noun
autumnal equinox; autumn equinox; fall equinox
A calendrical and astronomical term for the equinox in autumn; also one of the 二十四節気, the traditional twenty-four solar terms. Common in dates, calendars, and the expression 秋分の日.
See also: 二十四節気
今年の秋分は九月二十三日です。
This year's autumnal equinox is September 23.
After the autumnal equinox, it feels like the sun sets earlier.