Map of the city of New York and island of Manhattan as laid out by the commissioners appointed by the Legislature, April 3, 1807, published in 1811, from the collection of the New York Public Library

Last year, the Manhattan street grid celebrated its 200th birthday.

The grid was originally proposed in 1811, by Gouverneur Morris, surveyor John Rutherfurd, and New York State Surveyor General Simeon De Witt, four years after the city council appointed them "Commissioners of Streets and Roads," charged with master-planning the city's expansion from its dense base on Manhattan's southern tip.

In their proposal, the Commissioners explained that they had deliberately avoided embellishments such as "circles, ovals, and stars," in favor of a regular grid of twelve north-south avenues criss-crossed with east-west streets. Their scheme, they wrote, would ensure "free and abundant circulation of air" to combat disease, and had the added benefit that "straight-sided and right-angled houses are the most cheap to build."

They did not mention, however, that alongside the economic and public health benefits embedded in their design is also an astronomical calendar.

The sun creeps into view behind Madam Tussaud's. Manhattanhenge photographs by Nicola Twilley.

Twice each year, the setting sun aligns perfectly with the angle of the street grid (which is thirty degrees off from true north). The phenomenon has been dubbed Manhattanhenge, most notably by astrophysicist Neil deGrasse Tyson, and it is quite spectacular: for a few minutes, the enlarged red ball of the sun hovers above the pavement, nestled perfectly in the cradle of the dark skyscraper canyon, and sends a shaft of glowing orange light down each street before dropping out of sight.

Of course, this cosmic phenomenon is not limited to Manhattan. Any city built on a relatively uninterrupted grid will experience its own days of alignment, and, provided it has the right balance of street-width and skyscraper-height to produce a sun-sized notch, it, too, can be called a henge: hence Chicagohenge and Torontohenge, and no doubt others besides.

Manhattanhenge is also not limited to sunset; however, the sunrise alignments take place in December and January, and the cold weather, combined with the early hour and more cluttered eastern horizon, makes it a much less popular event.

But the sunsets of Manhattanhenge have gradually turned into minor public festivals, with camera and phone-waving crowds gathered in the middle of popular cross-streets (14th, 23rd, 34th, 42nd, and 57th), careless of traffic. Their behavior attracts yet more people, demanding to know what's going on, as well as a horn accompaniment from blocked taxis, and, combined with the astronomical light show, it feels as though Manhattan is sharing an unofficial 15-minute celebration (and pedestrian takeover of the streets).

Clouds and rain obscured the first Manhattanhenge of 2012 in May, but the July 11 full sunset was spectacular, and Venue got back from visiting an atomic clock in Boulder, Colorado, just in time to document it.

This idea that a city can be a clock, with its own solstices, seems to be an accident worth making intentional. From constellations to comets, what other cyclical astronomical events could be given a spectacular frame by the built environment? Landscape architecture blog Pruned, for instance, recently put out a call for speculative designs for a "pavilion for viewing the coming intergalactic collision between Andromeda and the Milky Way," but the list of possible overlaps between astronomy and the urban environment is all but infinite.