Astrology, in its traditional form, is a type of divination based on the theory that the positions and movements of celestial bodies (stars, planets, sun, and moon) at the time of birth profoundly influence a person's life. In its psychological form, astrology is a type of New Age therapy used for self-understanding and personality analysis (astrotherapy).

Ivan Kelly, who has written many articles critical of astrology, thinks that astrology

has no relevance to understanding ourselves, or our place in the cosmos. Modern advocates of astrology cannot account for the underlying basis of astrological associations with terrestrial affairs, have no plausible explanation for its claims, and have not contributed anything of cognitive value to any field of the social sciences.

Even so, astrology is believed by millions of people and it has survived for thousands of years. The ancient Chaldeans and Assyrians engaged in astrological divination some 3,000 years ago. By 450 B.C.E. the Babylonians had developed the 12-sign zodiac, but it was the Greeks--from the time of Alexander the Great to their conquest by the Romans--who provided most of the fundamental elements of modern astrology. The spread of astrological practice was checked by the rise of Christianity, which emphasized divine intervention and free will. During the Renaissance, astrology regained popularity, in part due to rekindled interest in science and astronomy. Christian theologians, however, warred against astrology, and in 1585 Pope Sixtus V condemned it. At the same time, the work of Kepler and others undermined astrology's tenets. Its popularity and longevity are, of course, irrelevant to the truth of astrology in any of its forms.

The most popular form of traditional Western astrology is sun sign astrology, the kind found in the horoscopes of many daily newspapers. A horoscope is an astrological forecast. The term is also used to describe a map of the zodiac at the time of one's birth. The zodiac is divided into twelve zones of the sky, each named after the constellation that originally fell within its zone (Taurus, Leo, etc.). The apparent paths of the sun, the moon, and the major planets all fall within the zodiac. Because of the precession of the equinoxes, the equinox and solstice points have each moved westward about 30 degrees in the last 2,000 years. Thus, the zodiacal constellations named in ancient times no longer correspond to the segments of the zodiac represented by their signs. In short, had you been born at the same time on the same day of the year 2,000 years ago, you would have been born under a different sign.

Traditional Western astrology may be divided into tropical and sidereal. (Astrologers in non-Western traditions use different systems.) The tropical, or solar, year is measured relative to the sun and is the time between successive vernal equinoxes (365 days, 5 hr, 48 min, 46 sec of mean solar time). The sidereal year is the time required for the earth to complete an orbit of the sun relative to the stars (365 days, 6 hr, 9 min, 9.5 sec of mean solar time). The sidereal year is longer than the tropical year because of the precession of the equinoxes, i.e., the slow westward shift of the equinoctial points along the plane of the ecliptic at a rate of 50.27 seconds of arc per year, resulting from precession of the earth's axis of rotation.

Sidereal astrology uses the actual constellation in which the sun is located at the moment of birth as its basis; tropical astrology uses a 30-degree sector of the zodiac as its basis. Tropical astrology is the most popular form and it assigns its readings based on the time of the year, while generally ignoring the positions of the sun and constellations relative to each other. Sidereal astrology is used by a minority of astrologers and bases its readings on the constellations near the sun at the time of birth.

According to some astrologers, the data support the hypothesis that there is a causal connection between heavenly bodies and human events. Appeals are made to significant correlations between astrological signs and such things as athleticism. However, even a statistically significant correlation between x and y is not a sufficient condition for reasonable belief in a causal connection, much less for the belief that x causes y. Correlation does not prove causality; nevertheless, it is extremely attractive to defenders of astrology. For example: "Among 3,458 soldiers, Jupiter is to be found 703 times, either rising or culminating when they were born. Chance predicts this should be 572. The odds here: one million to one" (Gauquelin 1975). Let's assume that the statistical data show significant correlations between various planets rising, falling, and culminating, and various character traits. It would be more surprising if of all the billions and billions of celestial motions conceivable, there weren't a great many that could be significantly correlated with dozens of events or individual personality traits.

Defenders of astrology are fond of noting that 'the length of a woman's menstrual cycle corresponds to the phases of the moon' and 'the gravitational fields of the sun and moon are strong enough to cause the rising and falling of tides on Earth.' If the moon can affect the tides, then surely the moon can affect a person. But what is the analog to the tides in a person? We are reminded that humans begin life in an amniotic sea and the human body is 70 percent water. If oysters open and close their shells in accordance with the tides, which flow in accordance with the electromagnetic and gravitational forces of the sun and moon, and humans are full of water, then isn't it obvious that the moon must influence humans as well? It may be obvious to some, but the evidence for these lunar effects is lacking.

Astrologers emphasize the importance of the positions of the sun, moon, planets, etc., at the time of birth. However, the birthing process isn't instantaneous. There is no single moment that a person is born. The fact that some official somewhere writes down a time of birth is irrelevant. Do they pick the moment the water breaks? The moment the first dilation occurs? When the first hair or toenail peeks through? When the last toenail or hair passes the last millimeter of the vagina? When the umbilical cord is cut? When the first breath is taken? Or does birth occur at the moment a physician or nurse looks at a clock to note the time of birth?

Why are the initial conditions more important than all subsequent conditions for one's personality and traits? Why is the moment of birth chosen as the significant moment rather than the moment of conception? Why aren't other initial conditions such as one's mother's health, the delivery place conditions, forceps, bright lights, dim room, back seat of a car, etc., more important than whether Mars is ascending, descending, culminating, or fulminating? Why isn't the planet Earth-the closest large object to us in our solar system--considered a major influence on who we are and what we become? Other than the sun and the moon and an occasional passing comet or asteroid, most planetary objects are so distant from us that any influences they might have on anything on our planet are likely to be wiped out by the influences of other things here on earth.

No one would claim that in order to grasp the effect of the moon on the tides or potatoes one must understand initial conditions of the Singularity before the Big Bang, or the positions of the stars and planets at the time the potato was harvested. If you want to know what tomorrow's low tide will be you do not need to know where the moon was when the first ocean or river was formed, or whether the ocean came first and then the moon, or vice-versa. Initial conditions are less important than present conditions to understanding current effects on rivers and vegetables. If this is true for the tides and plants, why wouldn't it be true for people?

Finally, there are those who defend astrology by pointing out how accurate professional horoscopes are. Astrology "works," it is said, but what does that mean? Basically, to say astrology works means that there are a lot of satisfied customers and one can shoehorn any event to fit a chart. It does not mean that astrology is accurate in predicting human behavior or events to a degree significantly greater than mere chance. There are many satisfied customers who believe that their horoscope accurately describes them and that their astrologer has given them good advice. Such evidence does not prove astrology so much as it demonstrates the Forer effect, and confirmation bias. Good astrologers give good advice, but that does not validate astrology. There have been several studies that have shown that people will use selective thinking to make any chart they are given fit their preconceived notions about themselves and their charts. Many of the claims made about signs and personalities are vague and would fit many people under many different signs. Even professional astrologers, most of whom have nothing but disdain for sun sign astrology, can't pick out a correct horoscope reading at better than a chance rate. Yet, astrology continues to maintain its popularity, despite the fact that there is scarcely a shred of scientific evidence in its favor. Even the former First Lady of the United States, Nancy Reagan, and her husband, Ronald, consulted an astrologer while he was the leader of the free world, demonstrating once again that astrologers have more influence than the stars do.