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 (May 30) 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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