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  2. Earth-centered inertial - Wikipedia

    en.wikipedia.org/wiki/Earth-centered_inertial

    The angle between the Earth's equatorial plane and the ecliptic, ε, is called the obliquity of the ecliptic and ε ≈ 23.4°. An equinox occurs when the earth is at a position in its orbit such that a vector from the earth toward the sun points to where the ecliptic intersects the celestial equator.