Michelle Thaller
What did you say your sign was? The real story behind the zodiac
I love it when someone asks me what my “sign” is (that is, my sign of the zodiac). I’m an Ophiuchus. I’m totally serious. I know it’s a bit cheeky of me, but that’s what you get for asking an astronomer a question like that.
So how can my sign be something you’ve never heard of and no astrological predictions exist for? The answer gets to the heart of what the zodiac really is, and what it means astronomically instead of astrologically.
Personally, I find it hard to imagine that the position of stars and planets in the sky could have anything to do with my daily life or personality traits. How could they? It’s completely true that at this very moment, all the planets in our solar system are exerting a tiny gravitational tug on me. But the pull of even the largest planet, Jupiter, is far less than the gravitational attraction between me and my computer. Jupiter is a lot more massive, but the computer wins because it’s much, much closer.
So what does the word zodiac really mean? Zodiac is an actual astronomical term for the path that the sun takes across our sky every day, from sunrise to sunset. Interestingly, the moon and all the visible planets take the same path across the sky as well, which doubtless seemed very important to ancient celestial observers. You can see this yourself on a clear night when the moon and a few planets like Mars, Venus, or Jupiter are up; they all seem to follow one behind the other in an organized arch across the sky.
Why do all the important celestial objects move along the same line in the sky? The answer to that goes back to the very beginning of our solar system, when the gravity of our young sun was bringing together dust and gas which would someday become the planets. As material was pulled closer and closer together by gravity, it began to spin into a disk surrounding the sun.
Eventually, all the planets condensed out of material from this spinning disk of dust and gas. With nothing in space to stop them or slow them down, the planets kept right on spinning around the sun, in the same orientation of the original disk. Because of this, all the planets orbit our Sun in the same plane, with some small deviations caused by gravitational interactions.
So, when you see the path of the sun, moon, and planets across the sky, you’re seeing the plane of the long-gone disk of material that became our solar system.
Now, the ancients had no problem figuring out which objects in the sky were planets and which were stars. All the familiar stars and constellations move across the sky once a day, in an orderly, predictable fashion. That once-a-day motion is caused by the Earth spinning on its axis once every 24 hours. Because of our spin, everything moves across the sky once a day.
But planets are a little more tricky. As they move across the sky once a night, they also change positions in the sky relative to the constellations. Hence, sometimes the planet Mars seems to be in the constellation Ares, but at other times it’s in Scorpio. The word planet even comes from an ancient word for “wanderer,” because the planets wander around through several different constellations.
And each constellation that the path of the sun, moon, and planets passes through is called, logically enough, a constellation, or “sign” of the zodiac. When this system was set up by the ancient Greeks, there were twelve officially recognized constellations of the zodiac.
Now, since the sun travels along this path in the sky as well, obviously some of the constellations are up in the sky during the day (and hence invisible to us), while others are up at night. The Sun makes one full circuit around the zodiac every year, (which is really caused by the Earth moving around the sun once a year), spending about one month in every sign. So, officially, your “sign” of the zodiac is the constellation the sun was in during the month you were born.
And here’s where astrology starts to have some real astronomical problems.
You see, things have changed since the ancient Greeks were around. It’s not that the constellations themselves have changed. Stars do move through space, but even after 3,000 years, the motions are too small to be easily noticed.
Instead, it’s something more close to home that’s messed things up. Namely, the moon. Our moon is actually quite exceptional compared to other moons in the solar system -- it’s much too big. The moon’s diameter is about one fourth of the Earth’s, huge compared to other planets and their satellites. Our over-sized moon exerts a strong gravitational force, which, among other effects like tides, changes the way the Earth spins on its axis.
We still spin once every 24 hours, but the angle we spin at changes. The scientific word is precession, and it’s usually illustrated with the example of a toy top, spinning around. The top spins quickly around its axis (usually too quick too see), but there’s also a slower spinning motion, as the entire body of the top seems to move around in a slow circle.
In other words, the angle the top is spinning at changes, or precesses. The moon’s gravity makes the Earth do the same as its tug changes the angle of our spin. In the case of the Earth, this motion is very slow. One full precession of the top, in the case of the Earth, takes 26,000 years.
What does this have to do with the zodiac? The angle of our spin axis is what determines what season it is on Earth (for more information, read my column “The Reason for the Seasons,” which can be found in the inset box to the right). So, as the angle of our spin changes, the seasons change with it.
What that means is that while Orion is a winter constellation today (that is, it’s visible in the night sky only during the winter months), in 13,000 years, or one half a precession cycle later, it will be up in the balmy summer evening sky.
In astronomical observation terms, the precession of the Earth’s axis means that the sun makes one complete, slower, cycle through the constellations of the zodiac every 26,000 years. And while 26,000 years is a very long time, the signs of the zodiac were set up about 3,000 years ago. That’s a substantial enough fraction of 26,000 years that we’re now beginning to find anomalies in our astrological system.
Like we said before, your sign of the zodiac is supposed to be the constellation the sun was in at the moment you were born. Unfortunately, the sky has shifted enough that the official signs are off by about one and a half months. A person born on May 8th, for example, is officially a Taurus. But the sun was actually in Aries at the time. And yes, that means (gasp!) that you’re not the sign you think you are.
This shift of the zodiac is also where the somewhat dated phrase “the dawning of the age of Aquarius” comes from. The position of the sun on the first day of spring used to be in Aries 3,000 years ago. Now it’s moved almost all the way through Pisces, and is about to enter the constellation Aquarius.
But getting back to the Ophiuchus thing. What’s that all about? Honestly, that’s more of a technicality, but I love it all the same.
You see, modern astronomers have divided the entire night sky into constellation regions, in order to make it easier to map and catalog the stars. These boundaries don’t have much to do with classical astronomy; they’re just clearly delineated regions in the sky which make organizing tons of data easier.
And for the constellation Ophiuchus, the official boundaries just barely snuck over that magical line in the sky, making it the 13th, “astronomical” constellation of the zodiac. And that’s where the sun was when I was born. I always did like to be different, which some might say makes me a typical Sagittarius.
Oh well.