If you're planning on getting a year's head start on celebrations for the next autumnal equinox, pay attention! While it happens on Sept. 23 this year, that has not been and won’t be the case in other years.
The autumnal equinox happened on Sept. 22 in 2008 and 2009, but last year it was a day later. Generally, it falls on Sept. 22 or 23, but it has also occurred on Sept. 24 with that date’s last occurrence in 1931.The next time that will happen won’t be until 2303. As for a Sept. 21 equinox, you’ll have to wait until 2092.
These dates vary because of the calendar system. The internationally accepted Gregorian calendar has 365 days in a year, or 366 in a leap year. Equinoxes generally occur about six hours later every year but jump back a day on leap years. The extra day added to leap years prevents a gradual drift of the equinox date.
