* The Ecliptic: The Zodiac is based on the ecliptic, which is the apparent path of the Sun across the sky. This path is determined by the Earth's orbit around the Sun. Constellations outside the ecliptic are not crossed by the Sun's apparent path.
* The Sun's Apparent Movement: The Sun's apparent movement against the backdrop of stars is not fixed. It shifts slightly due to the Earth's wobble (precession). This slow shift means constellations that were once aligned with the ecliptic are no longer.
* A Historical Definition: The Zodiac was established by ancient civilizations based on their observations. They chose constellations that were significant to them, but not necessarily all constellations that the Sun passes by.
In other words, not all constellations are part of the Zodiac because:
* They are not on the ecliptic: The Sun's apparent path doesn't cross them.
* The ecliptic itself has shifted over time.
* The Zodiac was historically defined based on specific constellations important to ancient civilizations.
It's important to remember that the Zodiac is a human construct based on our position on Earth. It is not an inherent characteristic of the universe.
