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Astrology is as old as astronomy, but where and when did this idea arise that events in the sky is related that on Earth? In fact, as seen in the pages on history , humans from prehistory came under a sky studded with bright points that all seemed to be at the same distance from the observer, our eyes not allowing us to see relief (i.e. distances) beyond a few tens of meters. The question of the meaning (and usefulness) of all these "lights" naturally arose.

Since prehistoric times and because of the precariousness of human existence, a constant concern was to try to predict the future. From a simple questioning on the periodicity of the seasons, on the return of spring, on the recurrence of astronomical phenomena (phases of the Moon, eclipses, ... ) to the question: " what will happen to me in the future? ". These questions were asked to astronomical observations. It is a real fact that causes and effects have been identified between the motion of celestial bodies and life on Earth: the arrival of spring and the increase of the declination of the Sun are clearly related! Concerning the link between the sky, the characteristics of individuals and their future it is not so clear.

Let us recall the superstitions of antiquity on the flight of birds : some movement was lucky , another unlucky. Astrology based on the same principle but much more complex, this complexity giving an illusion of science for those who do not have access to knowledge.

Thus, astronomers and astrologers merge for several thousand years until our knowledge of the things of heaven is sufficient to rule out any link between celestial movements and individual human life. This is during the seventeenth century, when, at the time of the founding of the Paris Observatory, a royal decree now banned astronomers to take care of astrology. Why a decree if things were so obvious ? Just because astrology reports more than astronomy and some astronomers funded their work through consultations in astrology!

How astrologers reason : the principles of astrology

The signs of the Zodiac

Sun, Moon ("luminaires") and the planets move through the zodiac , that is to say in the ecliptic. The inclinations of the orbits of this plane are low (0.8 ° to 3.4 °, except Mercury and Pluto 7° 17°) and it is sufficient to spot them by their ecliptic longitudes.

The Zodiac (or ecliptic ), described by the Sun in one year, is divided into 12 equal parts of 30° each, named "signs". To what historical reasons? The division of the year into four seasons is understandable (solstices and equinoxes) at least in our latitudes. However, the division of each season in 3 is arbitrary (3 moons ?). For example, it seems that people of megaliths (4000 years ago) have divided seasons in two: many alignments give sunrises (and sunsets) at the equinoxes and solstices, but some also provide intermediate dates.

The history of the names of the zodiac signs is not very clear, but it seems clear that some were named after the constellation corresponding to that time, such as Gemini. However, Libra, which contains no remarkable star (α and β are of 3rd magnitude) derives its name, according to Virgil, to equal day/night when the sun enters at the autumnal equinox.

The "theme" or "birth horoscope"

Il est défini par les positions des astres dans le ciel au moment de la naissance.

It is defined by the positions of the stars in the sky at the time of birth. Part of the theme reflects only the positions of the celestial bodies in the signs and between them (they are called "aspects"): it does not depend on the place, but only the date. Variations are slow: the fastest motion is the one of the Moon which puts 2.5 days to cross a sign.

Another part involves the horizon, through the system of "houses" detailed below. It will therefore depend on the place. It varies a lot faster, the sky moving from one sign to another in 2 hours. The distribution of the 8 planets, the Sun and the Moon in the 12 houses can therefore change up to 120 times a day.

As a result, few people have identical horoscopes, contrary to the claims of an argument often used (incorrectly). Even in a big city like Paris, it arises only about 200 children a day! The problem of twins, with identical theme and different destiny should however be problematic for astrologers...

The house system or the definition of 12 houses.

One defines "ascending" and its opposite the "descending" by the intersections of the horizon and the ecliptic, and the " mid-heaven" and its opposite "background of the sky" by the intersection of the meridian and the ecliptic. For the rest, astrologers do not agree. The most commonly used system is the one of Placidus of Titis (which dates from the 17th century, but taken from Ptolemy) cutting the sky around the north-south horizontal axis (not the pole axis North- South) in 12 time that the Sun travels in equal time from sunrise to sunset (the "daytime houses") or from sunset to sunrise (the "night houses "). Diurnal and nocturnal houses therefore have different durations. There are other systems, such as that of Maternus has 12 equal houses .

This house system will be a problem at high latitudes...

What is wrong with the birth astrological process?

The problem of high latitudes

Beyond the polar circles North or South:

- the part of the theme independent of the place remains valid : at the time of birth , the stars have a well-defined position on the zodiac .
- the horizon and the ecliptic are great circles , they intersect at two opposite points : there always has an ascending (and a descending), except for a singular event : the Arctic Circle , once a day , when the pole of the ecliptic passes at the zenith.
- there is always a middle of the sky, except at the pole where you can not define the meridian.
- at the end, only the definition of the houses does not work beyond the polar circles, when the sun did not rise or set, that is to say in the vicinity of the two solstices.

