How can you tell when you're going to have a Really Bad Day? Like millions of people, you may check your horoscope. You scan it until you come to your own "sun sign," and you see something like this:

Cancer (June 21-July 22)

What got away will be back in your charge. Focus on diplomacy. Taurus, Libra persons play roles.

Or this:

Virgo (Aug. 23-Sept. 22)

Make fresh start, stress independence, originality. Avoid heavy lifting, imprint style; be ready to give and receive love. You'll locate lost article and increase earning power.

Or this:

Sagittarius (Nov. 22-Dec. 21)

Home repairs could put dent in budget but are worth it. Domestic adjustment featured, includes where you live, marital status. Remember recent resolutions concerning fitness, diet, nutrition.

Or this:

Sagittarius (Nov. 22-Dec. 21)

Avoid scattering forces -- popularity rises, you could win contest. Elements of timing, luck ride with you. In matters of speculation, choose number 3. You'll amaze yourself.

What should you do now? Go out and buy a lottery ticket or spend the day in bed? Sometimes, it is difficult to make out what the stars are trying to tell you.

Suppose weather forecasting were done this way. Instead of saying, "Tomorrow will be sunny" or "Expect two to four inches of snow," the TV meteorologist would say something like, "Take special care with outer clothing. Direction of wind may be a factor. Temperature changes could affect financial affairs and love life."

That illustrates one crucial difference between science and occult beliefs such as astrology. Scientific predictions are crisp enough to verify or reject. As the philosopher Karl Popper put it, they are "falsifiable."

Of course, maybe it isn't fair to insist that astrological predictions be any more flawless or all-encompassing than weather forecasts or election polls. Horoscopes, however, incur no penalty for being wrong. Indeed, astrologers are the first to explain that there are no guarantees, noting that "the stars incline, they do not compel."

Serious astrologers claim, however, that, with exact information about an individual's birth date and time, down to the second if possible, they can be fairly specific.

That's all the basic input astrologers use: the date and time of your birth. (Long ago, some forms of astrology used the date of conception, but that turned out to be hard to pin down.) That's a bit like letting an automobile mechanic diagnose what's wrong with your car based solely on its gas mileage.

But astrology is an ancient system. The 12 sun signs that make up the zodiac (from the Greek for "circle of animals") date to Babylonian times. They are named after the creatures embodied in the 12 constellations located more or less directly overhead at noon on the equator in the course of a year as Earth orbits the sun [see illustration below].

In order, starting at the vernal equinox, they are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. Whichever is nearest to uppermost at the time of your birth becomes your sign.

This seems to be a straightforward scheme. But constellations are simply fanciful names that ancient peoples once gave to arbitrary patterns of stars in the sky. Each of those stars is utterly unrelated to the others in "its" constellation except in the human imagination.

Moreover, the zodiac constellations aren't all the same size, so it's somewhat arbitrary to allot a month to each. And others, such as Ophiuchus, are almost as big but didn't make the cut. Astrologers have not always taken the number to be 12; they may have settled on 12 because it divides 360 (the number of degrees in a circle, another Babylonian invention) exactly.

Because the tilt of Earth's axis changes over time, the date on which these constellations are actually overhead occurs about two months later than in the Babylonian era. This is no more than a bookkeeping problem, unless one inquires how the stars exert their influence.

In addition to the zodiac, astrologers divide the 24-hour day into 12 "houses." The houses are used to tell where the planets, a term that includes the sun and moon, happen to be in relation to one another and to the fixed stars, because each is supposed to be related to a particular personality type.

Astrology boasts of being a mature discipline, perfected since the time of the famous Egyptian astronomer Ptolemy more than 1,800 years ago. Of course, three planets have been discovered since then. But that's the least of the credibility problems astrology faces.

Perhaps the most profound is that even astrologers have no plausible explanation whatsoever for how arbitrary patterns of dots in the sky, or even comparatively nearby planets, might conceivably influence a human being at birth or any time thereafter.

Indeed, it's difficult to imagine one. For example, the gravitational attraction between, say, Jupiter and a newborn baby is weaker than that between the baby and its mother. Likewise, Jupiter's magnetic field on Earth is weaker than that produced by the electric current running through the baseboards in the maternity ward.

Astrology cites the existence of biological clocks to argue that the body tracks planetary motions and "remembers" the planetary configuration at birth. But aside from the sun and moon, which are highly visible and have strong tidal effects, there is no evidence that this happens with any heavenly body.

