Yes, you may be reading the wrong horoscope. No, it isn't NASA's fault your zodiac sign changed.

Horoscope
(Image credit: Eva Hambach/AFP/Getty Images)

There are actually 13 signs of the zodiac, and the Earth's axis has shifted since the Babylonians codified the horoscope dates some 3,000 years ago, so if you are a believer in astrology but also scrupulously exact about your science, you may have to change which star sign you were born under.

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NASA wrote about the the 13th zodiac sign, Ophiuchus, four years ago, and it also noted that other traditions recognized up to 24 zodiac constellations — so, options — but the information has been in the public domain for at least 20 years. Why does it pop up again every few years? Maybe it's something in the stars. But whatever the reason, it's always new knowledge to someone, so here is NASA's explanation:

When the Babylonians first invented the 12 signs of zodiac, a birthday between about July 23 and Aug. 22 meant being born under the constellation Leo. Now, 3,000 years later, the sky has shifted because Earth's axis (North Pole) doesn't point in quite the same direction.Now Mimi's Aug. 4 birthday would mean she was born "under the sign" of Cancer (one constellation "earlier"), not Leo.The constellations are different sizes and shapes, so the Sun spends different lengths of time lined up with each one. The line from Earth through the Sun points to Virgo for 45 days, but it points to Scorpius for only 7 days. To make a tidy match with their 12-month calendar, the Babylonians ignored the fact that the Sun actually moves through 13 constellations, not 12. Then they assigned each of those 12 constellations equal amounts of time. Besides the 12 familiar constellations of the zodiac, the Sun is also aligned with Ophiuchus for about 18 days each year. [NASA]

So there you have it. Disappointed? Have you considered the enneagram?

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Peter Weber

Peter Weber is a senior editor at TheWeek.com, and has handled the editorial night shift since the website launched in 2008. A graduate of Northwestern University, Peter has worked at Facts on File and The New York Times Magazine. He speaks Spanish and Italian and plays bass and rhythm cello in an Austin rock band. Follow him on Twitter.