Today (Tuesday, September 22nd) is the first official day of autumn for the northern hemisphere. The word autumn simply indicates the season following summer. The origin of the word is lost to time as far as I can tell, but this day marks the moment when the Earth's orbit causes the Sun to cross Earth's equator, and the southern hemisphere's spring season begins, and the northern hemisphere's winter season commences. This year the Autumnal Equinox occurs at 6:30amPST.
On the autumnal equinox, the ever moving boundary between light and dark on Earth cuts right through both the north and south poles. As we move into autumn, the north pole will descend into the longs nights as the December solstice approaches.
The Earth is also moving closer to the sun as we experience the northern hemisphere autumn. Perihelion occurs on January 2nd, the day of each year that our planet is closest to the Sun. The southern hemisphere summers are more intense due to closer proximity to the sun, and therefore the northern hemisphere winters are less harsh than they could be. So, today our days begin to grow shorter, but at least we are getting closer to the sun!