where the sun, moon, and planets were in relation to constellations if each constellation covered 30°, right? But apparently now, where those constellations actually are doesn’t matter in reality, it only matters where they used to be over 2000 years ago?

      In addition to this, why do some western astrologers actually take precession into account and claim to get just as much accuracy as those who don’t?

      As to Hindu versus Western and one group taking into account precession and the other not, my basic argument is that this is like saying Person A uses a totally different basic law of physics than Person B. If astrology wants to be considered a science, then it has to have a unified, cohesive set of guidelines, be testable, and ideally have a mechanism. Having two entirely different systems that both claim to be accurate while being based on grossly different extrapolations would indicate that there’s something else going on, that the apparent predictions of astrology are not actually based on those observations.

      Comment by Stuart Robbins — November 7, 2011 @ 10:57 pm | Reply

      • It does not make sense to you because you ahve neither studied astrology nor understand its methodology. Yes originally (2500 years ago) the sun, moon and planets were placed into their respective zodiacal constellations. But things evolve, just like in science where doctors no longer refer to Galen. In the end, astrologers have to produce the goods for their clients, and will always look for techniques that provide greater clarity and more information to their clients. This is what droved the evolution of astrology, just like in most fields. We don’t listen to rocks being knocked together anymore – we now have electric guitars. Astrologers first assigned the planets etc to the constellations, then they replaced the constellations by 12 sidereal signs of 30 degrees each overlayed upon the constellations. Then the Greeks developed the tropical zodiac about the time both tropical and sideral zodiacs temporarily aligned. They sent their knowledge of astrology off to India, and the Indians played with the tropical zodiac but in the end preferred the sideral zodiac (their indiginous astrology was sidereal). The islamic astrologers picked up on Ptolemy’s greek tropical zodiac and promulgated its use, with this knowledge passed on to Europe around 1200 AD which is why the tropical zodiac became the zodiac of choise in the west.

        You may say that western astrologers are using an archaic version of the sidereal zodiac fixed in time to around 2,000 years ago because you cannot recognise that both tropical and sidereal zodiacs have integrity, work but are very different. You cannot replace the tropical zodiac with the sidereal zodiacs or vice versa and interpret using the same techniques. Only Hindu techniques work with the sidereal zodiac and tropical techniques only work with the tropical zodiac. its like two languages – they are very different but they communicate a similar message.

        There is a small band of western siderealists that do use a sidereal zodiac without using Hindu interpretive techniques – but less than 1 % of western astrologers would do this and their numbers don’t seem to be growing. The reason they do it is because whatever techniques they have developed applied to the sidereal zodiac works for them. Both zodiacs offer a workable framework but the techniques applied to them are different.

        You should become an astrologers as many astrologers state your same argument that there should not be two zodiacs – only one can be correct. But then again you have all these astrologers successfully applying their craft in the west using the tropical zodiac, and Indians successfully doing the same with the sidereal zodiac, and multi-lingual astrologers like me that do both techniques. I use both zodiacs, they both provide excellent results, but both must be interpreted very differently – it took me about 15 years to comprehend how to interpret the sidereal zodiac whereas the tropical zodiac is far more intuitive

        Until String Theory etc came along, science could not find a unifying principle incorporating quantum mechanics and the theory of relativity, and i am not sure how many scientists accept string theory or m theory as the answer anyway. The problem with the tropical/sideral zodiacs is not a problem in practise – they both work. The problem is that no one knows why astrology works, including astrologers. Without a plausible underlying mechanism etc, then conflicting attrributes such as the sidereal/tropical controversy remain inexplicable. Scientists don’t devote any attention to this subject as they threw the baby out with the bathwater about 300 years ago and so will not devote any attention to something that does not exists anyway.

        In astrology, IMO, any astronomical view of any sort whatsover has the potential for astrological insight and provided the astronomical phenomena is strong enough – there usually is an astrological insight aligned with it. The astrological insights provided by the tropical zodiac are different to those provided by the sideral zodiac .. but they don’t conflict. Its a bit like two mug shots, one frontal, one on the side – they are very different, show different aspects, but cannot conflict if they of the same person.

