What Is 'Manhattanhenge?'
Oct. 12, 2012
-- It might look like just an ordinary picture of Stonehenge, but this is how the creators of the prehistoric monument wanted the site to be viewed, according to research using the latest 3D laser scanning technology. The groundbreaking analysis determined that the prehistoric monument was built to show off the solstices. In this view Stonehenge would look best when approaching from the Avenue, its ancient processional way to the north east.
Commissioned by the English Heritage, the laser-scan survey revealed in unprecedented detail the efforts made by prehistoric people at Stonehenge. "The result of the project were beyond all expectations. The investigation identified traces of stone working on virtually every stone," Marcus Abbott, head of geomatics and visualization at ArcHeritage, Hugo Anderson-Whymark, an Oxford-based expert on ancient worked stone, and colleagues wrote in the English Heritage report.
The laser-equipped researchers investigated the entire site. The laser scanner collected data with a resolution of 1 mm across the entire stone circle, and of just 0.5 mm for four stone surfaces of special interest. More than 700 surface features came to light.
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The laser highlighted prehistoric carvings from 4,500 years ago as well as damage made by modern visitors. Along with modern graffiti, this image shows scores of little axe heads and a possible dagger added when the slabs were already 1,000 years old.
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Overall, the laser scanning revealed 71 new Bronze Age axe heads, which bring the number of this type of carvings known at Stonehenge to 115.
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But the most interesting findings came from analysis of the stone surfaces. The study showed that the techniques and amounts of labor used varied from stone to stone. According to the researchers, these variations provide almost definitive proof that it was the intent of Stonehenge's builders to align the monument with the two solstices along a north-east/south-west axis. Indeed, the extremely straight and neat outline of the Great Trilithon, compared with all the other trilithons, shows that Stonehenge creators made deliberate efforts to shape it more carefully due to its special position on the solstice axis, just as they did for other stones that flank this axis.
The laser scanning showed that sides of the stones that flanked the solstice axis were most carefully worked to form very straight and narrow rectangular slots. To make them glisten in the sunlight, some stones had their crusts removed. These stones include two of the north-east facing sarsens in the outer circle, the Great Trilithon in the inner sarsen horseshoe, and an isolated upright stone in the south-west segment of the outer circle. By contrast, the stones in the south-western segment of the circle did not have their crusts removed.
The specially smoothed slabs created a dazzling light effect when the sun rays hit the stones. They would glisten in the dawn light on the longest day of the year and at sunset on the shortest This drawing shows Stonehenge in about 2300 B.C., after the construction of the sarsen outer circle and trilithons. Note the solstice axis.
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Today through Tuesday marks a biannual solar event called Manhattanhenge, where the rising or setting sun aligns with the east-west grid of Manhattan streets.
Astrophysicist Neil deGrasse Tyson coined the term back in 2002, inspired by the famous Stonehenge site in the United Kingdom, where the sun sets in alignment with the stones every summer solstice.
Technically, "Manhattanhenge" occurs around the summer solstice, not on the solstice itself. That's because of the orientation of Manhattan's famous grid pattern — established by the the Commissioners' Plan of 1811 — is not perfectly aligned with the geographic north-south line; it's rotated 29 degrees east, shifting the dates of alignment.
If that alignment had been perfect, Manhattanhenge would have occurred on the equinoxes every year: the first day of spring and autumn, respectively.
(Historical side note: The goal of the 1811 plan was "a free and abundant circulation of air" to stave off disease. The right angles were also favored because "straight-sided and right-angled houses are the most cheap to build." The rigid Manhattan grid has been much-maligned over the last 200 years, but recently has come back into favor with city planners.)
This kind of alignment is not unique to Manhattan; any city with a uniform street grid will have dates where the sun aligns with those streets, including Chicago, Toronto and Montreal.
But Manhattan also boasts a clear view of the horizon, looking across the Hudson River toward New Jersey. Plus you've got all those tall buildings lining the streets, creating the perfect vertical frame to show the setting sun to best advantage.
Tyson has been outspoken in the past about astronomical inaccuracies in film and television. For instance, you can always spot a fake sunrise onscreen, because the sun will move up and to the left as it rises. In reality, the sun always rises up and to the right. Directors tend to film a sunset for such scenes and then just run it backwards to portray a sunrise, thinking nobody will notice. (And they'd probably get away with it, too, if it weren't for those meddling astrophysicists!)
The term Manhattanhenge technically applies to the setting sun phenomenon that flanks the summer solstice, usually around May 28 and July 12, although the precise dates vary slightly year to year. A similar alignment occurs with the rising sun around the winter solstice, usually Dec. 5 and Jan. 8.
Note: This is a re-published version of an article published on Discovery News in 2012.
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