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).
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