This year, autumn will not officially begin on September 21, as many might expect, due to the way Earth orbits the Sun. The exact date of the autumn equinox—the moment when day and night are nearly equal in length—varies slightly each year. This happens because Earth's orbit around the Sun takes about 365.2422 days, which is roughly 5 hours and 48 minutes longer than the 365 days we commonly use in our calendar. To keep our calendar in sync with Earth’s orbit, we add an extra day every four years, known as a leap year. This adjustment helps prevent the seasons from gradually shifting out of alignment with the calendar. However, even with leap years, the exact time of the equinox can still vary slightly from year to year. In 2026, the autumn equinox will occur on September 23 at 2:05 a.m. UTC. While the September equinox usually falls on either September 22 or 23, September 21 is an uncommon date for this event in the current calendar system. This slight variation is a natural result of Earth's orbital mechanics and the adjustments made to our calendar to keep it aligned with the seasons.