It seems to have started with this article in the Minneapolis Star-Tribune last weekend, in which one astronomer made some statements about the zodiac. Parke Kunkle is on the board of directors of the Minnesota Planetarium Society and teaches astronomy at Minneapolis Community and Technical College. Kunkle told the Star-Tribune the Earth's relation to the sun had changed since the Babylonians first created the zodiac.
We got in touch with Kunkle and asked him what he actually told the Star-Tribune. He said he was asked by the Star-Tribune to give them a few bits of information about astronomy, not realizing the article would become a huge discussion of astrology and the relationship between astronomy and astrology. And the main stuff he talked to the Star-Tribune about has to do with the phenomenon of "precession."
If you take a toy top and spin it, it spins around an axis and that axis tends to point in different directions. It moves around. That's what we call precession. So in Earth's case, right now, Earth's spin axis points towards Polaris, the North Star. But in 3000 BC, the Earth's axis pointed towards a different star, Thuban. And that majestic motion takes about 26,000 years. so if you went from 3,000 B.C. and waited 26,000 years, you'd have the north star Thuban again.
This phenomenon was first noticed around 130 B.C. by a Greek astronomer, Hipparchus of Nicea. And as a result, if you actually look at what stars were positioned behind the sun on a particular date, that would have been very different 5,000 years ago than it would today. "We're in a different constellation now and that is the typical sun sign," based on the sun's position when you were born.
And no, Parke Kunkle didn't tell the Star-Tribune that the zodiac ought to include 13 signs instead of 12 — especially since he doesn't believe in astrology at all. (He highly recommends Phil Plait's page about astrology.) He did mention that astronomers tend to reckon the sun's position with 13 constellations instead of 12, and Ophiuchus is the 13th. But in the current astrology zodiac, there are just 12. "I just mentioned that it's there, and astronomers actually count it... So if you actually watch the stars in the background of the sun, it actually does go through the constellation of Ophiuchus." He adds that the Babylonians probably had totally different constellations anyway.