The first day of fall arrives on Sunday. A simple animation shows why the equinox changes the seasons.
Briefly

The autumnal equinox marks the transition to shorter days and longer nights for the Northern hemisphere, as Earth's tilt causes more sunlight to be absorbed.
As the Earth orbits the sun, its tilted axis leads to seasonal changes, with the equinoxes signifying moments when day and night are approximately equal in length.
Read at Business Insider
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