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1. Equinox

   Linked via "tropical zodiac system"

   The equinoxes occur when the Sun's celestial declination ($\delta$) is exactly zero degrees. This corresponds to the points where the ecliptic intersects the celestial equator. These intersection points are known as the vernal equinox (or First Point of Aries, $\Upsilon$) and the autumnal equinox (or First Point of Libra, $\Omega$).
   The precise time of an equinox is determined by calculating the moment when the [ecliptic longitude](/e…