Prof. Darin Hayton teaches a course in science history at Haverford College and is affiliated with the Philadelphia Area Center for History of Science (PACHS).

Writing in the PACHS Smorgasbord blog, he observes that, throughout the twentieth century, opponents of astrology have seemed baffled.

“They regularly express shock and surprise that despite a rational, modern scientific worldview people continue to believe in astrology,” he observed. “Critics have displayed a remarkable doggedness in putting forward the idea that astrology is an intentional fraud perpetrated on a gullible populace.  But exactly what is accomplished by calling astrology rubbish?” He asks.

Such attacks may rally the faithful but fail to persuade others, he suggests.

Prof. Hayton says it is not his intention to defend astrology or astrologers, but thinks it disingenuous that critics rely upon a small set of rhetorical straw-man positions to refute astrology.  A straw-man fallacy occurs when an actual position is ignored and a distorted, exaggerated or misrepresented version is substituted for it.

The strategy has not been particularly effective for the science side, he noted.

Critics of astrology claim it is impossible to understand how the stars can affect human affairs, and doubly difficult to suggest a mechanism to account for the influence of the zodiac signs, which continue to change their positions among the stars.

A phenomenon known as precession of the equinoxes is responsible for the shifting sky map.  As the Earth rotates on its axis, a slight “wobble” effect causes the  apparent position of the constellations to advance about one degree every 72 years.

In the latest brouhaha over astrology, Minnesota astronomer Parke Kunkle made headlines by claiming that astrologers must be reading the wrong charts.  Because of precession, a person born with the sun in Pisces today would have been an Aquarian if born on the same calendar date 2,000 years ago.

But this is the same straw-man fallacy that opponents have used for decades to debunk astrology, Hayton points out.

“In Western astrology, the astrological signs are determined by the position of the sun in the sky, not the absolute positions of the constellations.  Refuting a doctrine that astrologers don’t hold seems, at best, ancillary and probably irrelevant to any attack on astrology,” he said.

Such attacks aren’t new.  In a 1930 Popular Science article debunking astrology, author Jesse Gelders claimed that astrologers are ignorant charlatans because they fail to realize (or acknowledge) that precession exists.  Others have made similar points over the years.

In fact, precession was identified by Hipparchus, a Greek astrologer, more than 100 years before the birth of Christ.  Historical documents indicate that astrologers have been aware of the phenomenon ever since, Hayton says.

“What is served by the denigrating rhetoric used to brand astrologers as frauds and charlatans? Surely it would be more effective to adopt a more conversational approach rather than labeling astrologers and their customers irrational, superstitious dupes,” he suggests.