University of Pittsburgh
January 18, 2011

It’s the Earth, Stupid! Moon Not at Fault for Zodiac Discrepancy, Says Pitt Astronomer

Contact:  412-624-4147

PITTSBURGH—Zodiac calendars have been out of sync with the stars since before there were newspapers, and it’s not the moon’s fault, says Jeffrey Newman, an assistant professor of physics and astronomy in the University of Pittsburgh’s School of Arts and Sciences. The real reason for the discrepancy in Zodiac calendars is that the Earth’s axis changes direction like a wobbling top because of the uneven distribution of the planet’s considerable mass. As a result, the stars visible at night in a given month change in a 26,000-year cycle. The planet is tilted at a 23.5-degree angle, so as it “wobbles”—in a process known as the procession of the equinoxes—the poles shift in a circular direction. The Earth’s north pole currently points toward the star Polaris, but 12,000 years ago, it pointed to the star Vega, which will reclaim its title of pole star in another 14,000 years, Newman said. 

The Zodiac calendars printed in newspapers still use dates appropriate to the direction the axis tilted in Babylonian times, about a month off from the actual positions of the Sun and constellations today, he said. “Astrology columns say that I’m a Gemini, but the Sun was actually in Taurus when I was born,” Newman said. Contact Jeffrey Newman at 412-592-3853 (cell), 412-624-1345 (office), or janewman@pitt.edu, or through Pitt News Representative Morgan Kelly at 412-624-4356 (office), 412-897-1400 (cell), or mekelly@pitt.edu. 

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