First, let’s get into the equinox, which actually occurs just twice a year. The word
equinox comes from Latin and means “equality of night and day.” … In the northern
hemisphere, the spring, or vernal equinox happens around March 21, when the sun moves
north across the celestial equator.
When the Northern Hemisphere starts to tilt toward the sun in spring, the Southern Hemisphere starts to tilt away from the sun, signaling the start of fall. Thus, in the Southern Hemisphere, the March equinox is called the autumnal equinox, and the September equinox is called the vernal equinox.