Asterisms and Constellations and how not to confuse them with Tropical Signs.

If you are going to write about something, especially if you intend to lay bare somebody else’s ignorance, it pays to actually know what you are talking about otherwise you could well end up looking like a total idiot, as does Anna Culaba in her article on the RYOT website, The Stars and Your Astrological Signs Have Been Lying to You This Whole Time. I should point out that Ms Culaba is by no means the first person to publically embarrass themselves pontificating on this subject, in fact it’s a reoccurring theme much loved by scientists and science fans who want to take a cheap shot at astrology. Indeed, as we will see later Ms Culaba, in her article, is in fact just regurgitating the content of a BBC website. So what exactly does our intrepid science fan say in her blog post?

My horoscope for today (I’m a Virgo) according to Astrology.com reads, “Today, explore an aspect of an unfamiliar religion or culture. Today is a day to make plans and aim high.” There are only two things that are keeping me from leaving work right now: one, I don’t really believe that the stars can determine what will happen in my life and two, I wasn’t really born under the star sign that the world told me I was born into. According to the BBC, about 86 percent of people are actually born under a different sign than the one they think. This is because 2,000 years ago, when the Ancient Greeks first created the zodiacs, the star signs corresponded to the position of the sun relative to the constellations that appeared in the sky the day people were born. Unfortunately, during that time people didn’t know of the phenomenon known as the precession. Live Sciences reports that the precession is when the Earth continually wobbles around its axis in an almost 26,000-year cycle thanks to the gravitational attraction of the moon. Thanks to this phenomenon, the constellations some people live and die by have actually drifted away from us. This means that constellations are now actually off by a month. So if you were born between August 11 to September 16 you’re not the picky and critical Virgo that you thought you were — you’re really an ambitious Leo whose strength of purpose allows you to accomplish many, many things. And if you’re astrological world hasn’t been rocked enough, if you thought you had your star sign wrong, wait until some of you realize that there’s actually a 13th zodiac sign known as the Ophiuchus. According to the BBC, the Ancient Greeks deliberately left out the original zodiac so that ancient astrologers would be able to divide the sun’s 360 degree path into 12 equal parts. Where does Ophiuchus fit into the zodiac calendar? It goes between Scorpio and Sagittarius, so if you were born between November 30 and December 18 consider yourself an Ophiuchus. You’re probably very secretive and good at hide and seek.

I have reproduced the whole of Ms Culaba’s screed here to save me having to quote it in little bits, merely removing the links from the original. If you read it through you what will discover is the central claim that astrologers were too stupid to realise the astronomical phenomenon of precession and so you were not actually born under the star sign that they claim you were. There are two general points to be made here, firstly astrologers were well aware of precession and secondly Ms Culaba and the source she is quoting don’t know the fundamental difference between constellations and tropical signs. So for the benefit of Ms Culaba and all others who are confused by the topic we will have a Renaissance Mathematicus guide to asterisms, constellations, the zodiac and tropical signs.

If you go out on a dark night with a clear sky in an area with little or no light pollution (and if you have never done so you should, it’s awesome) and look up in the heavens you will see a myriad of stars looking down on you in a vast blue black vault. If you are not a trained astronomer you will probably find no means of orienting your gaze in this confusion of twinkling lights. This problem was confronted by all human cultures since the dawn of human existence. The human brain seems to be programmed for pattern recognition and so, like children with a join up the dots picture book, all cultures started to create pictures by imagining lines joining up or outlining eye-catching groups of stars and giving these pictures names. These pictures, and they exist in all human cultures, are known technically as asterisms. These asterisms help the observing eye gain orientation when traversing the vast dome of the night sky and early astronomers started compiling lists of the most prominent such join-up-the-dots-pictures or asterisms in order to use them as a scaffolding for mapping the heavens. Those asterisms contained in such formal lists are called constellations. Our modern, western list of constellations has its origins in ancient Babylonian astrology/astronomy and comes down to us via the ancient Greeks and the medieval Islamic astronomers. In his Syntaxis Mathematiké, Ptolemaeus lists 48 constellations by name. Currently, the International Astronomical Union (IAU) recognises 88 named constellations. We now need to turn our attention to the origins of the zodiac.

