Whether you care about horoscopes or not, you probably know your zodiac sign. You’ve probably known it for most of your life.

Zodiac signs were originally based on the stars. But over thousands of years, our view of the stars has shifted. That means, if you account for this shift, your sign might not be what you think.

Below, we’ll tell you what it would be instead.

There are three reasons the zodiac signs no longer line up with the constellations they’re named after.

1. Earth’s wobble

The Earth wobbles like a top. A spinning top starts to wobble soon after it is set into motion. The Earth does the same thing, only more slowly.

It takes 26,000 years for the North pole to trace out a complete circle in the sky, pointing at different stars along the way. Scientists call this wobbling motion axial precession.

This wobble means that our view of the stars shifts by one degree every 72 years. Over centuries, this difference builds up.

And it’s not just the Northern stars that shift in our view because of Earth’s wobble, but all stars — including the zodiac constellations.

Take the spring equinox, usually around March 20, the first day of spring in the Northern hemisphere (and the start of the zodiac calendar in Western astrology).

This shift in our view of the stars was discovered by Hipparchus over 2,000 years ago. Since you can’t see stars during the day, he waited for a lunar eclipse — when the moon is directly opposite the sun — and used the moon’s position to work out where the sun was.

By comparing his measurement with earlier ones, he found that our view of the stars shifts by about one degree per century — not too far from modern measurements.

Today, Western astrology uses the tropical zodiac system, which is based on the positions of the stars more or less as they would have appeared to Hipparchus, and not as they appear today.

That means that the zodiac signs familiar to Americans are in sync not with the stars, but with the seasons: Aries starts on the first day of spring, even though the sun is now in front of Pisces then.

In contrast, the Indian system of astrology uses the sidereal zodiac, which accounts for Earth’s wobble and aligns zodiac signs to the stars.

While these two systems were initially aligned, they have been drifting apart ever since. Western astrologers are well aware of this mismatch, but they don’t see a problem with basing the signs on the stars as they were two millennia ago.

“Astrologers using the tropical zodiac are just using what they consider to be an equally valid system,” said Dorian Greenbaum, a historian of astrology who teaches at the University of Wales Trinity Saint David.

2. Constellations differ in size

The zodiac signs were created around 2,500 years ago by the Babylonians.

Their star catalogs listed at least 17 zodiac constellations. But they eventually simplified these into the 12 zodiac constellations we know today, each 30 degrees wide, as if slicing the sky into 12 equal slices.

But constellations aren’t really the same size. In 1928, astronomers divided the sky into 88 officially recognized constellations, each one shaped like its own puzzle piece.

“They are not nice equal pieces,” said Stacy Palen, emeritus professor at Weber State University. “They’re like jagged shapes that are not symmetric in any way.”

Based on these boundaries, the sun spends more than twice as much time in front of Virgo as in front of Cancer. And it spends only a week in front of Scorpio — if you include Ophiuchus, that is.

Which brings us to the last reason the 12 signs don’t align with the zodiac constellations.

3. Ophiuchus

Ophiuchus is the 13th constellation along the sun’s path, according to astronomers. (It even has its own emoji: ⛎.) Ophiuchus means “serpent bearer” in Ancient Greek, and is usually depicted as a man holding a snake. If you squint, you can kind of see why.

So for people born during the sign of Scorpio 2,000 years ago, Ophiuchus was more likely behind the sun on their birthday. (And because of Earth’s wobble, most Sagittarians today were also born when Ophiuchus was behind the sun.)

We don’t really know why the Babylonians left out Ophiuchus from their zodiac signs. They may have originally had a different name for it. But historians believe that when Babylonians simplified their zodiac system, they wanted the 12 zodiac signs to match the 12 months of their calendar. Ophiuchus didn’t make the cut.

A ‘shape-shifter’

Astronomy and astrology have little in common today, and there’s no scientific basis to the idea that the movements of the stars and planets influence our future or our personalities. But the two disciplines started out as the same thing thousands of years ago.

“If you were an astronomer, you were also an astrologer,” Professor Greenbaum said.

The Babylonians viewed the planets as gods, and planetary motions as omens that could foretell the fortunes of kings and kingdoms. This motivated them to look for patterns in the sky.

Even by the 17th century, many astronomers were also practising astrologers. Johannes Kepler, who discovered how planets move in ellipses, probably learned astrology at college, and created horoscopes for friends and patrons. Galileo practiced astrology and sold horoscopes on the side.

“Their side hustle was to cast horoscopes for their rich patrons because that paid the bills,” said Tyler Nordgren, an astronomer and author.

Eventually, during the Enlightenment, astrology was divorced from astronomy and was no longer considered a legitimate science, Professor Greenbaum said.

“It was kicked out of the universities,” he said. “But there were still practitioners.”

Today, we understand the laws governing the motions of planets and stars well enough to send spacecraft to distant worlds, detect gravitational waves and take pictures of a black hole. At the same time, over a quarter of Americans believe that the positions of the stars and planets can affect their lives.

So why has belief in astrology endured, while other methods of divination like ornithomancy (finding omens in the behavior of birds) or tyromancy (fortune telling with a block of cheese) have drifted into obscurity?

“Astrology is a shape-shifter,” Professor Greenbaum said. “Astrology goes along with whatever’s in vogue and manages to survive.”

Because those constellations are behind the sun, they’re in the daytime sky, and so you can’t actually see them on those dates. You’ll need to wait until they’re in the night sky, about six months from the date you entered.