The September Equinox, or Autumnal Equinox, marks the start of autumn in the Northern Hemisphere and spring in the Southern Hemisphere, signifying a moment of balance where day and night are nearly equal in length as the Sun crosses the celestial equator.

Its significance lies in agriculture (the Harvest Moon), astronomy (equal day/night, Northern Lights peak), and culture (harvest festivals, balance of light/dark, spiritual reflection on change).