I’m a pound.
What does that mean? If you’re an evidence-based thinker, it means nothing. However, if you believe in astrology, it means that the sun’s influence on me (unidentifiable and uncertain and inexplicable as it is) means that I was born at a time of year ruled by the “sun sign” Libra. the sign’s relationship to the actual constellation of Libra is vague at best. If you believe in astrology, you and i should have a few words.
But what does it mean to say that the sun is “in” Libra? Why focus on that constellation and not, say, Orion, which is, in almost every way, objectively cooler?
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It boils down to two things: our solar system is flat and its motion is relative.
Let’s hit the “relative motion” section first. Consider our familiar Earth, for example. Our planet revolves around the sun once a year. From our point of view, stuck on Earth, the sun appears to be spinning we once a year Physically, this is not the case, but perceptually, it is, which is why, heliocentrism aside, we still geocentrically say something like “the sun sets” rather than “the Earth rotates so that the horizon rises to block the sun.” It’s enough: we don’t feel in our guts that the Earth spins once a day or revolves around the sun at 100,000 kilometers per hour.
The other stars are much, much further away than the sun, so they appear fixed in the sky relative to each other. Our meaning-seeking brain naturally interprets the patterns in these “fixed” stars as recognizable images. what we call constellations (literally, “collections of stars”). Well, they are mostly Recognizable: Although Orion looks like a human and Scorpius looks like a scorpion, Libra is made up of four main stars in a strange rhombus.
As the earth rotates, we see these stars rise and set every day. If the earth were fixed in space relative to the sun, we would see the same constellations in the sky every night throughout the year. Instead, as the Earth moves around our star, from our point of view, the sun is constantly moving behind the constellations, taking a full year to travel across the sky and return to where it started.
Here is the flatness of our solar system. The Earth, like all the other major planets, orbits the Sun in a flat, nearly circular ellipse, so the apparent motion of the Sun against the fixed stars draws a line around the sky—an ellipse. Earth in the center. We call this path the ecliptic.
This movement does not change significantly from year to year, from century to century; the sun follows the same well-worn path through the same constellations. The names are familiar: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius (not “Scorpio”, please), Sagittarius, Capricornus, Aquarius and Pisces. Many of these constellations represent animals, and the ancient Greeks referred to these constellations collectively as zodiac cycle, or “animal circle”. Hence, we now call it the zodiac.
The movement of the sun through the zodiac (pronounced zodye-a-kul) constellations create a calendar like; our star sets above Pisces at the end of March, for example. This is complicated by a long-term bump in the Earth’s rotation called precessionwhich is created by the gravitational pull of the moon and the sun. For millennia, the times of the sun’s position in a given zodiacal constellation have been ignored, creating a disconnect between what astrologers call “sun signs” and the actual constellations. Three thousand years ago, when the ancient Greeks first used the signs, the sun was in Libra at the end of September. But because of precession, that has since changed, so our star was actually in the constellation Virgo when I was born.
It is important to understand that the constellations we know are not natural, but rather a product of the random positioning of the stars to recognize patterns in the human brain. Sometimes different cultures see different patterns, and it happens that many modern societies use mostly the same ones as the ancient Greeks. But even so, the origin is a little vague. For example, the Greeks considered Libra to be part of Scorpius—his claws, specifically—while the Babylonians believed that Libra was a set of scales or scales without a scorpion.
This means that while the ancient Greeks believed there were only 11 zodiacal constellations, not 12, Libra came in much later to round them out to a couple of dozen.
But it gets worse. The true path of the sun, the ecliptic, passes through more than these 12 constellations. Ophiuchus (“snake bearer”) lies between Sagittarius and Scorpius, and the sun actually spends about 20 days—most of a month—within its confines. That’s longer than the sun spends in Scorpius! So Ophiuchus is arguably more deserving of being in the zodiac than some poisonous arthropods, but it happens to have fainter stars in a fainter pattern, so it’s left out.
And we’re not done yet, because while the solar system is flat, it’s not quite like that. In other words, the other planets orbit the sun mostly in the same plane as Earth but not exactly. Jupiter’s orbit around the sun tilts slightly more than Earth’s by a degree. Venus is tilted by more than three degrees. The moon’s orbit is tilted by more than five degrees! This means that the moon and planets can appear either north or south of the ecliptic, and occasionally within the boundaries of other constellations outside of the 12 canonical zodiacs. There are a dozen other constellations that can move moons and planets, including Canis Minor, Pegasus, and even our old friend Orion.
So no matter how you slice it, the zodiac—from the constellations of its members to the meaning our pattern-projecting brain assigns to those particular groups of stars—is complete.
This is not to say that the zodiac is not a useful construct. it is! Like other constellations, the zodiac provides a framework that we can use to navigate the sky. For an astronomer with some knowledge of the sky, knowing that Jupiter is in Taurus (as I write this, for example) means that the giant planet is visible in the autumn and winter sunsets, when the bull-shaped constellation is most visible. it is easily seen in the Northern Hemisphere. If you want more details, there are coordinate systems we can use to zero in on a given positionbut if you want to go outside and be under the night sky, the zodiacal constellations provide a “sufficient” set of celestial directions. Also, many of them have bright stars in obvious patterns that are easy to spot and identify; so it’s fun to watch them.
And that’s not Taurus.
