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What is the zodiac?

You might associate the word zodiac with astrology, but it has an honored place in astronomy, too. It’s defined by the annual path of the sun across our sky.

We are surrounded by stars.  Because Earth orbits in a flat plane around the sun, we see the sun against the same stars again and again throughout the year.  Those constellations, which have been special to people throughout the ages, are the constellations of the Zodiac.  Image via Professor Marcia Rieke

We’re surrounded by stars, but some are more special to us than others. Because Earth orbits in a flat plane around the sun, we see the sun against the same stars – the constellations of the zodiac – again and again at regular constellations are the constellations of the Zodiac. Image via Marcia Rieke

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. You might think that dates in a horoscope correspond to when the sun passes through each constellation. But they don’t, much of the time because astrology and astronomy are different systems. Plus, a closer examination of the motion of the Earth, the sun, and the stars shows the zodiac to be more intricate than you might imagine!

As 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. Its motion is entirely an illusion, caused by Earth’s own motion around our star.

Sun's path through the zodiac.

As the Earth orbits the sun, the sun appears to move against the background stars (red line). The constellations (green) through which the sun passes define the zodiac. Image via Tau’olunga/Wikipedia

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 approximate date of 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. The wobble is quite slow, requiring 26,000 years to go around once. But, as the years go by, the effect accumulates.

The wobbling Earth.

Tidal forces from the sun cause Earth’s axis to wobble over a 26,000-year period. The wobble changes where in Earth’s orbit the solstices and equinoxes occur. Image via NASA/Wikipedia.

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 2,000 years, and the sun will be sitting in an entirely different constellation!

On the June solstice 2,000 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.

The changing location of the vernal equinox.

The wobbling of Earth’s axis causes the location of the equinoxes to occur earlier every year. Here, the location of the sun at the vernal equinox (March 21) is shown to drift over a 6000 year period. Image via Kevin Heagen/Wikipedia.

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 13 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 along the sun’s path in the sky, track the orbit and wobble of Earth and remind us of astronomy’s humble roots.

Bottom line: You might associate the word zodiac with astrology, but it has an honored place in astronomy, too. The zodiac is defined by 12 constellations that lie along the annual path of the sun across the sky.

Christopher Crockett