The Main Reason No One Should Trust Astrology

Source: Denis OREA/Shutterstock

I make no apologies for being an advocate of open inquiry. But I also make no apologies for being an advocate of science. After all, science has brought us such advances as modern medicine, an understanding of the inter-connectedness of life on this planet, the ability to create extraordinary architectural masterpieces, the moon landing, and way more.

In recent years, I've gotten to collaborate with State University of New York at New Paltz astrophysicist Raj Pandya on various projects. Professor Pandya runs several astronomy-related programs in his role as Director of the John R. Kirk Planetarium on our campus. In fact, he is a master at leading stargazing events, seemingly knowing the name and story of nearly each and every star up above!

Last summer, during a stargazing event that I was fortunate to attend, Professor Pandya pointed out something remarkable. This was the fact that the zodiac signs that drive the beliefs of many who adhere to astrology are actually based on what the sky looked like in about 600 B.C. and, in fact, because of a natural wobbling of the earth over time, the constellations demarcated back then—that correspond to Capricorn, Aries, Pieces, etc.—are all, in fact, incorrect. They no longer match up with dates that they had matched up with in 600 B.C. In short, everyone who believes in astrology is actually off by approximately a month in terms of what their sign, so to speak, actually is.

As Professor Pandya puts it:

"The Sun’s position in the sky with respect to the background stars slowly drifts over time due to the Earth’s orbit. The apparent annual path that the Sun traverses from our perspective is called the ecliptic. Constellations that lay on the ecliptic (which will not be visible when the Sun is passing through them) are zodiac constellations. There are traditionally 12 zodiac constellations that span approximately 30 degrees in the sky each, which means the Sun spends roughly one month in each constellation.

"Around 2000 years ago, a person’s zodiac or “astrological sign” was originally determined by what constellation the Sun was in on their birthday. (Note that the Sun being in the foreground of this constellation during this time means it would not be visible on the person’s birthday.) The dates that the Sun passed through each constellation were observed, and these dates became associated with each 'astrologic sign.'

"Many people still use these traditional dates to look up their “astrologic sign.” However, the actual dates that the Sun currently passes through a given zodiac constellation has shifted by about one month since the traditional dates were determined due to the Earth’s precession, or wobble. As the Earth rotates, it also imperceptibly wobbles, like a spinning top right before it falls over. This wobbling motion is called precession and is caused by the Moon’s gravitational effect on Earth’s equatorial bulge. The result is that the Earth’s axis of rotation is not pointing at a fixed position in space, but is instead slowly tracing out a circle. Therefore, the dates that the Sun appears to pass through each zodiac constellation must also change. The change is slow, as one complete circle of precession takes about 26,000 years! However, it is significant enough that in the ~2000 years since the traditional dates for each “astrologic sign” were defined, the Sun’s position along the ecliptic on a certain date has shifted west by about 30 degrees, or one whole zodiac constellation. To better illustrate this, I used the virtual planetarium software called Stellarium to tabulate when the Sun actually passes through a given zodiac constellation, and compared it with the traditional dates associated with each 'astrologic sign':

Data from Stellarium showing shifts in zodiac signs across the past two millenia

Source: Professor Raj Pandya, used with permission

"One interesting result here is that the Sun actually passes through a 13th constellation that was not traditionally recognized as a zodiac constellation (this is because the official boundaries for each constellation were not internationally agreed on until 1928). So, if you were born between November 30 and December 17, the Sun was actually in the zodiac constellation of Ophiuchus on your birthday, not Sagittarius as you might think if you Googled what your 'astrologic sign' is.

"Another thing to note is that not all zodiac constellations equally cover 30 degrees of the sky, some are larger and some are smaller. The largest zodiac constellation is Virgo, hence, the Sun spends more time in Virgo than any other constellation of the zodiac. So more people will have birthdays during the time when the Sun is in Virgo than any other zodiac constellation, simply because of how the constellations boundaries are drawn. Conversely, the ecliptic only goes through a small section of Scorpius, so only people born within a narrow five-day period in late November were born when the Sun is passing through Scorpius."

Hey, I hate to be a Debbie Downer, but with an upsurge in interest in astrology, I kind of felt like someone needed to set the record straight!

For decades, social psychologists have studied what we call the Barnum Effect (see Tabacyk et al., 2010). This phenomenon is based on the late P.T. Barnum's famous quote: There is a sucker born every minute. And it applies to the psychology of belief in the supernatural. Some of the classic work on this topic has been conducted on belief in astrology, basically showing that if you write a personality description that is broad and general enough, you can pretty much get anyone to believe that it corresponds to their personality in particular. Social psychologists have often pointed to the Barnum Effect to help us understand why so many people believe in astrology.

Personality Essential Reads

Why Do Antagonistic People Think They’re So Perfect?

Do Only Jerks Eat Meat?

Bottom Line

The large-scale belief in astrology may actually owe very much to the Barnum Effect: The fact that people will pretty much believe that any personality description matches them in particular if the description is framed in very general terms. Not only does astrology lean heavily on such broad-sweeping, general descriptions of people but, above and beyond that, due to the natural changes in the night sky across time (due to the earth's natural wobble), the zodiac signs that characterize modern astrology are, in fact, actually all wrong and are shifted by about a month.

That all said, you've got to be wondering. And the answer is this: My sign is Aquarius. But, based on the data provided by Professor Pandya, I'm actually Capricorn. Now what do I do with that!?

Facebook image: Denis OREA/Shutterstock