Viewed from the earth, and before the beginning of the so-called space age that was the only way possible to view the heavens, the sun appears to orbit the earth once every year. In fact the year is defined as the time it takes for the sun to orbit the earth. The path the sun follows on its way around the earth is called the ecliptic and is tilted at approximately 23 degrees to the earth’s equator. This tilt, known as the obliquity of the ecliptic, is the reason why we have seasons on the earth. The six planets visible to the naked eye and know in antiquity – Moon, Mercury, Venus, Mars, Jupiter and Saturn – all appear to orbit the earth in the plane of the ecliptic making this imaginary belt around the heavens very important for the study of astronomy. The earliest known mapping of the ecliptic is contained in a set of Babylonian clay tablets known as the MUL.APIN, which date from around 1000 BCE. Here the path of the moon’s orbit is described or mapped with 17 or 18 (the text is somewhat ambiguous) constellations and stars. The moon’s orbit is tilted at about five degrees to the ecliptic. This mapping was still in use around 700 BCE. By around 500 BCE the 17/18 constellations/stars had be replaced by twelve constellations of varying sizes. Circa 420 BCE the Babylonians had replaced those twelve constellations with twelve equal divisions of the ecliptic comprising 30° segments. These segments were named after the constellations they replaced and form the zodiac that was taken over by the Greeks and made its way down to us. Those segments are known technically as tropical or sun signs, form the basis of zodiacal astrology and are abstract geometrical segment of the ecliptic and not constellations. The constellations slowly circle the heavens due to precession, the tropical signs do not! If an astrologer says you were born under the sign Virgo it means that the sun was in the 30° segment of the ecliptic that bears the name Virgo at the moment of your birth. This has nothing apart from the name in common with the constellation Virgo.

It is not the astrologers who display ignorance of the precession of the equinox, to give the phenomenon its full name, but Ms Culaba who displays total ignorance of both astronomy and astrology. This is not a very good situation to be in if you are going to write about the history of science and yes we are talking about the history of science here, the zodiac with its tropical signs was originally conceived for astronomical purposes. Ms Culaba might be excused because she did not originate this particular piece of history of science rubbish but is merely regurgitating false information from what she obviously thought was a reliable source, the BBC.

Here we have the presenter of Stargazing Live, a high prestige BBC science programme, Dara O Brian presenting the world with high-grade bullshit under the BBC’s banner. O Brian and his co-presenter Brian Cox should know better and I find it a total disgrace that the fee payers money is being wasted on such rubbish under the guise of educational television, both the presenters and the Beeb should be thoroughly ashamed of themselves.

11 Comments

Filed under History of Astrology, History of Astronomy, Myths of Science

11 responses to “Asterisms and Constellations and how not to confuse them with Tropical Signs.

  1. > The constellations circle the sphere of the fixed stars due to precession

    You lost me there. The constellations are composed of fixed stars, so how do they move relative to the fixed stars?

  2. The astrologers I know are well aware of the procession of the equinox and the difference between asterisms and tropical signs. The personality traits they associate with the signs reflect the constellations that were in the signs during the Hellenistic period, however, which is why LIbras are supposed to be well balanced, Leos courageous, etc. There are all sorts of systems, used by practicing astrologers so I wouldn’t be surprised if some of them have dispensed with or altered the usual interpretive symbolism, but garden-variety astrology harks back to the old pictures. Or am I missing something?

  3. Baerista

    “Circa 420 BCE the Babylonians had replaced those twelve constellations with twelve equal divisions of the ecliptic comprising 30° segments. These segments were named after the constellations they replaced and form the zodiac that was taken over by the Greeks and made its way down to us. Those segments are known technically as tropical or sun signs, form the basis of zodiacal astrology and are abstract geometrical segment of the ecliptic and not constellations.”

