Astrology Debunked (Georges Charpak, 2004)

NOTE: The following article is taken from the book Debunked!: ESP, Telekinesis, and Other Pseudoscience, pp. 1-10:


Truth is Drawn from a Well

“The proof that astrology works, and that it works well, is that my horoscope has accurately predicted things that actually happened to me.” How many times have we heard such remarks? How many personal experiences of this type get presented as evidence of the validity of astrology?

Well, let’s be clear on the subject: Yes, horoscopes work—they work well, in fact. But the validity of the horoscope does not imply the validity of astrology. Many people are convinced of the validity of astrology because their horoscope “works.” These people believe that the occurrence of predicted events, which they have witnessed, justifies the validity that they ascribe to the “Science of Signs.” They are especially convinced that their horoscope gives them a solid foundation for an understanding of themselves and provides guidance for how to act and on their destiny.

For such people, their horoscopes are meaningful, but in truth, the horoscopes take on meaning from the believers, not for believers. It is difficult to get this point across, since it runs contrary to personal experience—“You can’t say it’s not true, because it’s happened to me.” The individual who reads his horoscope is convinced that he is dealing with his horoscope, that that horoscope is destined for him, and that it was created specifically for him by a supernatural force. There’s no recognition that the satisfaction of the client is a source of feedback, adding to the credibility that the fortune-teller can claim for himself and his “science” and consequently for his motives and his effect on the client.


A Convincing Demonstration

Twenty years ago, during a class on paranormal phenomena and the occult, one of us asked the students to write the following information on a piece of paper: their first and last names; the date, time, and place of their birth, and the theme of their latest dream. All of this was handwritten. The request implied that some kind of star-based calculation would be made using the birth data or that a handwriting analysis would be made based on the written material, in either case possibly augmented by interpretation of the most recent dream.

A week later, each student received an individualized description of his or her personality followed by the question, “How good is this description of your personality?” The concordance of the description with the student’s real personality (or self-perception of it, anyway) was rated as excellent, good, fair, poor, bad, or none. Overall, 69% of the students judged the description of their personality to be excellent, good, or fairly good.

The result is especially convincing because I had been introduced to the students as a “scientific demystifier”—a pretty lousy introduction, by the way—and not as an astrologer or other supernatural leader, which would have certainly enhanced the believability of the personality descriptions and thus the percentage of “successful matches.” It’s also a particularly good demonstration when you consider that, when we asked one of the students to read aloud the personality description that we had prepared for him, the others all thought it was theirs. They couldn’t have been more similar—in fact, the much-touted “individualized” descriptions were prepared before the students’ data were collected, and they were exactly identical for all the students! This was a simple but enlightening demonstration of one of the many “effects” that occur so often when it comes to “paranormal” phenomena. If you ever want to try this illuminating experience, use the model personality description below; simply adding a name to complete the “personalization.”

  • You need others to like and admire you, yet you are apt to be critical of yourself.
  • Although you have certain character flaws, you are generally able to make up for them.
  • You possess substantial untapped potential that you haven’t exploited for your own benefit.
  • Some of your aspirations tend to be pretty unrealistic.
  • You are outwardly disciplined and you display self-control but inside you tend to worry and be insecure.
  • Sometimes you have serious doubts as to whether you have made the right decision.
  • You prefer a bit of change and variety and are annoyed when you encounter restrictions or limitations.
  • Sometimes you are extroverted, affable, and sociable, while at other times you are introverted, cautious, and reserved.
  • You are proud of being an independent thinker and don’t accept the statements of others without satisfactory proof.
  • You find it unwise to reveal too much about yourself to others.

Similar descriptions were used and tested for the first time by the psychologist Bertram Forer, who based his wording on an astrology book. The effectiveness of such wording in a real-life setting demonstrates clearly the power of what some have called the “well effect.”

Forer effect

The Well Effect

The well effect can be summed up as follows: The vaguer a statement is, the more numerous are the people who will recognize themselves in it—and the more completely the description will be seen to “fit” them. The descriptive statements may be profound, all right, but they are only deep in the sense that a well is deep—deeply hollowed out, that is, empty. In fact, experience has shown that vague, general statements are more convincing than specific descriptions made by professional psychologists because of what sociologists might call the Barnum effect. Barnum’s circus shows were constructed so that there was something for everyone so that everyone would find their way there, which created the shows’ success. Moreover, studies have shown that, when it comes to analysis of serious personal problems, “yes” and “no” answers picked in advance completely at random are perceived as very encouraging answers to specific questions by the people posing them! (You can read more about this phenomenon in Ray Hyman’s “Cold Reading: How to Convince Strangers That You Know All about Them,” in Paranormal Borderlands of Science [Buffalo: Prometheus Books, 1981]).


