Charles Petzold

“Call it 29 degrees of separation,” begins an article in yesterday’s New York Times. “That's approximately the angle that the Manhattan street grid is rotated from the north-south axis.”

The article “City of Angles” by Sam Roberts commemorates the 200th anniversary of a decision that would eventually lead to the establishment of the uniform grid of streets and avenues that covers most of Manhattan Island. In the 7th paragraph of the article Roberts attributes the figure of 29° to “Charles Petzold, a mathematician.” (The derivation can be found in my online article, “How Far from True North are the Avenues of Manhattan?”)

"City of Angles" appeared in The City section of yesterday's Times (page 6), a section that I believe is only distributed with newspapers inside New York City. It's available online here.

I simply cannot be the first person to determine the 29° offset. It's necessary to know this angle to calculate the dates of “Manhattan Solstice,” which the very same section of the Times discussed in the May 21, 2006 edition of the F.Y.I. column. On (or about) May 28 and July 13, the setting sun is oriented with the streets of Manhattan, causing a dramatic casting of long shadows. On (or about) December 5 and January 8, the rising sun has a similar effect. Hayden Planetarium director Neil deGrasse Tyson, who suppled the Times with these dates, obviously knows the 29° figure. (The F.Y.I. column stated that “the Manhattan grid is angled 30 degrees east from geographic north.”)

In “City of Angles,” Sam Roberts says that the 29° offset is “the reason that, looking west on the first day of summer, you couldn't see the sun set down the middle of any crosstown street, but you could have on May 28 and can again on July 13.” Actually, if the Manhattan grid were oriented with the points of the compass, the sun would rise and set parallel to the streets on the two equinoxes, not the solstice. On the vernal and autumnal equinox, the sun rises at due east and sets at due west everywhere in the world. During the summer months, the sun seems to rise north of due east, and set north of due west, and during the winter months, the sun seems to rise south of due east, and set south of due west. The maximum deviation occurs on the summer and winter solstices, but the actual angle of deviation depends on the latitude of the observer. In New York City, this maximum deviation obviously has to be greater than 29° to result in two winter days and two summer days of Manhattan Solstice.