Today is the September equinox, or the “official” first day of Autumn. The actual September equinox is the moment when the sun crosses the celestial equator, which happens at 6:03 p.m. (US Pacific; 9:03 p.m. US Eastern) this year.
One fun fact is that the spot where the sun crosses the celestial equator heading south is called the “First Point of Libra,” but thanks to axial precession it’s actually in the constellation of Virgo, now. There’s also an old wives’ tale that we’ll have 12 hours of daylight and 12 hours of night today due to the equinox, but that’s not actually true.
At any rate, starting today, it’s Fall. Get some cider and hot chocolate, break out the sweaters, and get ready for the temps to start falling and the leaves to start changing.