According to a recent Gallup poll, over 55% of American teenagers and over 40% of all Americans believe in some aspects of astrology. What is astrology? Astrologers believe that a person’s character and future depend on the position of the Sun, Moon, and planets with respect to the background stars and constellations at the moment of his or her birth. They make forecasts using horoscopes (descriptions of celestial objects’ positions at the time of birth) to predict a person’s future and to provide insight into character. Every person is assigned an astrological (Sun or zodiac) sign based on the date of birth (mine is Taurus), that supposedly corresponds to the constellation the Sun was in at the time of birth. Astrologers complain that this is an over simplification and that true astrologers will take into account all the important ingredients, including the exact time of birth, to make a horoscope, but since most of us only read the 12 zodiacal horoscopes based broadly on birth month (as published in the Culver City News and LA Times), I will discuss these here.

The astrological birth sign corresponds to one of the twelve original constellations in the zodiac (Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capriconus, Aquarius). Each year the Sun makes an apparent path through these zodiacal constellations. The traditional dates corresponding to Sun signs were assigned by dividing the zodiac up into 12 roughly equal parts nearly centered on the appropriate zodiacal constellation. (A Taurus is defined as someone born between April 20 and May 20). Unfortunately, for believers in astrology, the direction of Earth’s orbital inclination (or tilt) to the ecliptic wobbles (like a spinning top). This wobble, called precession, causes the Sun’s position against the background stars to shift about 1 degree every 72 years (taking about 26,000 years to precess all the way around). Since the original development of the zodiac over 3000 years ago, the Sun’s position has shifted essentially one-twelfth of the way around. Thus, the position of the Sun at birth is shifted one zodiacal sign from what is assigned by astrologers (a Pisces is actually an Aquarius, and the Sun is actually inside the constellation Taurus between May 16 and June 21, so I am really an Aries). To make matters worse, the Sun crosses 13 constellations, including Ophiuchus. The paths of the eight planets (not including Pluto) cross 21 different constellations.

Astrology had a major influence on the history and development of astronomy. Ancient astronomers were motivated to measure the positions of the stars and planets and to keep track of eclipses for astrological reasons (the term horoscope is from the Latin horoscopium, which, in turn, is from the Greek words for boundary (the horizon) and target [object of observation]). Astrologers’ terminology, measurements, and techniques were the foundation of astronomical knowledge. Many famous 16th and early 17th century astronomers, such as Tycho Brahe, Johannes Kepler, and Galileo Galilei , practiced astrology to augment their income. However, these early scientists knew that their horoscopes were vague and based on probabilities and they used their knowledge of psychology and current events to make their horoscopes valued by their patrons. Kepler wrote about the role astrology played in these early astronomers lives - “Nature, which has conferred upon every animal the means of subsistence, has given astrology as an adjunct and ally to astronomy.”

From a modern astronomer’s point of view, astrology is a pseudoscientific belief system with its roots in astronomical observations and ancient beliefs about the role of the heavens on our fates. Astrology has no scientific basis, and horoscopes should be read only for fun and entertainment. An astrologer cannot tell you anything about your personality or your future. For an accurate prediction about future celestial events, consult an astronomer, not an astrologer. I can tell you the exact time of the next sunrise and the next full moon, the precise moment when summer will arrive, and when and where to see the next solar eclipse. For those who wonder why people believe in such things as astrology, I recommend Carl Sagan’s Demon Haunted World, a wonderful book and a passionate call for critical thinking in our daily lives. Can you add in the Kepler quote, and we’ll consider it complete?