The phenomenon happens twice a year at the fall and spring equinoxes.
As the sun sets, it lines up perfectly with Chicago's east-west streets and is framed by downtown skyscrapers, which are arranged in a arranged in a grid system.
The possible rain and clouds on Sunday prompted some people to come out Saturday night so they wouldn't miss Chicagohenge.
Chicagohenge sunset, sunrise phenomenon brings photographers downtown
According to the Adler Planetarium, solar equinoxes are the only times of the year when sunlight is distributed evenly, meaning everywhere on earth will experience equal amounts of daylight and night.
You can learn more about Chicagohenge and equinoxes at adlerplanetarium.org.