Tuesday, May 22, 2007

Manhattanhenge coming up

Here we go again -- our favorite meteorological phenomenon is on its way. Charlie Ridgway, one of Central Park's faithful amateur astronomers, sent me the article as a reminder.


by Neil deGrasse Tyson, © 2001

Manhattan-henge: Sunset down 34th Street
Sunset looking down 34th Street. One of two days when the sunset is exactly aligned with the grid of streets in Manhattan.
Photo © Neil deGrasse Tyson, 2001.

What will future civilizations think of Manhattan Island when they dig it up and find a carefully laid out network of streets and avenues? Surely the grid would be presumed to have astronomical significance, just as we have found for the prehistoric circle of large vertical rocks known as Stonehenge, in the Salisbury Plain of England. For Stonehenge, the special day is the summer solstice, when the Sun rises in perfect alignment with several of the stones, signaling the change of season.

For Manhattan (a place where evening matters more than morning), there are two special days—May 28 and July 11—when the Sun sets in exact alignment with the Manhattan grid, fully illuminating every single cross-street for the last fifteen minutes of daylight.

Had Manhattan's grid been perfectly aligned with the geographic north-south line, then our special day would be the Spring equinox, and if we so designated, the Autumn equinox—the only two days on the calendar when the Sun rises due East and sets due West. But Manhattan is rotated 30 degrees east from geographic north, shifting the days of alignment elsewhere into the calendar.

Upon studying American culture, and what is important to it, future anthropologists might credit the Manhattan alignments to cosmic signs of Memorial Day and, of course, the All-Star break. War and Baseball.

Because Manhattan is so small (13 mile long) compared with Earth's distance to the Sun (about 93 million miles), the Sun's rays are essentially parallel by the time they reach Manhattan, allowing the Sun to be seen on all cross streets simultaneously, provided you have a clear view to New Jersey. Some major streets cross the entire island from river to river without obstruction, including 14th, 34th, and 42nd Streets. While the May 28 and July 11 sunsets qualify as the exact days for this auspicious moment, a few days either side of these dates also work.

Sunset on Manhattan-henge occurs at a cross-street on and around these dates:

  • May 28 at 8:10 PM
  • July 11 at 8:27 PM

Sunrise on Manhattan-henge occurs at a cross-street on and around these dates:

  • December 10 at 7:11 AM
  • January 2 at 7:22 AM