    You seem to be saying that the tropical zodiac was already introduced by the Babylonians, but I’m not sure this is in fact true. Pre-Ptolemaic astronomical sources normally distinguish the sun’s entry into Aries from the vernal equinox, which in a tropical zodiac would be one and the same. It seems to me that the crucial innovation was only made in the second century by Ptolemy, who in light of precession (a phenomenon first gestured at by Hipparchus) decided that zodiacal signs should be redefined as segments of 30 degrees starting from the vernal equinox. He subsequently founded his own astrological system on this new definition of a zodiacal sign and it’s thanks to Ptolemy that newspapers still use tropical signs for their weekly horoscopes. But this was a disturbing step to many astrologers at the time, who were used to casting horoscopes according to sidereal signs (or what they considered to be such). In fact, the main reason why precession was so widely ignored or rejected by ancient astronomers/astrologers (very few sources even mention it) appears to have been precisely because it was wedded to the absurd notion of a tropical zodiac. This, at least, is Alexander Jones’s take on things (“Ancient Rejection and Adoption of Ptolemy’s Frame of Reference in Longitudes”, in: Ptolemy in Perspective, New York: Springer 2010), and I think he’s right.

    • It’s been a while since I read up on the history of the zodiac but all the sources I read suggested that the tropical zodiac came into being in the fifth century BCE. I haven’t read the source you quote but I will certainly take a look at it. Thanks for the tip.

      Not that any of this changes the fact that Ms Culaba and O Brian are in this day and age confusing tropical signs and constellations

      • Baerista

        Persumably the crucial distinction here is between (a) establishing a zodiac based on the constellations visible in the sky, but making sure that each sign has 30 degrees regardless of the actual dimensions of these constellations; and (b) counting the first 30 degrees from the vernal equinox rather than the beginning of the sidereal sign of Aries. According to my understanding (a) was introduced by the Babylonians in the fifth century, but (b) was introduced by Ptolemy. The term “tropical zodiac” normally denotes (b).
        You are, of course, completely justified and correct in your misgivings re: Culaba and O Brian.

      • I ordered the Alexander Jones volume through Interlibrary Loan and picked it up from the library this afternoon on my way to a meeting with my friend Darrel Rutkin, the historian of astrology who, by chance has a paper in the same volume. As Darrel is writing the definitive history of Western astrology I thought I could ask him about the origins of the tropical zodiac. Can’t think why I didn’t think of that before. I explained the problem to him and asked him who was right and he answered,”both of you”.

        It would appear that I was correct in that the Babylonians did indeed create the tropical zodiac in the fifth century BCE, naming each 30° section for the constellation that occupied that part of the ecliptic at that time. However the Babylonians had not discovered precession yet, which was first discovered by Hipparchus in the second century BCE. Ptolemaeus is the oldest surviving text that draws attention to the problem of the drifting apart of the tropical zodiac and the sidereal zodiac, about 4 or 5 degrees by his time and to nail his astrology to the tropical zodiac.

        What I didn’t mention in my post is that traditional Hindu astrology is based on the sidereal zodiac. There is also a revival or renaissance in Western sidereal astrology that started in the 20th century.

  4. Ian H Spedding

    I see the Sun orbiting the Earth every day. Except when it’s cloudy. But then I assume it still is because it has been “confirmed to such a degree that it would be perverse to withhold provisional consent.” Of course, I could be wrong but that’s what it looks like.

  5. Pingback: Whewell’s Gazette: Vol. #44 | Whewell's Ghost

  6. Baerista

    Thanks very much for your post and for following up on this issue, Thony. However:

    “It would appear that I was correct in that the Babylonians did indeed create the tropical zodiac in the fifth century BCE, naming each 30° section for the constellation that occupied that part of the ecliptic at that time.”

    I already acknowledged this in my last post and I think the issue is that the kind of zodiac the Babylonians introduced in the fifth century BCE is and should be described as a “tropical zodiac”. A tropical zodiac identifies 0 degrees of Aries with the vernal point. I don’t know of anyone who did this before Ptolemy. In Babylonian astronomy, 0 degrees Aries and the vernal point are always separate – as far as I am aware. Hope this clears things up.

  7. Baerista

    Sorry, I should have written “is not and should not be described.” Cheers.

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