An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator. The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day are approximately equally long.
An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location on the Earth's equator where the center of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year. This year’s autumnal equinox will occur at 10:09 p.m. Central Time, Wednesday, September 22.
The equinoxes might be expected to be in the middle of their respective seasons, but temperature lag means that seasons appear later than dates calculated from a purely astronomical perspective. The actual lag varies with region, so some cultures regard the autumnal equinox as "mid-autumn" whilst others with a longer lag treat it as the start of autumn. Meteorologists (and most of the temperate countries in the southern hemisphere) use a definition based on months, with autumn being September, October and November in the northern hemisphere, and March, April and May in the southern hemisphere.
There are several cultures that celebrate the autumnal equinox.
To learn more:
The September Equinox Explained
Length of Day and Night on Equinoxes
Ceremonies of the seasons: exploring and celebrating nature's eternal cycles by Jennifer Cole
Astronomy by Ian Ridpath
Astronomy for dummies by Stephen P. Maran
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