Star sign or constellation? The difference, and the “13th star sign”

Every few years, the same headline does the rounds: “The star signs have shifted, you're not even a Leo anymore!” Or: “There's a 13th star sign!” Anyone who then looks up their birth date is left feeling confused. The good news is that the fuss isn't caused by a flaw in astrology, but by a mix-up between two things that simply sound alike: constellations and zodiac signs. Once you understand the difference, you can scroll right past the next headline without a second thought.

What is a constellation?

A constellation is a region of the sky. The International Astronomical Union (IAU) has divided the entire celestial sphere into 88 constellations with precisely defined boundaries, much like countries on a map. These regions have grown up historically, which is why they are very different in size: Virgo is enormous, while Scorpio is comparatively small.

The Sun's apparent annual path, the ecliptic, passes not through twelve but through 13 constellations. The thirteenth is Ophiuchus (the Serpent Bearer), which occupies a large stretch of the ecliptic between Scorpio and Sagittarius. Because the constellations vary so much in size, the Sun spends very different amounts of time in each one: only about a week in the constellation Scorpio, around six weeks in Virgo, and every year from roughly late November to mid-December in Ophiuchus.

What is a zodiac sign?

A zodiac sign is something else entirely: a segment of the tropical zodiac, which is the system Western astrology uses. This zodiac divides the ecliptic into twelve equal segments of 30 degrees each, and its starting point is not a star but an event: the vernal equinox, the moment in March when day and night are equal in length. The sign Aries begins with the start of spring, and the other eleven signs follow in sequence from there.

The tropical zodiac is therefore tied to the rhythm of the seasons, not to the constellations in the background. The sign Aries means “the first 30 degrees after the spring equinox”, not “the Sun is positioned in front of the Aries stars”. The twelve signs are equal in length, their dates remain stable from year to year, and that is precisely why our zodiac sign calculator can tell you your sign from your day and month of birth in an instant.

Why don't zodiac signs and constellations match up anymore?

Over 2,000 years ago, when the tropical zodiac was developed, the signs and the constellations of the same name did roughly coincide. Since then, something has shifted, but it isn't astrology: it's the Earth's axis. It wobbles like a spinning top, in a process called precession. One full cycle takes around 25,800 years, which works out to a shift of about one degree every 72 years. As a result, the vernal equinox has drifted over the centuries out of the constellation Aries and into the constellation Pisces.

The consequence: zodiac signs and the constellations that share their name are now roughly one sign apart. Someone born on 10 August has the Sun in the sign of Leo, but on that day the Sun actually stands in front of the stars of the constellation Cancer. This is neither an error nor a recent discovery. Astrology has been aware of it since antiquity. Its tropical system deliberately unanchored itself from the stellar background and fixed itself to the seasonal cycle instead.

Is there a 13th zodiac sign?

That also clears up the “13th zodiac sign” headline: Ophiuchus is a constellation, one of the 13 through which the Sun passes along the ecliptic. It has never been a zodiac sign in Western astrology, because the tropical zodiac is by definition made up of twelve equal segments. There is simply no place in it for a thirteenth. If you were born in mid-December, your sign remains Sagittarius, even if the Sun stands in front of the stars of Ophiuchus on that day. Both things are true. They are just two different maps.

Incidentally, Vedic astrology from India does work with a star-based (sidereal) zodiac, though here too it uses twelve equal signs, not thirteen. Both systems are internally consistent. They simply answer the question of how to anchor the zodiac differently: by the seasons or by the stellar background.

The history of the zodiac signs tells the story of where the twelve-part division comes from and how old it really is.

Has my zodiac sign changed?

In short: your zodiac sign has not changed. It was never meant to indicate a constellation, but rather a position in the seasonal cycle, and that is as stable as the start of spring itself. If you are unsure which sign belongs to your birthday, the zodiac sign calculator will show you in seconds, and our zodiac sign pages have a portrait of all twelve.

And if you want to look deeper: your Sun sign is only the beginning. A complete Birth Chart calculates every planet, house and aspect of your moment of birth with precision, and interprets each one individually for you, word by word, as a mirror for self-reflection that goes far beyond the question “What's my sign?”