The well effect explains, in large measure, the success of horoscopes. “In some ways, you are one of the stronger people”: this is a statement that is empty and void of meaning, yet it will be accepted as fundamentally true in a horoscope, as each reader interprets it in the context that gives it meaning for him. the reader will think, “My knowledge of foreign languages is strong,” “I have a great ability when it comes to fixing things,” “My muscles are strong,” or any of so many other possibilities. And this is without even taking into account that the elementary principles that can improve the receptivity of the readers. For example, don’t tell people what you know (or think you know) to be true about them, just tell them what you wish were true about them.

Of course, astrologers count on the fact that the public quickly forgets the predictions. The supermarket tabloids print dramatic predictions at the end of every year, often involving such events as a presidential assassination or the second coming of Christ in the following year or two, and people have been reading similar predictions for decades. In France, a prediction concerning a former prime minister is little remembered: “Despite a generally positive picture for the year [1993], the first fifteen days of January and of September as well may pose serious problems for Pierre Beregovoy.” For this prediction we are indebted to a famous astrologer, Elizabeth Teissier, writing in Your Horoscope for 1993. For those unacquainted with subsequent events, Pierre Beregovoy killed himself on May 1, 1993, by a gunshot wound to the head. In the United States, thousands died in the World Trade Center disaster of September 11, 2001. The horoscopes of any number of the victims contain predictions supposedly useful for guiding behavior that day—“a good day for a fresh start in your love life”—and the next day! Individual deaths on that day were not predicted, let alone a mass disaster; if astrology can’t unambiguously predict an event of that magnitude in your life—and still makes further predictions for the next day—what good is it?  Yet no one remembers that predictions of good days and bad days for the rest of the year were cast for these victims.

Clearly, astrologers make full use of the well effect. A prediction from Elizabeth Teissier again: “Populations in the world will suffer violence in the following month, given that Venus and Pluto…” But they also use other tricks to lend credence to their pronouncements, always centered on the big three—Love, Money, and Health. And intellectual honesty does not have to be the foremost characteristic in the star charts that predict the traits of the astrologers themselves. Skill and craftiness are often plainly evident. For example, Darah was one of the four sons of a great Moghul emperor, and he was obsessed by astrology. A soothsayer predicted that he would stake his own life on his prediction that Darah would be crowned the heir to the emperor. When someone expressed astonishment at the temerity of the prediction, the astrologer said, “One of two things will happen: either Darah will ascend the throne and my fortune is assured, or he will lose his bid and be assassinated, and then I will have nothing further to fear from him.” Beyond the well effect, astrologers don’t hesitate to have two irons in the fire or, more subtly, one iron with two different sides.

Let’s not leave out that astrologers also count on the fact that “to err is human” but constant fallibility is not. Neither perpetual fallibility nor perpetual infallibility is characteristic of anybody. People readily accept one side of this and disregard a claim of infallibility because errors inevitably appear. But people are a lot less aware that a few instances of “getting it right” are inevitable. The contrary—always being wrong—would be extraordinary. Even an astrologer will sometimes make predictions that turn out to be true. So, predict merrily along—some of your statements will always pan out.

Élizabeth Teissier, astrologer.
Élizabeth Teissier, astrologer.

Astrology in a Vacuum

Astrologers actually know very little about what goes on in the skies. In Your Horoscope in 1993, Elizabeth Teissier asked, “How does such a horoscope [for a group readership] work, and how can it be justified? How can it be conceivable that a Capricorn born January 9, 1960, should be under the same planetary influences as another born January 9, 1924, for example? Here is the answer to such questions: In its apparent travels around Earth, the Sun ends up in the same place in the sky on the same date each year” (italics added). This is totally false! On two identical dates in different years, the Sun is not in the same spot in the sky at all.

On a given date in different years, our planet is not at the same point in its orbit around the Sun. As we will explain more fully later, the phenomenon of the precession of the equinoxes causes a shift in position. To put it in numbers, as an approximation, we can say that there is a difference of about 22,000 miles between Earth’s location on any specific date in two successive years, which is a distance of about three times the diameter of our planet. Thus, contrary to the astrologer’s statements about the same planetary influences on January 9th of 1960 and 1924, the earth would not occupy the same place in its orbit around the Sun, at all. Between those two dates there would be a shift of about 780,000 miles!

For the most part, astrological soothsayers work with what is called the “tropical zodiac.” This is based on the sun and has nothing to do with the stars, which are the basis for the “sidereal zodiac.” When astrologers put together an astral analysis, they generally use the twelve signs of the tropical zodiac, defined as the twelve equal rectangles into which the celestial sphere is divided. The starting point for this division is called “gamma,” which is the intersection of the ecliptic with the celestial equator, corresponding to the spring equinox.

