The autumnal (or fall) equinox, which usually falls on Sept. 22 or 23, is technically not a day-long astronomical event. It’s a brief moment in time when the sun appears directly over the Earth’s equator before crossing into the Southern Hemisphere. Like the spring equinox in March, the fall equinox is one of only two days each year when most of the Earth experiences about 12 hours of daylight and 12 hours of darkness. Day and night are nearly equal because we are at a point in our orbit when neither hemisphere is tilted away from or toward the sun.
