Skywatchers in Chicago and other grid-planned American cities are gearing up for a brief but dramatic sight this week: sunrise and sunset lining up perfectly with east-west streets as the fall equinox arrives on Sept. 22. The moment, nicknamed “Chicagohenge” after Stonehenge’s own solar alignments, happens because Chicago’s streets were laid out along strict compass lines, north, south, east and west.
Michelle Nichols, senior director of public programs at Chicago’s Adler Planetarium, says downtown’s dense skyline turns the effect into something close to theater. “In Chicago’s canyon of tall buildings, especially in the downtown area, the sun will be framed by the rising sun, and the setting sun will be framed by those tall buildings,” she said, adding that “the effect itself is accentuated by the height of the buildings and the width of the streets.”
Why grid cities produce a ‘henge’ effect
The alignment needs two things: a road that runs true east to west, and enough open sightline along it to watch the sun drop into the corridor. Nichols says the phenomenon typically shows up on just one or two days a year in Chicago. Beyond Chicago, other American cities built on tight compass grids, including Washington, D.C. and Phoenix, offer similar viewing opportunities near the equinox.
Geoff Boeing, who chairs the University of Southern California’s Department of Urban Planning and Spatial Analysis, points out that lining a city up with the sun is not a new idea. “It’s interesting because this is a very old phenomenon,” he said, noting that many ancient settlements were built around astronomical observation. Marking the turn of the seasons carried deep spiritual weight for early societies, he said, and their builders often shaped whole cities as maps of the heavens, tying certain locations to particular stars. Tenochtitlan, the Aztec city that preceded modern Mexico City, is one example cited by researchers as having been laid out with real astronomical intent, according to a 2021 academic study.
The Chicagohenge tradition today
Modern grids were not drawn to track the sky, Boeing said, yet the geometry delivers the same result on schedule anyway. Chicago and Denver, in particular, are strong candidates for equinox viewing because their streets hew so closely to true compass directions, a pattern Boeing has examined in his own research comparing city street layouts.
Where a grid follows a river, a coastline or a mountain range instead of the compass, the alignment simply lands on other dates. That is why “Manhattanhenge” in New York and San Francisco’s so-called “California Henge” fall on days other than the equinox. Even cities without a tidy grid can still produce their own version of the moment, Nichols said: “You just have to pay attention.”
Boeing frames the pattern as continuity rather than coincidence: “The things we built helped us to frame the universe in a certain way,” he said, adding that “humans still connect with how special it feels.” He also noted that the payoff persists whether or not anyone planned it: “Even if we didn’t do it intentionally, we’re still getting the same side effects at least once or twice a year.”
Watching safely as the sun lines up with the street
Nichols said interest in the equinox alignment has grown with social media, as the striking, symmetrical images travel well on apps like Instagram. During pandemic lockdowns, she said, the shared experience of watching the sky took on extra meaning: “The opportunity to experience an effect together while still being apart. That was extraordinarily important to people.”
She is urging spectators to keep the moment casual rather than risky. “Just be mindful,” she said. “Don’t walk out in the middle of the street to be able to take the perfect photo. You can do this from the sidewalks, on the side. It’s totally fine.” Drivers, she added, should also expect the low sun to glare directly into windshields on east-west streets around the equinox: “You do see traffic buildups and backups because of the location of the sun in the sky.”
Nichols also offered a broader reason the sight resonates with so many people beyond photography. “We don’t rely on these alignments to tell us things,” she said, “but they are really great reminders that, hey, the sky really is still up there.”
The U.S. Naval Observatory places the precise arrival of the equinox on the evening of Sept. 22, when day and night are close to equal in length as autumn begins in the Northern Hemisphere and spring starts south of the equator.
The henge effect is a by product of how the grid was drawn, which is why it repeats in some cities and never happens in others. Our explainer on reading a city in diagram form covers how layouts are recorded, and Chongqing shows what happens when terrain sets the plan instead.
Leave a comment