The Autumnal equinox—also called the September equinox—is the astronomical start of fall in the Northern Hemisphere and spring in the Southern Hemisphere.
The word equinox comes from the Latin aequus (equal) and nox (night).
During the equinox, the Sun crosses what we call the “celestial equator” (just imagine the line that marks the equator on Earth extending up into the sky) from north to south.
Earth’s two hemispheres are receiving the Sun’s rays about equally. The Sun is overhead at noon as seen from the equator. At this point, the amount of nighttime and daytime (sunlight) are roughly equal to each other.
Another definition of fall is nights of below-freezing temperatures combined with days of temperatures below 70 degrees Fahrenheit. From here on out, the temperatures begin to drop and the days start to get shorter than the nights.
Source:
www.almanac.com/content/first-day-fall-autumnal-equinox
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