Tuesday, July 11, 2006

It's tomorrow -- Neil Tyson on Manhattan-henge


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 pre-historic 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 rose in perfect alignment with several of the stones, signaling the chage of season.

For Manhattan, a place where the evening matters more than the morning, that special day comes on July 12; one of only two occasions in the year when the Sun sets in exact alignment with the Manhattan grid, fully illuminating every single cross-street for the last fifteen minutes of daylight. The other occasion is May 28th. 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 the New Jersey horizon. Some major streets cross the entire island from river to river without obstruction, including 14th, 23rd, 34th, and 42nd streets. While the July 12 sunset qualifies as the exact day for this auspicious moment, the surrounding days will also work, as the point of sunset migrates slowly south from day to day along the horizon, bringing with it ever-shortening daylight hours.

Sunset on Manhattan-henge begins at 8:20PM, at a cross-street near you.

As always, keep looking up,

-Neil deGrasse Tyson

Department of Astrophysics
& Director, Hayden Planetarium
American Museum of Natural History
Central Park West at 79th Street
New York, NY 10024