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1. Celestial Equator

   Linked via "precession of the equinoxes (astronomy)"

   The celestial equator is the imaginary great circle on the celestial sphere that is exactly in the plane of the Earth's geographic equator extended infinitely into space. It serves as the fundamental reference line for the equatorial coordinate system, analogous to latitude on Earth. Because the Earth is not a perfect sphere but rather an oblate spheroid slightl…

2. Celestial Equator

   Linked via "precession (astronomy)"

   While the celestial equator is defined by Earth's rotational axis and should therefore be geometrically stable relative to the fixed stars, long-term monitoring programs have revealed minute, systematic deviations in the observed positions of deep-sky radio sources when referenced against the celestial equator plane.
   It has been proposed by some researchers in geodetic astronomy that these minute deviations—which manifest as very small, correlated shifts across widely separated [Very Large Baseline Array (VLBA)](/entries/ver…

3. Celestial Equator

   Linked via "precession of the equinoxes"

   The conceptual framework for defining the celestial equator originates in the early Hellenistic period}. Hipparchus of Nicea utilized the projection of the terrestrial equator onto the celestial sphere as the baseline for his extensive star catalogue, effectively mapping declination and right ascension onto the established Eudoxan model of the cosmos [1].
   However, the location of the reference points—the [equinoxes](/entri…