SIGNS AND CONSTELLATIONS ARE NOT THE SAME!
It is very important to note that there is a difference between the signs of the zodiac and the actual constellations in the sky. The signs of the zodiac have been used as a fixed coordinate system while the constellations themselves have changed over the years. The constellations have been used for centuries to keep track of the movements of the planets and moons along the ecliptic.
To understand why constellations no longer align with their corresponding signs, we need to know a little bit more about how the Earth moves. And something about how we measure time.
Time is a fiendishly difficult thing to define, especially if we insist on using the sun and stars as a reference. Our calendar is, for better or worse, tied to the seasons. June 21—the summer solstice above the equator and the winter solstice below—marks the day the sun appears at its most northerly point in the sky. At the June solstice, the North Pole is most tilted towards the sun.
What makes this complicated is that the North Pole is not always pointing in the same direction relative to the backdrop stars. Our planet spins like a top. And like a top, the Earth also wobbles! A wobbling Earth makes the North Pole trace out a circle on the celestial sphere. Now, the wobble is quite slow—it takes 26,000 years to wobble around once—but as the years go by, the effect accumulates.
Over the course of one orbit around the sun, the direction of the Earth’s axis drifts ever so slightly. This means that where along our orbit the solstice occurs also changes by a very small amount. The solstice actually occurs about 20 minutes earlier than one full trip in front of the backdrop stars!
Since we tie our calendar (and astrologers tie the signs) to the solstices and equinoxes, the Earth does not actually complete an entire orbit in one year. The seasonal or tropical year is actually a hair less time than one full orbit (sidereal year). This means that, each year, where the sun is relative to the stars on any given day—June 21, for example—drifts a very tiny amount.
But wait about 2000 years, and the sun will be sitting in an entirely different constellation!
On the June solstice 2000 years ago, the sun was sitting almost halfway between Gemini and Cancer. On this year’s June solstice, the sun will be sitting between Gemini and Taurus. In the year 4609, the June solstice point will pass out of the constellation Taurus and into the constellation Aries.
The signs more or less aligned with their corresponding constellations when the modern Western Zodiac was defined about 2,000 years ago. But in the intervening centuries, the slow wobble of the Earth’s axis has caused the solstice and equinox points to shift roughly 30o westward relative to the constellations! At present, signs and constellations are about one calendar month off. In another two thousand years or so, they’ll be about two months off.
To complicate matters more, the constellations – unlike the astrological signs – are not of equal size and shape. The stars that make up a constellation are not, for the most part, physically related. The constellations are just patterns that our ancestors saw as they gazed skyward and tried to make sense of it all. Today’s constellations are specific to ancient Greek culture. Most of them were introduced by the Greek astronomer Ptolemy in the 2nd century (who in turn borrowed them from ancient Babylonian texts). Different cultures have seen patterns in the sky unique to their history. Some constellations are shared by many cultures (Orion is a notable example), but most are not.
The modern constellation boundaries weren’t defined until 1930 by the International Astronomical Union. With the current boundaries, there are actually thirteen constellations that lie along the sun’s path. The extra one not listed in any horoscope is Ophiucus, the Serpent Bearer, who sits between Sagittarius and Scorpius. Whereas the signs remain fixed relative to the solstices and equinoxes, the solstices and equinoxes drift westward relative to the constellations or backdrop stars.
While the Zodiac may not be a great predictor of love, fortune, and health, it is a great tool for better understanding the motions of the sun, the Earth, and even the cultures that have come and gone on our little planet. The Zodiac signs, derived from the constellations which lie along the sun’s path in the sky, track the orbit and wobble of Earth and remind us of astronomy’s humble roots.