The Zodiac, the 12 signs listed in a horoscope, is closely tied to how the Earth moves through the heavens. The signs are derived from the constellations that mark out the path on which the sun appears to travel over the course of a year. In principle, dates in a horoscope should correspond to when the sun passes through each constellation. But they don’t, much of the time. And a closer examination of the motion of the Earth, the sun, and the stars shows the Zodiac to be more complex than you might imagine!
What is the ecliptic?
As the Earth orbits the sun, the sun appears to pass in front of different constellations. Much like the moon appears in a slightly different place in the sky each night, the location of the sun relative to distant background stars drifts in an easterly direction from day to day. It’s not that the sun is actually moving. The motion is entirely an illusion caused by the Earth’s own motion around our star.
Over the course of a year, the sun appears to be in front of, or “in”, different constellations. One month, the sun appears in Gemini; the next month, in Cancer. The dates listed in the newspaper’s horoscope identify when the sun appears in a particular astrological sign. For example, March 21 through April 19 are set aside for the sign Aries. But your astrological sign doesn’t necessarily tell you what constellation the sun was in on the day you were born.
If only it were that simple!
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!
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