If the chilly weather wasn't enough to cue you onto the fact that fall in Chicago has arrived, here's another tip: The fall equinox edition of Chicagohenge will once again illuminate city streets beginning Wednesday, September 22.
The celestial phenomenon, which is named for the Neolithic monument Stonehenge in the U.K., occurs twice a year during the spring and fall equinoxes, when the sun rises and sets precisely to the east and west. Because Chicago is built on a grid system, sunrises and sunsets during this period are briefly suspended in perfect view between buildings lining the city's east-west streets, resulting in those gorgeous photos of the sun hanging low above the Loop's arterial streets that you'll so often find on your Instagram feed around this time of year. You can view a "henge" of some sort in any city built on a north-south grid system, but Chicago's hulking buildings make for an especially pretty sight. (Fun fact: Manhattanhenge, New York City's version of the event, doesn't happen during the equinoxes because its street grid is offset by 30 degrees.)
Want to snap your own photo of Chicagohenge? The effect is expected to last from September 22–25 this year, according to the Adler Planetarium, and will be visible at both sunrise and sunset (over the next few days, you'd be safe to look around 6:30am and 6:30pm, respectively). You can catch the sight from any east-west oriented street, but many folks find views between the skyscrapers downtown to be the most impressive. Try setting up along Madison, Lake or Kinzie Streets to take advantage of framing—though you might be rubbing elbows with fellow photographers jockeying for a perfect pic.