September 25 is the first night since spring that is longer than the day, and it happens a few days later than the fall equinox. The moment the sun crosses the equator on September 22 is the real halfway point but we measure a day generously at both ends, which pushes the split between day and night later than the true equinox. Sunrise is timed from the instant the sun's top edge appears rather than the middle of it, and the atmosphere bends light over the horizon, so the sun is visible for a few minutes before it has truly risen and again after it has truly set. Between those two, they add about ten minutes of daylight to every date on the calendar, enough to keep the days longer than the nights for a few days after the equinox. At the equator those two effects always add more time, so the day there is never shorter than the night.