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updated 5/29/2009 12:19:10 PM ET 2009-05-29T16:19:10

For 15 minutes around sunset on two days this summer, the sun will set in exact alignment with the cross streets of Manhattan's street grid, making the city's towering buildings function something like a modern-day Stonehenge.

They call it Manhattanhenge.

The first Manhattanhenge opportunity comes this weekend: On Saturday at 8:17 p.m. EDT the ball of the sun will be half above the horizon, half below if you look west down a major cross-street (34th Street and 42nd Street are good viewing locations). On Sunday, May 31, the entire solar sphere will be visible just above the horizon at 8:17 p.m. EDT.

The second opportunity comes later in the summer, with another half-sphere sunset on Sunday, July 12, at 8:25 p.m. EDT and a whole-sphere viewing on Saturday, July 11, at 8:25 p.m. EDT.

These times are calculated every year by the astronomer Neil deGrasse Tyson, director of the Hayden Planetarium in New York, who coined the term "Manhattanhenge."

The "henge" comes of course from Stonehenge, the prehistoric monument in the Salisbury plains of England. The large structure of stones and earthen mounds is thought to be a burial ground that was oriented to face the midsummer sunrise and midwinter sunset.

Manhattan's street grid doesn't run geographically north to south, but instead aligns itself with the direction of the island. If the grid did run north-south, Manhattanhenge would fall on the spring and autumn equinoxes, the only two days during the year when the Sun rises due-east and sets due-west. (The equinoxes occur when the sun sits directly over the Earth's equator and the length of day and night are roughly equal.)

Because Manhattan's grid is rotated 28.9 degrees east from geographic north, the days of alignment with the cross streets are also shifted.

Manhattan's street grid was laid down by the Commissioners' Plan of 1811, which was adopted by the New York State Legislature.

New York isn't the only city that can have its own "henge" events: Any city crossed by a rectangular grid has days where the setting Sun aligns with the streets. But a clear view of the horizon and straight streets are needed, and New York might be the only city that fits the bill.

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