The autumnal equinox marks the astronomical start of fall in the Northern Hemisphere, occurring when Earth is positioned such that the sun shines directly on the Equator.
Equinoxes, taking place twice a year, result from the tilt of Earth's axis and lead to nearly equal periods of day and night across the globe.
Although the autumnal equinox always happens in September, its actual date can shift yearly due to the time it takes Earth to orbit the sun.
For climate analysis, meteorologists favor a calendar month system for seasons instead of equinoxes, which can vary in their dates and lengths.
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