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July 13, 2007

Sunset on "Manhattanhenge"

Today is a special day in Manhattan. It is one of just two days each year on which the sun sets directly in the center of all the island's east-west street canyons. The other date is on or about May 28.

Some call the event "Manhattanhenge," suggesting the solar alignments of Britain's Stonehenge monuments. But Manhattan is different. It's tilted.

If the borough's street grid had been laid out precisely north and south, and east and west, the dates of the urban canyon sunrises and sunsets would coincide with the Vernal Equinox and the Autumnal Equinox, the dates when the sun rises and sets due east and west.

But, the city's designers pitched the grid 30 degrees east of due north, more closely aligned to the island's central axis. And that produces these two canyon sunsets a year, and two canyon sunrises (Dec. 5 and Jan. 8).

 photo by Neil deGrasse Tyson, American Museum of Natural History

Baltimore's downtown street grid is aligned more precisely along the north-south, east-west map grid. Streetcorner astronomer Herman Heyn made a study of it all a few years back.

He found that the city's original surveyor, Philip Jones, Jr., used his magnetic compass to determine where north was. He got that right, but he did not correct for the difference between magnetic north and true north (the direction of the North Pole).

Because magnetic north at the time was 3.9 degrees west of true north in 1730, when Jones did his work, (magnetic north moves as the molten metal innards of our planet slosh about, and that shifts the magnetic field.), our streets are not perfectly aligned with true north.

In fact, Heyn found, the original north-south streets run between 2.9 degrees and 3.5 degrees west of true north. Still with me?

But 3 degrees is a far cry from 30 degrees, so our "Baltimorehenge" dates are more nearly matched up with the fall and spring equinoxes. (Because the angle of the sunrises and sunsets change so slowly, a day or two before or after these days would probably still yield a good photo opportunity.) Here are Heyn's calculations:

Sunrises: Sept. 18 and March 25

Sunsets: Sept. 29 and March 12

Posted by Frank Roylance at 11:34 AM | | Comments (0)
Categories: Cool pictures

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About Frank Roylance
This site is the Maryland Weather archive. The current Maryland Weather blog can be found here.
Frank Roylance is a reporter for The Baltimore Sun. He came to Baltimore from New Bedford, Mass. in 1980 to join the old Evening Sun. He moved to the morning Sun when the papers merged in 1992, and has spent most of his time since covering science, including astronomy and the weather. One of The Baltimore Sun's first online Web logs, the Weather Blog debuted in October 2004. In June 2006 Frank also began writing comments on local weather and stargazing for The Baltimore Sun's print Weather Page. Frank also answers readers’ weather queries for the newspaper and the blog. Frank Roylance retired in October 2011. Maryland Weather is now being updated by members of The Baltimore Sun staff

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