        Comment by Terry MacKinnell — November 8, 2011 @ 6:36 pm

  10. Terry, one need not know how astrology supposedly works to know that it doesn’t work.

    Comment by Phil — November 26, 2011 @ 4:58 am | Reply

  11. The topic was not on the validity of astrology, but on the zodiac used. If astrology does not work, it does not matter one iota if the zodiac used is the sidereal/constellational zodiac or the modern tropical zodiac. I am not interested in getting involved in discussions of the validity of astrology because it is like a discussion between Muslims and Christians arguing about who the true Messiah is. I was merely trying to explain in response to a query how astrologers view a situation in non-existent astrology to someone who did not know the inner workings.

    Also, further to my previous post, the argument aginst the modern tropical zodiac in favor of the old constellational zodiac also ignores the Tropic of Capricorn and Tropic of Cancer. These geographical coordinates are based on the tropical zodiac, not the old sidereal/constellational zodiac.

    Comment by Terry MacKinnell — November 26, 2011 @ 8:05 pm | Reply

  12. Let me say first off that you’re site is a real gem. I was led here after listening to Brent Miller on C2C discuss geographical pole shifts. I thought he was talking about magnetic pole shifts at first, until I realized he wasn’t. Amidst reading your articles concerning the whole topic, I decided to browse and found this, perhaps I can share what I know about the subject. Mind you, I am no astrologer, and do not claim to be. I have been interested in astrology for a little over one year now, which I was exposed to during my experiences with Ayurveda (an ancient, holistic healing system which came from India), so my background is non western. Astrology was inseparable from Indian culture until the “modern era”, but still remains a part of daily life for some. My studies have shown that Indian astronomers were aware of precession, as the stories the Ramayana and Mahabharata state where the vernal equinox was in relation to the “fixed stars” at the time they were written, which was in Taurus. Of course it then precessed through Aries during the time of Ptolemy, then to approximately six degrees Pisces now. In Vedic astrology there are two major categories, mathematics and interpretation, and they give value to each other. The situation is such that it is not a matter of which Zodiac is calculated correctly, but which Zodiac is to be interpreted using the principles of astrology. Vedic astrology, better defined as Jyotish (the science of light), accounts for both Zodiacal starting points, and uses the Zodiac derived for different purposes and under different rules of interpretation. The tropical (sayana) zodiac is used for mundane events (weather, climate, etc.), while the sidereal (narayana) zodiac is used for natal horoscopy, under a daunting extent of interpretive rules and regulations.

    Also, the great majority of Jyotish is math. In fact, in ancient times a prospective student of Jyotish would be forced to master physics, astronomy, geometry, and advanced mathematics before even thinking about astrology/horoscopy. Signs are mathematical divisions of the sky. The rulerships are based on the sequence the planets appear in our solar system. The houses are sometimes determined by mathematical divisions, and sometimes just based on the sign. Of course, the planetary longitudes are mathematically derived and converted to a certain zodiac based on the precession. A subtler part of Jyotish called divisions (vargas) utilizes divisions of a sign to create more horoscopes used for interpreting specific areas of life in question based on the divisional equation used. There is a laundry list of other astrological forces to be derived from employing various equations on the natal chart longitudes of certain points. In fact, even the constellations of a Jyotishi’s sky are spaces understood by mathematical calculation and not necessarily by the size of star groups. Is this understanding of astrology different from what our preconceived notions understand it as (i.e., a belief that meaning about our physical, mental, and spiritual lives is derived from a connect-the-dots game our ancestors played with the sky)? Does this open new doorways for the understanding of astrology? I will not answer these as I can’t, as they are different for everybody.

    Everything you said in this article is correct. Consequently, I also agree with everything in this article. Astrology should adhere to astronomy, or, what we see in the sky, but that is only the beginning. I think there is an amount of philosophical background as well as objective information that must be accounted for before one seriously decides to believe in it or not. Of course, knowledge informs opinion, but the stifling amount of information out there usually forces us to form opinions before we have the adequate knowledge. Further, opinions directly influence action, which makes the ill-informed the most dangerous to any subject. I am not suggesting anyone believe in astrology if they do not want to and/or their evidence speaks against it, but I am suggesting that astrology is worth looking in-depth into, and that reliable sources are absolutely fundamental in that decision process, especially for astrology. Here are some reliable sources I am not affiliated with for astrology, astronomy, and related subjects…
    http://www.saptarishisastrology.com (hundreds of articles concerning relevant topics)

    I hope I have stayed on topic. Thank you for all your research and knowledge.

    Comment by Noah — December 8, 2012 @ 4:44 pm | Reply

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