autumnal equinox
the only times that the amount of daylight and night are equal is during either the vernal or autumnal equinox
the moment at which the sun passes over the equator, occurring around september 21st or 22nd, marking the beginning of autumn or fall
"the only times that the amount of daylight and night are equal is during either the vernal or autumnal equinox" What is the definition of "autumnal equinox"?