Sunday, July 09, 2006

The return of Manhattanhenge

A photograph of "Manhattanhenge" as seen from 34th street. The photo is from Dr. Neil deGrasse Tyson, director of the Planetarium in New York.

Did you miss Manhattanhenge on June 21st? You have one last chance this year to see the sun setting exactly at the western end of every one of Manhattan's streets that run from east to west [not, of course, the Avenues that run from north to south]. It will happen again
next Wednesday, July 12th:

Tom Clabough, an amateur astronomer who is often to be found at the northeast corner of Central Park's Great Lawn, ready to point out sky happenings to all interested passers-by, sends the following note of explanation:

[IMAGE: The apparent path of the Sun across the sky]
The red line represents the path of the Sun during summer solstice. The green, winter solstice, and the blue, both spring and fall equinoxes. Note how in summer the Sun rides high in the southern sky as it rises and sets NORTH of the east-west line. And in winter, the Sun rides low in the southern sky rising and setting SOUTH of the east-west line (true east-west). During the two equinoxes the Sun is at an intermediate inclination, rising and setting precisely east and west.
Now, recall that the northerly aligned avenues of Manhattan are actually skewed about 30 degrees to the east of true north, as the easterly streets are skewed ~30 degrees south of true east. If the street grid were in line with the compass, the "stone-henge effect" would occur at the spring and fall equinoxes, since this is when the Sun rises and sets due east and due west. However, since the grid is rotated ~30 degrees, the effect happens to occur 22 days before reaching summer solstice, and again 22 days on the return from summer solstice, summer solstice being the Sun's northern most rising and setting points.
In the event the diagram did not appear, here is a direct link to it: