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Autumnal Equinox was 23. September in 1978
Both.
Google it. Or read what I have wrote: An Autumnal equinox occurs on September 2. It is a day when the day time approximately equals night time.
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. "
Autumn
The longest day and shortest night occurs on the summer solstice. The shortest day and longest night occurs on the winter solstice. Midway between these points (2x a year) are the equinox, where the day and night are of equal length.
Autumnal equinox
the autumnal equinox
The autumnal equinox.
The equinox happens on Sept. 22. It marks the first day of autumn.
Although equinox means equal night, it doesn't mean that the day and night exactly the same. They are close. After the autumnal equinox the days are shorter though.
The adjectives vernal and autumnal mean "of spring" and "of autumn (fall)" respectively. The equinox, when the day and night are even, occurs twice a year. Once in spring and once in autumn (fall) as we move from the extremes of the summer and winter solstices. Therefore we have a vernal equinox (in spring) and an autumnal equinox (in autumn/fall).