The precession of the equinoxes and the 13th sign of the zodiac

The widely used argument that astrology is worthless because it ignores the precession (offset of a sign and a half compared to the constellations since antiquity) is not valid because astrology does not give particular influence of the stars themselves (at least in general we must not forget that there is no one astrology). It is also mentioned and refuted in most astrology books.

Let us recall the origin of this problem: the ecliptic is fixed in space (on the durations of interest here), but the axis of rotation of the Earth precess on a period of about 25800 years. The equator - terrestrial or celestial - is driven in this precession, as are its intersections with the ecliptic: the equinoxes. Precession therefore comes from the choice of the origin of longitude. If we had chosen in the ecliptic, a "fixed" origin in space ( relative to the stars ) , the constellations would have remained in their sign over time . However, the influences of the Sun and Moon on the Earth (seasons, tides) would gradually shifted with respect to signs and their associated constellations. It is the choice of a calendar following the seasons that made us adopt a mobile origin compared to the stars. We measure the positions relative to the intersection of the ecliptic and the equator : the vernal point (denoted " γ " in reference to the symbol of Aries) .

The scale of the ecliptic into 12 signs of 30° (tropical zodiac or seasonal, opposed to zodiac of constellations) is therefore nothing but a marker in the sky. With this choice, despite the precession, the fair weather , for example, comes back ( in the northern hemisphere ) when the Sun " enters the sign of Aries " (and not in the eponymous constellation). It was also at this time that we have tides. But in this system the constellations are moving referred to the signs: we can not have everything!

Note that this use of purely geometric tropical zodiac removes some other problems raised by opponents to astrology:
- the three-dimensional nature of constellations whose appearance is the result of a projection on the sky of stars located at arbitrary distances from Earth .
- the very unequal lengths of the intersections of the ecliptic zodiacal constellations .
- the omission of the "13th sign " (Ophiuchus between Scorpio and Sagittarius) in the band of the zodiac.

We must not forget that the current boundaries of the constellations is very new: it dates from a resolution of the International Astronomical Union in 1922.

Where astrologers are caught inconsistency is when the interpretation of the theme retains the symbolic of constellations that were used to name signs: strength Leo, Gemini dual character , etc. . (but we must recognize that this is rare) , and also when the motion of the vernal equinox in the constellations is considered " Age of Aquarius " . One should choose!

The "influence" of the celestial bodies

If the stars themselves are not expected to have a direct influence on human beings, it is not the same for the bodies of the solar system. We will see, however, that peoples present during childbirth have a gravitational influence much greater than that of celestial bodies! We give below a comparison of the intensity of the gravitational force (proportional to Masse/Distance2) and the tidal force (proportional to Masse/Distance3) exerted by the Moon, the Sun, the planets (at their closest to the Earth ), the Eiffel Tower or a mountain (at ~ 1 km) and the obstetrician (at 1 m) on a newborn human being (though note that astrologers generally avoid invoking a gravitational origin for so-called astral influences):

 Mass (kg)   Distance (m)   Force of the gravitation (Lune=1)   Force of the tide (Moon=1) 
Moon 7 x 10 22 4 x 10^8 1 1
Sun 2 x 1030 1.5 x 10^11 200 0.5
Mercury 3 x 1023 9 x 10^10 1 x 10^-4 4 x 10^-7
Venus 5 x 1024 5 x 10^10 5 x 10^-3 4 x 10^-5
Mars 6 x 1023 8 x 10^10 2 x 10^-4 1 x 10^-6
Jupiter 2 x 1027 6.5 x 10^11 1 x 10^-2 6 x 10^-6
Saturn 6 x 1026 1.5 x 10^12 6 x 10^-4 2 x 10^-7
Uranus 9 x 1025 3 x 10^12 2 x 10^-5 3 x 10^-9
Neptune 1 x 1026 4.5 x 10^12 1 x 10^-5 9 x 10^-10
Pluto 1 x 1022 6 x 10^12 6 x 10^-10 4 x 10^-14
Mountain ~1012 2000 0.5 100 000
Eiffel tower ~2 x 108 500 2 x 10^-3 1 600
Obstetrician 1 ~ 100 (he is fat !) 1 2 x 10^-4 80 000
Obstetrician 2 ~ 50 (he is thin !) 1 1 x 10^-4 40 000

Is Astrology a science ?

If one considers the astronomical calculations made ​​by astrologers, scientific rigor is there: often they require the most accurate ephemeris and are at least in some cases very attached to that determination of horoscopes of birth is accurate. But this part of astrology has nothing to do with the interpretation "divination" that will be the purpose of the making of horoscopes. The quality of the preliminary calculation is there to hide the fanciful and arbitrary side of the suite. It is this suite that should be support by truly scientific considerations .

