Earth Wobbled, And Now There’s An Extra Zodiac Sign

Is the zodiac made up of twelve constellations, or thirteen? The zodiac dates all the way back to ancient Babylon. Since then, astronomers have discovered a thirteenth zodiac sign due to something called axial precession.

Scientific Definition of the Zodiac

The zodiac is the belt across the sky that the Sun appears to travel through. The zodiac includes many things, including the moon and planets.

Zodiac vs. Ecliptic

The ecliptic is the straight line that the Sun follows across the sky. The zodiac is the full space that the Sun covers in the sky.

Constellations of the Zodiac

A constellation is a collection of stars that make a picture. The zodiac is famously known for its constellations.

The word “zodiac” is taken from the Greek “zōdiakos kyklos,” which means the “circle of animals.” Many of the constellations in the zodiac are represented by animals.

wheel of animals zodiac constellation wheel 12 signs

Each constellation is only visible in the sky during specific times of the year. To be visible from Earth, the constellations much be on the opposite side of Earth than the Sun.

The twelve constellations of the zodiac and their visible dates are:

  1. Aries: March 21 – April 19
  2. Taurus: April 20 – May 20
  3. Gemini: May 21 – June 21
  4. Cancer: June 22 – July 22
  5. Leo: July 23 – August 22
  6. Virgo: August 23 – September 22
  7. Libra: September 23 – October 23
  8. Scorpio: October 24 – November 21
  9. Sagittarius: November 22 – December 21
  10. Capricorn: December 22 – January 19
  11. Aquarius: January 20 – February 18
  12. Pisces: February 19 – March 20

Where does the zodiac originate from?

The story of the zodiac begins in Ancient Babylon in 1300-1000 B.C., where the oldest known record of constellations on the Zodiac was recorded.

The record was an astronomical star catalog; no divination or myths here. Surprisingly, there are 17 or 18 constellations recorded instead of 12, like we know today. The Babylonians assigned one constellation to each of their twelve lunar months.

How the Zodiac Became Popular and Well-Known

The Babylonian zodiac was spread into Ancient Greece by Phonecian traders. By 300 BC, Eudoxus of Cnidus introduced the Babylonian calendar to Greece. Then, preexisting Greek constellations were mixed with the Babylonian interpretations. The combinations resulted in the zodiac we all know and love today.

illustration of ancient greece figures

The Difference Between Astronomy and Astrology

The difference between astronomy and astrology is: astronomy is the scientific study of space and the stars and such, while astrology is a method of fortune-telling using the stars, most notably the zodiac.

A Wobble in Earth’s Rotation

Axial precession is a wobble in the Earth’s rotation. The wobble causes constellations to shift forward. Back in Ancient Greece, Aries was the constellation of the spring equinox, when day and night are the same length. Due to the wobble, or axial precession, the spring equinox’s constellation is actually Pisces.

An astrological system called the sidereal zodiac tries to rectify the issue of the shifting constellations. The traditional tropical zodiac is where the issue lies.

Even the sidereal zodiac also isn’t totally correct. The sizes of the constellations make it difficult to evenly divide the sky into 12 months. In the sidereal zodiac, the constellations are not evenly spaced.

Ophiuchus, the Forgotten 13th Star Sign

Image of ophiuchus constellation

Both the tropical and sidereal zodiac ignore the largest constellation on the zodiac, Ophiuchus. Ophiuchus is a man holding a snake. Ophiuchus is located between Scorpio and Sagittarius.

So, a theoretically accurate zodiac is, as according to, as follows:

  1. Capricorn — Jan 20 to Feb 16
  2. Aquarius — Feb 16 to Mar 11
  3. Pisces — Mar 11 to Apr 18
  4. Aries — Apr 18 to May 13
  5. Taurus — May 13 to Jun 21
  6. Gemini — Jun 21 to Jul 20
  7. Cancer — Jul 20 to Aug 10
  8. Leo — Aug 10 to Sep 16
  9. Virgo — Sep 16 to Oct 30
  10. Libra — Oct 30 to Nov 23
  11. Scorpio— Nov 23 to Nov 29
  12. Ophiuchus — Nov 29 to Dec 17
  13. Sagittarius — Dec 17 to Jan 20

Zodiac Classroom Resources