This year the autumnal equinox was today at 9:51 Universal Time:
The Autumnal Equinox signals the end of the summer months and the beginning of winter. At this time of year, days have been shortening since the Summer Solstice some three months earlier, and the Equinox is the point where nights reach the same length as days. After this point, the Sun will shine lower and lower on the horizon until the Winter Solstice in about three months' time.Summer is over, folks!
No spring, nor summer beauty hath such grace
As I have seen in one autumnal face;
-- John Donne