Since the influence of the stars is not measurable or even observable (see the gravitational effects which are only be put into evidence), astrology does not intend to provide (or even attempt) a theoretical explanation of the influence of the celestial bodies on humans. So we have to test the validity only the statistical analysis that is ​​much more difficult, and can almost always be question.

Again, do not let the astrologers move the problem : it is up to them to prove that astrology works and not to astronomers - and other scientists - to prove that it does not work. Now there has been many experiences, all negative, although some astrologers claim that they are positive. Without giving here a statistics lesson, one must recall the basic conditions necessary to ensure the scientific validity of any analysis of this type:
(1) to rigorously define the experimental protocol before the experiment and stay on it;
(2) check the significance of the results (confidence tests, analysis of possible bias, etc..
(3) to publish all the results, clearly, under control.

In all cases where these rules were followed, statistics have shown the futility of predictions and astrological themes.

What is a scientific rationale?

To understand why the astrological process is not scientific, let us understand what a scientific rationale.

To be scientifically valid, a phenomenon does not have to be observed (measured) and explained: one of the two is enough. Many phenomena have been studied perfectly well before we have a theory (tides, heredity ... ). But in all cases, it was a huge phenomenon whose existence was obvious. It is very interesting to note that in astrology, after 3000 years and a lot of literature on the subject, the more optimistic are still trying to identify the existence of the phenomenon. This seems to be a characteristic of false sciences: dowsing , telepathy, dowsers , etc... At best, it would still be marginal phenomena. For us (scientists) , it is obvious of course. But it should worry astrologers ...

The issue of the impact of this type of debate on public (scientific analysis of the phenomenon "astrology") is that the argument is developed convincingly in our system of thought (scientific) with our knowledge basis. It is not suitable to others. At best it will not be convincing, at worst it will tax us dogmatism (terrorism of the single thought).

During the reflection "is astrology a science?", inevitably we arrive on other issues , more fundamental , or relative to the public:
- what in the scientific process ?
- what to reasoning, scientific evidence ?
- how think the common people?
- how to get the scientific message ?

Some attempts to answer ...

The scientific approach is a (measurable) method of analysis of reality , trying to "explain" in terms of a small number of mathematical laws. We can credited the science of non-dogmatic theories: they are conditional and must be disproved to be scientific ( "falsifiability" according to K. Popper). An exception may sometimes suffice to completely call into question a theory (the Michelson experiment and the existence of the ether, for example). The exception invalids rule (and does not confirm!). In addition, scientists apply (somehow) a coherent collective action leading to the accumulation and development of knowledge, cross-checked and criticized until consensus. It is the "city of scholars" of Bachelard. In its fields of application, science has some predictive power and obvious practical applications. However, it establishes restrictions on the scope of validity and is not a substitute to the "intuitive" "global" perception of the world, which is translated into mysticism, faith , etc... These have an issue different from that of science and do not seek assimilation with it.

Astrologers, however, have much to gain from a scientific appearance, but astrology has stagnated for centuries in the qualitative and arbitrary. Just consider for example the lack of consensus (and even the need of consensus ) between different "schools" : taking into account whether the precession, the number of celestial objects to be considered, etc. . Worse, the interpretation of the same theme by several astrologers is usually different. These differences themselves demonstrates irrefutably the arbitrary nature of astrology which looks like more a superstition than science .

Reasoning and scientific evidence must result from a specific (analytical, inductive/deductive) approach and a basis of knowledge associated with a powerful query and comparison system. They allow one who has received a scientific education to submit new information to this particular analysis grid, and integrate or criticize it. Note that this approach does not require a combination of encyclopedic knowledge (facts and dates), often cited as proof of credibility by astrologers.

The difficulty of science to convince the "common people" is the intellectual approach often very different: even when it is rational, this approach is generally methodologically incorrect. For example , it may be convinced by the arguments of astrologers about them by showing one particular case that works, and even better that he wants to believe in astrology (remember the quote attributed to Voltaire " An astrologer can not have the privilege to be always wrong").

Finally, the "common people" is not asking for science, it needs images, dreams, perhaps hope. It is even more "producer" of mental models on science, far away from the modern reality and closer to the image of Epinal of the scholar of the XIXth century, continuing his research in isolation atop his ivory tower (note that this outdated image is fairly widespread among scientists themselves). This shift makes the dialogue difficult as the transmission of knowledge.

Under these conditions, how to get the scientific message? The whole problem of public education and the popularization of science has arisen (see below).

But after all, why try to convince the public? Why popularize science? Why fight astrology, demonstrate its futility, rather than simply let those who want it to believe? The answer requires a look to the place of astrology in our society.

