The Roman scholar Sextus in 200 A.D. described how absurd the basic ideas of astrology are when he wrote concerning people born at the same time and thus with the same astrological chart "some have been kings while others have grown old in chains. Thus, though many throughout the world were born at the same time as he, none were equal to Alexander the Great, nor to the philosopher Plato."
Today there have been many experiments with such "time twins" (people born within minutes of each other and thus expected by astrologers to have the same traits) with results that are completely inconsistent with any belief in astrology. These studies found no similarities in personality scores, interests, occupation, appearance, handwriting, names, or life events. For ordinary twins the time interval between births is the same whether they are fraternal or identical so they should be equally alike according to astrology. Yet, the fraternal twins are only as similar as any two siblings would be, but the identical twins do share many characteristics as a result of their genetic identity.
There is much disagreement among astrologers concerning which factors in a horoscope are important, how many planets to use, whether asteroids are to be included and even the location of the signs in the sky. Trying to be more modern, most astrologers use Uranus, Neptune and Pluto as part of the horoscope chart. This implies that all horoscopes done before their discovery were incorrect. The qualities assigned to these planets were merely due to the myths associated with the names that astronomers choose for these newly discovered bodies.
Some astrologers take into account the precession of the Earth's rotation axis (which causes a precession of the vernal equinox) but most do not. This disagreement is described as using either sidereal signs or tropical signs. Aries would be described as aggressive in both systems but the area of the sky to which the meaning is applied is not the same. One group would believe that a certain area of the sky means "intense" while the other side believes the same piece of the sky means "relaxed" (Scorpio vs. Libra). Astrologers do not even agree on whether sun signs should be used at all.
Hundreds of tests of the basic ideas of astrology have been conducted in the past 50 years with always negative results. For example, John McGervey at Case Western Reserve University looked at biographies and birth dates of over 6000 politicians and over 17,000 scientists to see if people who chose these professions would cluster among certain signs. He found the signs of both groups to be distributed completely at random. Psychologist Bernard Silverman of Michigan state University looked at the marriage and divorce data of almost 3000 couples to determine whether the compatible or incompatible claims of astrologers had any basis and found no correlation.
Since some astrologers think the Sun sign alone is not enough to make predictions, a test was conducted by Shawn Carlson in conjunction with an astrological organization. Subjects provided all information about birth date, time, and location and also took the California Personality Inventory test and listed their occupation, interests etc. Twenty eight professional astrologers (who had approved the procedure in advance) were each sent one birth data horoscope and three personality profiles, one of which belonged to the subject whose birth data they knew. Their task was to interpret the horoscope and select which of the three disparate personality profiles belonged to the person born on that date. The results were exactly what would be predicted by random chance, they got only one out of three correct.
French statistician Michel Gauquelin sent the horoscope for one of the worst war criminals and mass murderers in French history to several hundred people and asked them how well it fit them. Ninety four percent of the people in the study said they felt the horoscope described them accurately. Geoffrey Dean, a researcher in Australia who has conducted many experimental tests of astrology, gave horoscope readings that were the opposite of what the chart actually stated (extrovert instead of introvert, passive instead of aggressive). The subjects in the study said the readings applied to them with just as high a percentage (95) as people to whom non reversed readings were given.
A well known British astrologer, Suzanne Lilley-Harvey, did detailed charts for Prince Charles and Princess Diana before their wedding and wrote "they have very fundamental rapport... harmonious communication, general emotional and social compatibility, a strong social/cultural/spiritual bond, ... ability to work together in a very practical way". After their divorce, astrologers saw in their birth charts only trauma, anger, rebellion, and disaster. None predicted Princess Diana's tragic death. forecasts.
Astronomers Philip Culver and Roger Ianna tracked the published predictions of well known astrologers for 5 years. For 3,000 specific predictions (concerning famous people, politicians etc.) only about 10 percent came true, so they were wrong 9 out 10 times and their "hits" could have been guessed at by anyone aware of the news and political forecasts.