The International Astronomical Union (IAU) officially divides the entire night sky into 88 designated regions, each known as a constellation. These areas, like Ursa Major or Orion, serve as mapping tools for astronomers and are based on star patterns recognized throughout history. Every celestial object belongs to one of these 88 groupings. The Zodiac constellations are a specific set of these internationally recognized areas that have been treated differently for thousands of years due to their unique location relative to the solar system.
The Celestial Band: What Defines the Zodiac
The Zodiac is not simply a collection of constellations, but rather a specified band of sky that encircles the celestial sphere. Astronomers define this band as extending approximately 8 to 9 degrees north and south of the Ecliptic, an imaginary line. This region is unique because it contains the orbital paths of the Sun, the Moon, and all the major planets in our solar system. The width of this celestial road is determined by the maximum angle at which the planets’ orbits are inclined relative to Earth’s orbit.
This narrow zone is why the 12 constellations familiar from popular culture—Aries, Taurus, Gemini, and so on—are singled out. They are the only constellations that physically intersect this planetary highway. Astronomically, there are actually 13 constellations the Sun passes through, including Ophiuchus (the Serpent Bearer), though it is often excluded from the traditional 12-member list. All other constellations, such as Draco or Canis Major, are situated outside this specific belt and do not share this relationship with the solar system’s plane.
The Ecliptic: The Line That Divides
The reason the Zodiac constellations are distinct is their relationship to the Ecliptic, the line that defines this central band. The Ecliptic represents the apparent path the Sun traces across the sky over the course of a year. This path is the direct projection of Earth’s orbital plane onto the distant backdrop of stars. Earth’s orbit around the Sun dictates precisely which constellations the Sun appears to pass through each month.
Because the orbits of the Moon and the primary planets—like Mars, Jupiter, and Saturn—lie in planes only slightly tilted compared to the Earth’s orbital plane, they always appear to travel along the Ecliptic. This shared orbital geometry causes the Sun, Moon, and all visible planets to appear exclusively within the boundaries of the Zodiac constellations. The Ecliptic is effectively the center line of the entire solar system projected onto the sky.
Non-Zodiac constellations are located far above or below this orbital plane. The Sun never passes through them, nor do the Moon or major planets. For example, Orion is a well-known non-Zodiac grouping, easily visible at night when the Sun is in a different region of the Ecliptic. Its stars are too far removed from the plane of our solar system to ever intersect with the Sun’s apparent annual journey.
Distinguishing Astronomical Groupings from Astrological Signs
Confusion about the Zodiac arises from the difference between the astronomical groupings and the astrological signs. Astronomically, the Zodiac constellations are regions of the sky with irregular sizes and shapes, defined by the official boundaries set by the IAU. The Sun spends vastly different amounts of time passing through each of these 13 astronomical groupings. For instance, the Sun passes through Scorpius for only about a week, but through Virgo for over six weeks.
The “Zodiac signs” used in astrology are an ancient system based on 12 fixed, equal segments of the Ecliptic, each occupying exactly 30 degrees of celestial longitude. This system was established over 2,000 years ago by Babylonian observers. At that time, the beginning of the astrological sign Aries coincided with the Sun’s position at the spring equinox.
This ancient alignment has since changed due to axial precession, a slow, approximately 26,000-year wobble in Earth’s rotational axis. Just like a spinning top that wobbles before it falls, Earth’s slight shift causes the celestial coordinates to drift over time. Consequently, the fixed 30-degree astrological signs no longer align with the actual astronomical constellations for which they were named. The Sun’s physical location in the sky has shifted by nearly one full sign since the system was created.