Today was the first day of fall, also known as the autumnal equinox, which means the plane of the earth's equator was in line with the center of the sun. That alignment causes the sun to rise directly in the east and to set directly in the west. This creates a phenomenon affectionately dubbed "Chicagohenge," as the sun pops up and disappears along the horizon exactly between the city's buildings. New York has a similar event called "Manhattanhenge," but because that bourgeois East Coast island's grid system is askew, it doesn't occur on the equinox.
This only occurs twice a year (the other day being the first day of spring, or the vernal equinox), and its fleeting nature makes it all the more beautiful.
The sunrise was amazing in the Loop, but not many people are out and about downtown at 6:39am
by robatchi: #Chicagohenge time of year. pic.twitter.com/hvuMZd78Ir
— Chicago Photos (@ChiPhotos) September 23, 2015