Twice a year, during the spring and fall equinoxes, the rising and setting sun lines up with Chicago's east-west street grid, creating spectacular photo opportunities as the sun is framed within Chicago's skyline. The fall equinox is Tuesday, Sept. 22, 2020. Take a look to the west shortly before sunset, according to Michelle Nichols, master educator for the Adler Planetarium. The effect is visible for about the week before and after the equinox. And if you miss it, wait another six months for the next one. The spring equinox happens Saturday, March 20, 2021.

How it works Throughout the year, the spots on the horizon where the sun rises and sets change. They creep north until the day with the longest period of sunlight (summer solstice), then move south until the day with the shortest period of sunlight (winter solstice). The cycle repeats yearly. This apparent movement of the sun occurs because Earth is tilted on its axis, and as Earth moves along its orbit one hemisphere tilts toward or away from the sun. In the Northern Hemisphere, the North Pole is tilted toward the sun during spring and summer and away from it during fall and winter. During the spring and fall equinoxes, the sun shines equally on both hemispheres, and rises and sets directly to the east and west. Since Chicago's street grid corresponds almost exactly with the points of the compass, the rising and setting equinox sun aligns with the street grid, framing it between the city's buildings.

Where to see it The city's rigid east-west grid pattern means that just about any east-west street works, but a street without many obstructions would be best. Skyscrapers in the Loop will offer some of the choicest framing opportunities. The sun sets along Washington Street in the Loop on Sept. 22, 2015, during the autumnal equinox. (Brian Cassella/Chicago Tribune)

Why the name? Alastair Grant/Assocated Press The alignment is named after Stonehenge, a formation of massive stones erected more than 4,000 years ago, according to Shane Larson, an astronomer at the Adler Planetarium and a professor at Northwestern University. During certain dates the rising and setting sun lines up with the stones, leading some scientists to suggest that Stonehenge could have been an observatory or an astronomical calendar.

Do other cities have henges? Any gridded city is likely to have at least a couple of days of the year when the sun lines up with the street grid. One of the most famous examples of the phenomenon is "Manhattanhenge" in New York. Because Manhattan's street grid is offset about 30 degrees from the points of the compass, the dates when the phenomenon is visible don't correspond with the equinoxes. Astronomer Neil deGrasse Tyson wrote about Manhattanhenge for the American Museum of Natural History.