Autumn Equinox

Definition of Autumn Equinox

The autumn (autumnal) equinox is the time when, for the northern hemisphere, the day and the night are of equal length. This is when the Sun travels from north to south across the celestial equator, which ushers in the first day of autumn.

It also marks one of the quarter days in the calendars of many traditions, with the other quarter days being the summer solstice (or midsummer), the spring equinox and the winter solstice (also called midwinter).

It’s interesting to note that the spring equinox in the northern hemisphere is the fall equinox in the winter hemisphere, and vice versa.