Nonetheless, astrology is a complicated subject with its own technical jargon and fundamental principles. For example, people born under a particular sign or planet are supposed to have certain general personality characteristics. These rules are never hard and fast, however.

For example, a Leo is "proud, forceful, a born leader." But not always: the author of this article, though a Leo, has never had any luck leading anyone. Tables have been compiled that show for which combinations of astrological signs two people of the opposite sex are incompatible. Unfortunately, astrologers do not agree on which combinations these are.

But then, as we have seen, astrology does not make exact predictions. In fact, it's considered unethical to do so.

The code of ethics of the Vienna-based Astrological Association of Northern Virginia distinguishes between " . . . the improper use of astrology as a pretense of powers to predict specific future events with absolute certainty and its proper use as a technique for determining the probabilities in human events." But in fact, astrology doesn't determine probabilities either. None of its laws is quantitative.

Astrology and astronomy have very little in common beyond the first five letters of their names, which come from the Greek word for "star." Astrologers rarely write about astronomy, and vice versa. This would be surprising if there were overlap between the two fields but not in light of the deep gulf that separates them.

Unlike famed German astronomer Johannes Kepler (1571-1630), who cast horoscopes for the gullible when no other source of funding was available, most astronomers will not even discuss astrology. Even showing curiosity on the subject is a career-inhibiter.

There have been exceptions, however. George Abell included brief instructions on casting a horoscope in his widely used introductory astronomy text, Exploration of the Universe (6th ed., Saunders, 1991). Bart J. Bok, formerly director of Kitt Peak Observatory, and Lawrence E. Jerome, a science writer, wrote articles in The Humanist that later were published as Objections to Astrology (Prometheus Books, 1975).

Roger B. Culver of Colorado State University and Philip A. Ianna of the University of Virginia have written The Gemini Syndrome (Pachart Publishing House, 1979; revised and reissued as Astrology: True or False?, Prometheus Books, 1988).

Jerome's article, which provoked an enormous controversy, at least among astrologers, argued that astrology has more in common with magic than with science. Indeed, in Babylonian times when western astrology began, there was no science.

The authority figures who claimed responsibility for foreseeing and explaining trends or events by searching the skies for ominous meanings were priests of the Babylonian religion. Their practices also included haruspicy -- searching for omens in the entrails of animals -- which most people today would have no trouble dismissing as superstition.

The Greeks and their Hellenistic successors in Egypt, who included Ptolemy, took over this body of belief and refined it. As it matured, however, astrology ceased to be an observational discipline.

Thus, astrologers often cannot agree on even the basics -- the number of houses (8, 12, 24 or some other); when the "Age of Aquarius" started (proponents cite dates varying from 1781 to 2740 B.C.); where boundaries between various constellations should be reckoned, and so on.

As might be expected, astrology takes different forms in other cultures. Chinese and Indian astrologers divide the zodiac into 28 houses, not 12. In Europe, astrology was largely unknown after the fall of the Roman Empire until humanist scholars began borrowing from Arab culture in the late Middle Ages.

According to Christopher McIntosh in The Astrologers and Their Creed (Praeger, 1969), " . . . The beginning of the Renaissance in Italy marks the zenith of astrology. . . . Kings, popes, generals, physicians -- all made use of the services of the astrologer."

But starting in the 18th century, astrology again fell on hard times. By then, most of its practitioners were fortunetellers, not scholars. They were the butts of ridicule. In a pamphlet titled Predictions for the Year 1708, satirist Jonathan Swift, who would later pen Gulliver's Travels, attacked an astrologer named Hewson, who prognosticated under the pseudonym "Partridge."

"I have consulted the star of his nativity by my own rules and find that he will infallibly die upon the 29th of March, about eleven at night, of a raging fever," Swift wrote. Partridge survived the fateful day, whereupon Swift issued another pamphlet entitled An Account Of the Death of Mr. Partridge, the Almanack Maker, Upon the 29th Instant. The death announcement was widely credited, and Hewson's publishers even struck him off their rolls.

The Reformation and the Enlightenment contributed to the distrust of traditional authority, including astrology, which became widely regarded as fraudulent. Astrological studies did not begin to regain popularity until the late 19th century. The Theosophical Society, founded in New York in 1875 by Madame Blavatsky and Col. Henry Olcott for the study of comparative religions and the paranormal, was instrumental in their revival.

With notoriety came scrutiny by scholars. However, because astrological predictions are not falsifiable, they are well-nigh impossible to test. Nonetheless, numerous studies have been conducted. A good list of recent papers can be found at http://www.cis.ohio-state.edu/hypertext/faq/usenet-faqs/bygroup/alt/astrology/ astrology:papers.html.

