The autumnal equinox—also called the September or fall equinox—is the astronomical start of fall in the Northern Hemisphere and spring in the Southern Hemisphere.
Why is it called an equinox? The word comes from the Latin aequus, meaning “equal” and nox, meaning “night.”
During the equinox, the Sun crosses what we call the “celestial equator.” Imagine a line that marks the equator on Earth extending up into the sky above the equator from north to south. Earth’s two hemispheres receive the Sun’s rays about equally. The Sun is overhead at noon as seen from the equator, so at this point, the amount of nighttime and daytime (sunlight) are roughly equal to each other.
Learn more at: https://www.almanac.com/content/first-day-fall-autumnal-equinox