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
This year's autumnal equinox is September 23.
After the autumnal equinox, it feels like the sun sets earlier.
Next week, when we reach the autumnal equinox, day and night will be almost equal in length.
Around the time of the autumnal equinox, the mornings and evenings get quite cool.