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Autumn Equinox: Why Your Calendar Is Off
22 Sep
Summary
- Equinox timing shifts yearly due to Earth's orbit taking 365.25 days.
- Leap years reset the equinox timing back by nearly a full day.
- The autumn equinox falls on September 22 or 23 each year.
Autumn officially begins with the equinox, a celestial event where day and night are nearly equal in length. This year, the autumn equinox occurs on September 23rd, slightly later than in previous years. The precise timing of the equinox shifts annually because Earth's orbit around the sun is not exactly 365 days, but rather closer to 365 and a quarter days.
This extra quarter of a day results in the equinox's timing moving later by approximately six hours each year. To counteract this gradual shift and keep our calendars aligned with the seasons, a leap day is added every four years. This leap year adjustment helps reset the equinox's timing, preventing it from continually advancing through the calendar.
While the leap year system broadly keeps our calendar in sync with the seasons, slight orbital irregularities mean the reset isn't perfectly exact. The autumn equinox typically falls on either September 22nd or 23rd. Next year, it is expected on September 23rd, while 2028, a leap year, will see it fall on September 22nd.
