By Whether or not you believe in horoscopes, you probably know your zodiac sign. But the stars that once defined those signs have shifted over time, meaning your real sign might not be what you think.The 12 zodiac signs were originally based on constellations behind the sun as seen from Earth. Ancient astronomers used these patterns to track time and predict the future. For instance, 2,000 years ago, the sun appeared in front of Scorpio on Nov 3, but today, it is Libra.There are three reasons zodiac signs no longer align with their namesake constellations: Earth’s slow wobble, which shifts our view by about 1° every 72 years; the uneven sizes of the constellations; and the overlooked 13th constellation, Ophiuchus.

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This shift, known as precession, was discovered over 2,000 years ago by the Greek astronomer Hipparchus. Unable to see stars in daylight, he observed lunar eclipses (when the moon sits opposite the sun) to determine the sun’s position. By comparing his results with earlier records, he found that our view of the stars drifts by roughly 1° per century, a figure close to modern measurements.Today, Western astrology uses the tropical zodiac, which fixes the signs to the seasons rather than the stars. Aries, for instance, still begins on the first day of spring, even though the sun now appears in front of Pisces at that time. By contrast, Indian astrology follows the sidereal zodiac, which accounts for Earth’s wobble and keeps the signs aligned with the stars.

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Both systems began in sync but have drifted apart over the centuries. Western astrologers acknowledge the mismatch yet see no issue. “Astrologers using the tropical zodiac are just using what they consider to be an equally valid system,” said Dorian Greenbaum, historian of astrology at the University of Wales Trinity Saint David.Astronomy and astrology, once united, have long since parted ways. “If you were an astronomer, you were also an astrologer,” Greenbaum said. Babylonian astronomers viewed planets as gods and celestial motion as divine messages. Even in the 17th century, Johannes Kepler and Galileo wrote horoscopes because “that paid the bills”, per astronomer Tyler Nordgren.During the Enlightenment, astrology was expelled from universities and lost its scientific standing. Yet, belief persists. Why? “Astrology is a shape-shifter,” Greenbaum said. “Astrology goes along with whatever’s in vogue and survives.”

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LAST WEEK’S QUICK QUIZQuestion on Oct 27: How does the Moon’s gravity compare to Earth’s? It’s much weaker, causing objects to weigh far less on the lunar surface than they do on our planet?Answer: The Moon’s gravity is about one-sixth as strong as Earth’s