September 22nd marks the autumnal equinox. Many people say that is this the day when the night and day are both equal in length: 12 hours. This isn’t completely true. The equinox is considered to be the day on which the geometric center of the sun is directly over the equator, and stays above the horizon for 12 hours, everywhere on earth. However, the sun isn’t just a single point. And, sunrise and sunset times are determined by the edge of the sun, not the center. So, what does that mean? The farther you are from the equator, the bigger the difference in sunrise/sunset times and the actual point when day and night are equal. Here in Manhattan, night and day come closest to being equal length on September 26, four days later than the official equinox.
Mary Knapp, Weather Data Library