Friday, April 24, 2009

Manhattanhenge

Manhattanhenge
by Neil deGrasse Tyson, © 2001-2009



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 change of season.

For Manhattan, a place where evening matters more than morning, that special day comes on Saturday, May 30th this year, one of only two occasions 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 day is Sunday, July 12th. These two days give you a photogenic view with half the Sun above and half the Sun below the horizon—on the grid. The day after May 30th (Sunday, May 31), and the day before July 12 (Saturday, July 11) will also give you Manhattanhenge moments, but instead you will see the entire ball of the Sun on the horizon—on the grid. My personal preference is the half-Sun.

As you may know, had Manhattan's grid been perfectly aligned with the geographic north-south line, then the days of Manhattanhenge would be the spring and autumn equinoxes, the only two days on the calendar when the Sun rises due-east and sets due-west. But Manhattan's street grid is rotated 30 degrees east from geographic north, shifting the days of alignment elsewhere into the calendar.

Note that any city crossed by a rectangular grid can identify days where the setting Sun aligns with their streets. But a closer look at such cities around the world shows them to be less than ideal for this purpose. Beyond the grid you need a clear view to the horizon, as we have over New Jersey. And tall buildings that line the streets create a kind of brick and steel channel to frame the setting Sun, creating a striking photographic opportunity.

True, some municipalities have streets named after the Sun, like Sunrise Highway on Long Island and the Sunset Strip in Los Angeles. But these roads are not perfectly straight. And the few times a year when the Sun aligns with one of their stretches of road, all you get is stalled traffic solar glare temporarily blinds drivers.

So Manhattanhenge may just be a unique urban phenomenon in the world, if not the universe.

Note that a couple of years ago, an article in the New York Times identified this annual event as the Manhattan Solstice. But of course, the word solstice translates from the Latin solstitium, meaning stopped sun, in reference to the winter and summer solstices where the Sun's daily arc across the sky reaches its extreme southerly and northerly limits. Manhattanhenge comes about because the Sun's arc has not yet reached these limits, and is on route to them, as we catch a brief glimpse of the setting Sun along the canyons of our narrow streets.

IMPORTANT: For best effect, position yourself as far east in Manhattan as possible. But ensure that when you look west across the avenues you can still see New Jersey. Clear cross streets include 14th, 23rd, 34th. 42nd, 57th, and several streets adjacent to them. The Empire State building and the Chrysler building render 34th street and 42nd streets especially striking vistas.

Arrive a half-hour earlier than the times given below.

Half Sun on grid: Saturday, May 30 — 8:17 P.M. EDT
Full Sun on grid: Sunday, May 31 — 8:17 P.M. EDT
Half Sun on grid: Sunday, July 12 — 8:25 P.M. EDT
Full Sun on grid: Saturday, July 11 — 8:25 P.M. EDT

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