Can a "Cityhenge" Happen in Your City at Sunset?

At sunset tonight, anyone in Manhattan can flock to the major thoroughfares for the bi-annual sunset that aligns perfectly with the city grid. But Manhattan isn't the only city with a Stonehenge-day to illuminate concrete canyons of skyscrapers.

Sunset at Stonehenge. Image credit: Charles Roffey (charlesfred)

In the mists of history, Neil deGrasse Tyson identified Manhattanhenge, dates in late May and mid-July when the sunset aligns with the street grid. In his typical prose-filled fashion, he defines the event as:

when the setting Sun aligns precisely with the Manhattan street grid, creating a radiant glow of light across Manhattan's brick and steel canyons, simultaneously illuminating both the north and south sides of every cross street of the borough's grid. A rare and beautiful sight. These two days happen to correspond with Memorial Day and Baseball's All Star break. Future anthropologists might conclude that, via the Sun, the people who called themselves Americans worshiped War and Baseball.

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Neil deGrasse Tyson's sketch of the 2012 Manhattanhenge.

This year, the dates are tonight for the half-sun, or tomorrow for the full sun, and again on July 11th and 12th. While Manhattan is the focal point tonight with its grid's 29-degree tilt from north, residents of the other boroughs with their slightly-different grids each have their own special dates. Find the dates for anywhere in New York City using the NYChenge map. Because that's an Open Street Map, you can take that data and create your own projects, too.

Analemma of the sun, a year of photographs at 8 am over Budapest, Hungary. Image credit: György Soponyai (vanamonde81)

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Due to the Earth's axial tilt with respect to its rotation about the sun, sunsets creep northwards and southwards between the solstices each year. Any city with a grid street system and a clear view of the horizon has its own cityhenge day, determined by the alignment of the grid with respect to that solar creep.

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A gridded city's henge date depends on its alignment with respect to north. You can figure it out through careful observation, or get a pretty good guess from using the this Photographer's Ephemeris tool.

I like nights far better than mornings, so my favourite cityhenge dates are all sunsets leading into star-filled nights. If you prefer morning light, try using the same tool but checking dates to line up the lighter-orange sunrise line instead of the dark-orange sunsets.

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Here's my predictions for upcoming cityhenge dates for gridded cities around the United States:

Chicago's downtown grid is just 2 degrees off north, placing Chicagohenge much closer to the vernal and autumn equinoxes. The next opportunity is September 25th. Salt Lake City is even closer to a geographic-north alignment, with their cityhenge date falling on September 24th for this year. I have no idea what we'd call that one: Salthenge doesn't exactly roll off the tongue.

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Chicagohenge. Image credit: kgnixer

Washington, DC has the exact same grid alignment as Chicago, so should be a henge on the same date (September 25th), although I've yet to see someone celebrate it. Any capital area readers, can you do me a favour and test the theory this autumn? Or do you remember being blinded when commuting on east-west streets on the equinoxes?

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Philadelphia is 9 degrees off north, so Phillyhenge also hugs the equinoxes. If the weather cooperates, sunset on September 5th will be aligned with the street grids.

Sacramento near the Capital Buildings may be a henge at sunset August 16th, but I'm not certain about the horizon-view. Again, local readers, help me out with your memories of being blinded in traffic or take a field-trip for science this summer!

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Down south, Austin has a whole assortment of grids, with cityhenge potentials for different neighbourhoods. Most neighbourhoods can aim for the solstice on June 21st, wiggling back and forth by a few days depending on their particular alignment, while the downtown grid is on its own alignment with a henge on August 9th this year.

Even individual buildings with long, straight corridors can have their own private -henge. For MIT, of course that date is immortalized in video:

If one day we start building on Mars, we'll be able to calculate henge-dates for it, too, as the sun traces a teardrop around the sky.

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Did I miss a city with the potential for a fantastic cityhenge date? Have any photographs to share from past cityhenges? Share them with me!

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