These tests are mostly statistical, that is, they try to establish whether astrologers' assertions and predictions are correct more often than would be expected by chance alone. The impression from a brief overview is that almost all results are negative or inconclusive. Even those that imply positive correlations between predictions and outcomes are tantalizingly open to hedging or interpretation.

One statistical study of the relationship between birth date data and life experience deserves special comment. Over two decades in the middle of this century, French researchers Michel and Francoise Gauquelin assembled and analyzed horoscopes of more than 25,000 individuals. At first, they found no positive correlations. For example, the presence of Jupiter in conjunction with the midheaven point at a person's birth is supposedly "an excellent aspect for success in life."

Of 10,000 successful individuals whose horoscopes the Gauquelins examined, no more had Jupiter in conjunction with the midheaven point at birth than would be expected by chance.

Mars has long been astrologically associated with violence and killing. But the files of 623 notorious convicted murderers showed no correlations with the location of Mars by astrological house.

In A Scientific Basis for Astrology (Stein and Day, 1969), Michel Gauquelin wrote that "it is now quite certain that the signs in the sky which presided over our birth have no power whatever to decide our fates." A few years later, he added, "Whoever claims to predict the future by consulting the stars is fooling either himself or someone else."

Nevertheless, one modest positive correlation did appear: A significant number of sports champions was found to have been born when Mars was between the eastern horizon and the celestial meridian. The Gauquelins' 1969 book trumpeted this result as the "Mars effect." Professional astrologers embraced their findings and proclaimed that they placed the whole field on a scientific footing.

The Mars effect ignited a firestorm of criticism and rebuttal that continues today. A bibliography and blow-by-blow account of the affair is available on the Web at ftp://ftp.primenet.com/pub/lippard/mars-effect-chron.rtf.

Astrology was again becoming a staple of pop culture. The first newspaper horoscope probably appeared about the turn of the century, and the practice became widespread only after newspaper "chains" became common, beginning with the Scripps chain in 1878.

In his magisterial work American Journalism: A History, 1690 to 1950, Frank Luther Mott never mentions horoscopes but recounts how Joseph Pulitzer and William Randolph Hearst made newspapers more attractive by larding them with "entertainment features."

Feature syndicates came into existence soon afterward, and along with syndicated columnists, cartoonists and comic strips, began providing astrological advice. At least as early as 1924, Washington's Evening Star carried one, Mary Blake's "What Does Today Mean to You?"

McIntosh's book states that the first regular British astrological feature in England appeared in the Sunday Express of London in 1930 under the byline of R.H. Naylor. According to sources at the Newseum, the newspaper museum in Rosslyn, it began when Princess Margaret was born Aug. 31.

Competitors copied the practice, and daily horoscopes soon acquired a wide following there, too, especially after a timely airship disaster apparently bore out Naylor's prediction of danger in British aviation.

As former treasury secretary Donald Regan revealed, even President Ronald Reagan and his wife, Nancy, resorted to astrology in making important decisions. Fortunately, recent events have driven home the lesson that it's not always right to do something just because the president does it.

How reliable is such advice, presidential or otherwise? Consider the four horoscopes at the beginning of this article. For the record, they come from Sydney Omarr's column in The Washington Post on the days that the following people died:

Princess Diana ("Focus on diplomacy") Aug. 31, 1997.

Mother Teresa ("You'll locate lost article and increase earning power") Sept. 5, 1997.

Frank Sinatra ("Remember recent resolutions concerning fitness, diet, nutrition") May 14, 1998.

Joe DiMaggio ("Elements of timing, luck ride with you") March 8, 1999.

David L. Book, who wrote the Come to Think of It column for Horizon, is a consultant to the astronomy department at the University of Maryland and president of Enigmatics, Inc., a research and development company.

CAPTION: The Zodiac: A Celestial Field Guide

The zodiac, or "circle of animals," is made up of 12 constellations that lie along the sun's apparent path across the sky, called the ecliptic. All the planets except Pluto also appear in this band. As seen from Earth, the sun "passes through" a different contellation about every 30 days. If you were born when the sun was passing through the constellation Taurus, that would be your "sign."

However, this fanciful system was devised thousands of years ago. Since then, gradual changes in the Earth's axis of rotation have altered the constellations' apparent positions at various times of the year. For example, according to astrological tradition, the sun is supposed to be in Capricorn from Dec. 22 to Jan. 19. But if there were an eclipse of the sun on Jan. 1, you'd find it in Sagittarius.

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