Astrology and Society

Socio-psychological reasons for the success of astrology, that we can only touch on here, match the folowing needs :
- a link between man and the cosmos,
- some irrational (actually of wonderful)
- a response to the apparent "absurdity" of the world (formerly provided by religion, currently in failure as evidenced by the revival of various spiritualities)
- predictions whatsoever, that can assist and guide through the everyday problems.

However, if the social role of astrology is at the acceptable limit when presented as a grid symbolic interpretation (playful?) - even unfounded - of reality or a psychological help to solve through a "disguised chance" or a "disguised arbitrary", the situations of subjective choices where rational analysis is not enough, there are other very negative drifts:
- the direct economic exploitation (consultations and other scams on credulous victims, condemnable in the old Criminal Code) is more than questionable, as well as the attitude of the Government - accomplice - which merely tax practitioners of astrology (and other occult sciences) at several hundred (~ 6) millions euros per year! Worse , the "Française des jeux" - organism under the control of the Government - has recently offer a lottery on astrlogical themes, not hesitating to encourage obscurantism for more money.
- astrology is dangerous if it is used to manage public affairs: it had an important influence on U.S. President Ronald Reagan, and before him the wife of President Roosevelt, we recently learned that it was the case of François Mitterrand, to a lesser extent; local politicians say they refer to it (eg A. Santini, Mayor of Issy -les- Moulineaux, during a televised live debate on TF1 in May 1996). In the private sector, the situation is not better : the astrological sign interfers in the calculation of rates of certain insurance companies in the UK , etc. .
- finally, the use of astrology is highly disturbing when it is used for recruitment. This then gives rise to a selection unjust and reprehensible as sexual or racial discrimination!

If we criticize the social role of astrology (the astrologer), it is worth considering next the role of astronomy (the astronomer). The latter can not - and does not intend - provide an answer to the apparent "absurdity" of the world, it leaves room for religion and spirituality. It does not pretend to be a guide through the everyday problems. However, it can sharpen critical thinking and analytical ability, useful in the long term to develop individual responsibility. For example, astronomy can reveal contradictions between myths and reality on planets : Venus, once a symbol of love and beauty (and used in astrology to provide decision support about marriage), is actually a hell overwhelming pressure and temperature and sulfuric acid atmosphere. But beyond this revelation, we can find a warning to the modification of our Earth and its atmosphere, and an encouragement to the realization that our planet is a unique and fragile environment that must be respected and protect, rather than considered as an object of predation.

Finally (and especially?) , astronomy can offer much richer answers than those of astrology about the link man-cosmos, and it has an undeniable potential to inspire wonder. For example:
- The atoms that make us (most elements heavier than helium) have been produced by nuclear fusion in the heart of the first massive stars in our galaxy, are billions of years ago (this is a deep connection and well real between man and cosmos).
- The Olbers' paradox (the simple fact that the night is black) leads to the finite lifetime of stars ("the stars are not old enough" was the response approached by Edgar Poe !). Recent long exposure of the Hubble Space Telescope and "New Technology Telescope" of the European Southern Observatory wonderfully illustrate this "quest for the confines of the Universe."

One could multiply examples, but the problem is that the current astronomical popularization, with rare exceptions, is limited to the presentation (explanation) of phenomena and theories. But it seems possible to take into account the needs of the public without betraying the rigor of the process. First attempts in this direction exist: the first works of H. Reeves [1981, 1984, 1986] , the "Conversations on the invisible" to Audouze et al. [1988] , "The ears in the stars" of Boujenah et al. [1995], and some other ... Popularize more and better? The material is not lacking, especially in astronomy (this is more difficult in particle physics, for example).


Astrology offers a prediction of future personal and its interpretation of an old dream: the link between man and the cosmos. Unfortunately , this interpretation is unfounded and " adulterated " and the multiple forms of exploitation are subject to criticism. Astronomer, scientist or teacher, often questionned must fight it.

But especially the astronomer is able to provide a much better answer to that old dream of mankind. It is to bring scientific information to the public, but also rigorously intelligible and attractive (while avoiding the drift of a process of communication and self- laudatory advertising, as practiced by NASA example).

If the appearance of many popular books of quality reflects a genuine effort to information and education, an important work (accessibility and attractiveness of speechs) seems still needed by astronomers and scientists in general.

The goal of these efforts should be to integrate citizens to the "community of scholars". It is vital for everyone : scientists in the short term (financial support, utility, recognition ... ) and citizens in the longer term. Science is a priori neither good nor bad, but citizens will not have influence on the use that is made only if they are sufficiently informed .


  To know more, read the "Que sais-je ?" n°2481" « L'astrologie » de Daniel Kunth et Philippe Zarka

Credit : this texte is translayed and extracted from a more complete text of Philippe Zarka and François Biraud available on the Internet site of Paris Observatory(