At one time, a bit before the birth of Christ, calculations based on the tropical zodiac would have been about the same as those based on the stars and constellations that originally determined the characteristics of the various signs of the sidereal zodiac. This is no longer the case because the precession of the equinoxes has mixed things up by displacing the gamma point with respect to the starry background of the celestial sphere. This displacement has dragged along the astrological signs, which nowadays don’t correspond at all to their original stars. Today’s “tropically based” astrologers just blindly apply rectangular sign-zones, empty boxes that have nothing to do with anything and are devoid of any consistency or correspondence with the stars. If you want to know your real zodiac birth sign, rather than one fobbed off on you by the usual (tropical) astrologers, investigate Astronomic Zodiac. This resource is found on the Internet site Your true sign is simply the location of the Sun in the celestial sphere, as seen from Earth at the instant of your birth. This can be calculated rigorously from an astronomical point of view. But most astrologers, relying on the vacant rectangles of the tropical zodiac, really don’t practice astrology at all, but rather something we’ve got to call the study of emptiness—voidology, or astrology in a vacuum.

The 3 Magi
The 3 Magi

The Navel Gazers

Horoscopes are popular today because ours is a narcissistic civilization. Science makes only global or collective forecasts, while lots of people are interested only in their own personal destiny. For most people, it’s not hard to choose between the distant scientists who speak of generalities and the accessible astrologer who speaks exclusively to the individual about himself. The aura of exclusivity and uniqueness is certainly enhanced by the astrologer’s request for the complete details of the client’s birth. Exact spot, date, hour, and minute—all pertain to a single person, all pertain to me—so it must be that there’s a good correspondence between the results of the astrologer’s study and my own personality.

Our observations and perceptions depend in part upon what we are thinking at the moment we observe something. Our deepest desires and motivations, modified by our past experiences, are reinforced, whether consciously or not, by selection bias. Selection bias is a well-documented psychological principle. It means that we choose our magazines, newspapers, radio stations, television shows—all of our information sources—in such a way that our opinions are, for the most part, reinforced rather than challenged. And if, despite all that, we receive challenging information, we can always use subjective validation. Subjective validation is a psychological principle that allows us to absorb incorrectly any information that is contrary to our preferences and to interpret it in a different light. Subjective validation causes two events to be perceived as linked, when they are not, simply because a desire, a hypothesis, or a belief requires such a linkage. In the case of astrology, events are perceived as linked simply because the horoscope says the linkage exists. This perception of linkage, in turn, induces superstitious behavior founded upon the belief that one’s own actions determine the course of events, even when  this is not so in reality.

If the influence of the planets over our destiny is completely nil, however, it doesn’t follow that the horoscope is without effect. And the well effect, in particular, allows us to see why the horoscope holds such a sway over so many people.

The Precession of the Equinoxes

The precession of the equinoxes was discovered by Hipparchus of Nicea in the second century B.C.E. Earth is not perfectly spherical but flattened a bit at the poles and bulging a bit at the equator. Gravitational effects of the Sun and the Moon on the equatorial bulge cause the axis of the Earth’s rotation to shift. That is, the axis—the line between the poles—moves around. It pivots, a bit like a spinning top, very slowly, taking about 25, 790 years to go around completely. The picture is actually a bit more complicated, though. An additional phenomenon, called “nutation,” creates a little wave around the main circular motion of the axis, with a period of about 18.6 years.

In about 12,000 years, Earth’s axis will point to a new “North Star,” Vega, and today’s “North Star,” Polaris, will no longer mark the northerly direction.

The plane of the celestial equator—that is, the plane that includes Earth’s equator—obviously follows this pivoting of Earth’s axis. Therefore, its intersection with the ecliptic, which is the plane of Earth’s orbit around the Sun, must also move. But the location of that intersection determines the gamma point, the spring equinox. (Remember that the spring and autumnal equinoxes correspond to the two positions of Earth when the line from the Sun to Earth is perpendicular to the axis of Earth’s rotation. That’s why, at the equinoxes, day and night are of equal length all over the world.)

The problem is that the spring equinox serves as the reference point for the tropical zodiac. In other words, the gamma point moves slowly but surely through the celestial sphere and pulls along with it the signs of the tropical astrologers, who consequently are not working from the original constellations at all and who continue to be farther and farther away from them.

Here’s just one example among many: Those born when the Sun (as seen from the Earth) is in the constellation Leo are said to be “courageous, proud, and dominating.” But if that were true in Hipparchus’ day, two thousand years ago, it’s really difficult to understand what that sign has to do with us today. People born at the end of July are “Leos” according to the astrologers, but today’s Sun is not then in Leo at all but in Cancer.

The well effect is not limited to astrology. One can easily find many applications of this principle in every aspect of life