The signs of the Zodiac are counted eastward along the ecliptic (the plane of the planets) from the vernal equinox. The vernal equinox marks the first day of spring and occurs at the intersection of the ecliptic and the celestial equator. The vernal equinox also marks the zero point of the Zodiac.

Around 600 B.C., when this system was set up, the zero point was in Aries and was called the "first point of Aries." The constellation Aries encompassed the first 30 degrees of the ecliptic; from 30 to 60 degrees was Taurus; from 60 to 90 degrees was Gemini; and so on for all twelve constellations of the Zodiac.

But long before the ancients did their measuring, the Earth had been wobbling slowly around its axis in a 25,800-year cycle. This wobble—called precession—is caused by the gravitational attraction of the moon on Earth's equatorial bulge.

Over the past two-and-a-half millennia, this wobble has been sufficient to cause the intersection point between the celestial equator and the ecliptic to move west along that ecliptic by 36 degrees, or almost exactly one-tenth of the way around. This means that since astrologers started casting horoscopes based on this system, the signs have slipped one-tenth—or almost one whole month—of the way around the sky to the west, relative to the stars beyond.

Your morning newspaper horoscope ignores precession. If you were born between March 21 and April 19, your astrological sign is said to be Aries. But this was only true for a while, back when the system was set up in 600 B.C. Today, the sun is no longer within the constellation of Aries during much of that period. From March 11 to April 18, the sun is actually in the constellation of Pisces!

We'll prove it, and show you how precession changes your astrological sign over time.