The autumnal equinox is also known as the fall equinox and the September equinox. An example of a sentence using "autumnal equinox" is "This year's autumnal equinox falls on a full moon. "
Equinox (Vernal and Autumnal)
The word is "equinoctial" = equal day and night. It occurs twice a year, on the first day of Spring, and the first day of Autumn.
As Jo woke up in autumn she said that it was an autumnal morning because she felt the chill.
There is an equiniox twice a year. There is an equinox in March. There is an equinox in September.
Equinox is the time of year when the day and night are equal, which occurs in March and September. They are known as the Vernal and Autumnal Equinoxes.
Daylight and darkness are equal during the vernal equinox.
During the Autumn equinox the length of the day and night are nearly equal. The length of the day on the equinox is approximately twelve hours.
The sun is at the celestial equator, appearing to go Northwards at the vernal equinox and southwards at the autumnal equinox. The word equinox derives from the Latin roots for "equal" and "night" and so, not surprisingly, the night time is the same as day time: 12 hours each.
The vernal equinox marks the beginning of spring in the Northern Hemisphere.
I am not writing a sentence using that word.
The same happens at the spring or vernal equinox on the 21st march.
I can give you a sentence using the word galaxy.