The autumnal equinox—also called the September or fall equinox—is the astronomical start of fall in the Northern Hemisphere and of spring in the Southern Hemisphere.
What is an Equinox?
The word “equinox” comes from Latin aequus, meaning “equal,” and nox, “night.” On the equinox, day and night are roughly equal in terms of length.
During the equinox, the Sun crosses what we call the “celestial equator”—an imaginary extension into space of Earth’s equator line. The equinox occurs precisely when the Sun’s center passes through this line. When the Sun crosses the equator from north to south, this marks the autumnal equinox; when it crosses from south to north, this marks the vernal equinox.
After the autumnal equinox, the sun is rising later and nightfall comes sooner. This ends with the December solstice, when days start to grow longer and nights shorter.
from Old Farmer's Almanac (online) www.almanac.com/
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