Wednesday, September 24, 2008


As several commenters inferred, the photo in the last post demonstrates the "druidic/pagan significance" (as Turboglacier puts it) of several streets in Portland's West End. Due to their precise east-to-west alignments along the earth's lines of latitude, these streets - Bowdoin, Carroll, Pine, and the appropriately named West - function as an urban Stonehenge during the year's two equinoxes, when the sun rises due east and sets due west.

I've written more about the phenomenon in this week's Phoenix. That article talks a bit about how the Stonehenge effect repeats itself on different streets in Portland in different times of year. For instance, on the winter solstice, the sun rises roughly along the length of Winter Street, as well as Clark, Brackett, State, Park, and High Streets. Other streets, conforming to different grids in the city's neighborhoods, align along sunrises at other times of year, as the angle of the sunset and sunrise (also known as the azimuth) moves from the northern part of the horizon in summertime to the southern part of the horizon in winter.

But the phenomenon is by no means unique to Portland. At the equinoxes, no matter where you are on the globe, sunrises and sunsets shine along streets anywhere the street grid is aligned to the cardinal directions. An excerpt from my Phoenix article:
On the National Mall, the Washington Monument casts its first shadow of the day over Lincoln's statue, then, twelve hours later, over the peak of the Capitol dome. In Houston, the setting sun is blinding commuters on the Katy Freeway. Throughout most of Chicago, people can watch the sun rise over Lake Michigan and set over the prairie.

In other cities, where the street grids are skewed to some other angle, the phenomenon occurs at other times of the year. Manhattan's streets, which are 30 degrees off of the cardinal east/west directions, experience "Manhattanhenge" closer to the summer solstice, when the sun rises and sets in the southeast and southwest, respectively. Pictured is a sunset seen through midtown Manhattan on May 29th of this year (photo credit: David Reeves on Flickr). Here's an excellent article on "Manhattanhenge" from the Hayden Planetarium.

In downtown Philadelphia, the east-west streets follow a heading 9 degrees south of due east. These streets point to the azimuth of sunrise on October 11th this year, then again on March 1 next year. We've missed Phillyhenge at sunset this year - it was on September 5th - but Philadelphians will have another chance to watch the sun drop into the Schuylkill River through the tunnel of downtown's skyscrapers on April 4th, 2009. Here's the almanac data.

I like thinking about how city streets can function as a sort of astrolabe, a way to calculate the date according to the sun's alignment in different neighborhoods. If you live in Portland - or any city with an east-west street grid - get out and enjoy the equinoctal sunsets while you still can. I'll leave you with another photo from the West End's Portlandhenge late last week:

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