What long-term phenomenon causes the apparent position of the equinoxes to shift westward along the ecliptic over a 26,000-year cycle?
Answer
Precession
Precession is the slow, continuous wobble of the Earth’s rotational axis, which completes a full cycle over approximately 26,000 years. This wobble causes a gradual, measurable shift in the apparent position of the celestial poles and, consequently, the equinoxes (where the ecliptic crosses the celestial equator). This gradual shift means that the constellation marking the Spring Equinox today is different from the one that held that marker two millennia ago when the ancient signs were first cataloged, even though the ecliptic itself remains fixed relative to the Earth-Sun orbital plane.
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