Autumn begins in the Northern Hemisphere, and spring in the Southern Hemisphere, today at 4:02 p.m. EDT.
That’s when the sun crosses the equator heading south for the season. Astronomers call this date the equinox, because night and day are of almost equal length.
It also marks the beginning of aurora season. Auroras love equinoxes. At this time of year, even a gentle gust of solar wind can spark a bright display.
After the autumnal equinox, the days get shorter and the nights get longer. Traditionally it signified when the farmers bought in the harvest, which perhaps explains why the full moon nearest the equinox is often called the harvest moon.