What's your sign?

In January and February 2011, websites flared with astrology news, apparently precipitated by a January 10 column in the Minneapolis Star-Tribune, "What's your sign? Probably not what you think, thanks to the Earth's 'wobble.'" (The Star-Tribune has removed this article from their website. It would be a shame if it disappeared from the Internet entirely, so I have reproduced a scanned copy of the article and the article text at the bottom of this page.) In brief, this article says that astrological predictions are off because sun signs no longer correspond to zodiacal constellations, constellations that intersect the ecliptic (the apparent path the Sun takes through the sky over the course of a year), because of the precession of the equinoxes, which shifts the positions of the celestial pole and the ecliptic relative to the fixed stars. Like most so-called science "news", this is really old: the precession of the equinoxes was understood by Hipparchus. However, it's still worth asking 1) are sun signs really off? and 2) does this actually matter to astrologers?

Yes, sun signs are off. The Earth's axial precession has a period of about 26,000 years, and it's been over 2,000 years since the Babylonians defined the sun signs based on the zodiacal constellations, so the constellations have moved about 30 degrees relative to their signs. Each sun sign is now located in the zodiacal constellation with a name one sun sign behind. Worse, since precession isn't a rotation about the pole of the ecliptic, the ecliptic shifts north and south into parazodiacal constellations: for example, into Ophiuchus, "the Snake Bearer," a large summer constellation and the 13th sign mentioned in the article. Below is a screenshot from the planetarium software package Stellarium showing that not only does the ecliptic run through Ophiuchus, but also that it actually subtends a longer arc through that constellation than through Scorpio!

ecliptic arc extent in ophiuchus vs. scorpio

No, this doesn't matter to (most) astrologers. I think, reasoning from their premises, that they have a good case. Babylonian sun signs only corresponded roughly to zodiacal constellations in the first place, since zodiacal constellations don't all intersect the same length of arc of ecliptic. Because of this--and more importantly, because astrology is about the positions of solar system bodies, not distant stars--arcs and points on the ecliptic are astrologically significant because of their relationship to the motions of the Sun and the seasons, not because they happen to overlap some constellation. So sun signs should be reckoned from a point based on the motion of the Sun (which is immune to precession), not from some arbitrary point in a constellation. There is no correspondence between sun signs and zodiacal constellations now, there was only a mnemonic correspondence in 500 BC, and this is astrologically OK.

High-resolution scan of page 1 of the article.

astrology article page 1

High-resolution scan of page 2 of the article.

astrology article page 2
Minneapolis Star-Tribune
Star Tribune: Newspaper of the Twin Cities (Minneapolis, MN)
January 10, 2011

The stars might not actually be aligned in your favor
What's your sign? Probably not what you think, thanks to the Earth's "wobble."

Edition: METRO
Section: VARIETY
Page: 01E

Article Text:

A recent Harris Poll found that 31 percent of Americans believe in 
astrology. They're wrong -- although not necessarily in the way their 
detractors might cite. 

The ancient Babylonians based zodiac signs on the constellation the sun 
was "in" on the day a person was born. During the ensuing millenniums, 
the moon's gravitational pull has made the Earth "wobble" around its 
axis, creating about a one-month bump in the stars' alignment. 

The result? 

"When [astrologers] say that the sun is in Pisces, it's really not in 
Pisces," said Parke Kunkle, a board member of the Minnesota Planetarium 

Indeed, most horoscope readers who consider themselves Pisces are 
actually Aquarians. So instead of being sensitive, humane and 
idealistic, they actually are friendly, loyal and inventive. 

Or not. 

There is no physical connection between constellations and personality 
traits, said Kunkle, who teaches astronomy at Minneapolis Community and 
Technical College. "Sure, we can connect harvest to the stars," he said. 
"But personality? No." 

Astronomers have pooh-poohed astrology from the get-go, but Kunkle 
hedge! d when asked if astrology can bring people to his science. 

"Historically, people looked at the sky to understand the world around 
us," he said. "But today I don't think people who are into astrology 
look at the sky very much." 

But they do look at horoscopes. And now they have an explanation for why 
a day might not have turned out exactly as predicted. 


Astrology buffs should be using these dates, reflecting where the stars 
currently are aligned:

Capricorn: Jan. 20-Feb. 16.
Aquarius: Feb. 16-March 11.
Pisces: March 11-April 18.
Aries: April 18-May 13.
Taurus: May 13-June 21.
Gemini: June 21-July 20.
Cancer: July 20-Aug. 10.
Leo: Aug. 10-Sept. 16.
Virgo: Sept. 16-Oct. 30.
Libra: Oct. 30-Nov. 23.
Scorpio: Nov. 23-29.
Ophiuchus:* Nov. 29-Dec. 17.
Sagittarius: Dec. 17-Jan. 20.

* Discarded by the Babylonians because they wanted 12 signs per year.

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Will Vaughan. Last revision March 16, 2011.