Your zodiac sign is likely wrong. Here’s how to find the correct one

As a teenager, I loved reading my horoscope in gossip magazines. But even then my friends and I knew it was nonsense. For us, this was a fun pastimes for bored teens. So I was surprised when my hairdresser recently asked me my star sign. When I shared my opinion on astrology and horoscopes, she simply replied, “Typical Taurus.”

Astrology is currently experiencing a remarkable renaissance—especially on social media, where posts about “Geminis,” “Leos” and “Virgos rising” are everywhere. The trend may partly reflect how deeply people want to identify with personality types and, in the process, gain some insights into an uncertain future. And perhaps surprisingly, many astrology posts come from people who consider themselves to be scientifically inclined.

How can that be? Today modern science and astrology stand in stark contrast to each other. There is no evidence that your sign actually affects your day-to-day life. Yet the origin of horoscopes is not entirely unscientific. It’s actually a story that begins with careful observation of the cosmos and the creation of a calendar system thousands of years ago in the Middle East.

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What exactly are zodiac signs?

The zodiac signs, considered the foundation of modern astrology by many people, are based on 12 constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius and Pisces. These are by no means the only constellations visible in the night sky. In 1922 the International Astronomical Union (IAU) officially recognized 88 constellations, including those associated with the zodiac and other well-known examples such as the Big Dipper.

Thousands of years before the IAU’s founding, people in ancient Mesopotamia were pondering the constellations. They assigned particular importance to those that lay in the apparent path of the sun, also known as the ecliptic.

Thanks to Mesopotamia’s geographic location, it has historically had highly consistent weather patterns. There is a rainy season and a dry season. The wind’s direction can also be predicted depending on the time of year. And because certain constellations are visible in the night sky at different times of the year, people may have believed there was a connection between the stars and these seasons.

This idea isn’t such a stretch: after all, scientists today recognize that the sun and moon do indeed influence earthly events such as the weather and the tides. But distant stars, as we’ll discuss later, are a different story.

Toward the end of the fifth century B.C.E., Babylonian astronomers divided the ecliptic into 12 equal sections of 30 degrees each, analogous to their calendar’s 12 months of 30 days each. In doing so, they developed the first known astronomical coordinate system. They assigned each section a constellation located in the night sky along the ecliptic at that time of the year.

So began astrology. The zodiac signs and their associated superstitions spread from Mesopotamia to Greece and Rome and eventually became part of European tradition.

Times are changing—and so are star signs

In retrospect, it doesn’t seem so far-fetched that people in antiquity or the Middle Ages believed that constellations influenced us. What I don’t understand is how this belief has persisted to this day.

We now know much more about astronomy, physics and celestial mechanics. For example, scientists have long understood that constellations are composed of objects that are sometimes very far apart and have no connection to one another, except that they appear close together from our vantage point here on Earth.

Furthermore, their distance from Earth is so great that they—unlike the moon or the sun—cannot actually influence us or our planet. Stars are gigantic fusion reactors light-years away that do not shape our personalities or life events.

If you’re reading this article, I assume you don’t believe in astrology. But if you find yourself talking to someone who’s hooked on horoscopes, here are three scientifically grounded points you might raise:

There are actually 13 zodiac signs. If you look at the constellations along the ecliptic, you’ll find not 12 but 13 of them over the course of a year. Ophiuchus, the Serpent Bearer, was forgotten. Or rather it was probably intentionally left off the list because it didn’t fit into the Babylonians’ system of 12 months.

Not every zodiac sign corresponds to a month. The sun passes through the boundaries of the zodiac signs for different lengths of time while it travels along the ecliptic. For example, the sun only passes through the constellation Scorpius’s boundaries for six to seven days, while it takes 44 to 45 days to travel through the much larger Virgo constellation’s boundaries.

The zodiac signs change over time. More accurately, Earth’s axis of rotation performs a wobbling motion like a spinning top, a phenomenon known as precession. This movement shifts which constellations appear in the ecliptic at what time of year. While the constellation Aries was visible in the ecliptic starting on March 21, according to Babylonian time, today the constellation Pisces appears on that day instead.

Are you still determined to find your star sign?

A scientifically correct zodiac system does exist. In it, each constellation is located in the ecliptic in modern times.

This is quite different from the system that is still used for astrology today. But don’t worry, Scientific American is not going to be printing horoscopes based on the corrected zodiac—except, perhaps, as a joke.

This article originally appeared in Spektrum der Wissenschaft and was reproduced with permission. It was translated from the original German version with the assistance of artificial intelligence and reviewed by our editors.