The clock that stands still is right twice per day.

Over the years, in our studies of the seasons and equinoxes, we’ve encountered various human-made edifices with connections to the Sun. The ancient Irish sepulcher Newgrange features an aperture that aligns with the rising star on the winter solstice, dripping light into the funerary passageway. Famously, England’s Stonehenge faces the rising sun on the summer solstice and the sunset in the winter.

These structures were hard-coded calendars, alerting archaic people to the passage of time. They required ingenuity, precision, and long-term knowledge of the moving objects in the firmament. The creators of the henges were master horologists, even before we developed mechanical watches.

The rising summer solstice Sun behind the Heel Stone at Stonehenge - photo by Andrew Dunn

The famous idiom about stopped or broken clocks teaches us that timepieces don’t need to be precise to be right, though. If a clock’s hands stop at 12:12 and one looks just as 12:11 ticks over, one would imagine the clock to be in perfect working order.

The old crafters at Stonehenge needed to make a perfectly aligned clock, but it turns out humans dotted the Earth with modern versions that aren’t precise, yet they’re still right several times per year. 

In 1807, developers in New York City proposed a design for streets based on a grid plan. Four years later, the Commisioners’ Plan of 1811 took effect and the city began a (mostly) rigid “march uptown,” following a geometric, gridiron ideology.

An 1893 redrawing of the 1807 version of the Commissioners' grid plan for Manhattan by James Kemp

Fast forward to the middle of the 20th century.

At age 15, astrophysicist and science explainer extraordinaire Neil deGrasse Tyson took an expedition to Stonehenge led by Gerald Hawkins, an astronomer who first proposed the connection between the British structure and the Sun. The design and nature connection at Stonehenge impressed itself on the young scientist.

Fast forward again to 1997. Neil deGrasse Tyson had become the director of the Hayden Planetarium in New York City, giving him plenty of time to think about both astronomy and the stone topography of the metropolis. He noticed the grid system was rotated 29 degrees clockwise from a true east-west line, which got him thinking about celestial alignment. Plotted on a circle, which has 360 degrees, the west sits at 270 degrees. So, on days when the Sun slips below the horizon at 299 degrees (270 degrees + 29 degrees to the north), sunset aligns with the streets on New York’s grid!

A couple of days after the 299-degree sunset, a person can stand in the center of one of the east-west roads, look toward New Jersey, and watch the Sun as a full disk set between the skyscrapers. The urban canyons of New York City create a phenomenon known as Manhattanhenge.

Manhattanhenge - Photo by Aditya Chinchure

Tyson penned an article in Natural History, describing the phenomenon, and claims to have coined the term. Since then, Manhattanites and tourists have flocked to watch our star dive between tall buildings at specific dates.

Though most people know the Sun rises in the east and sets in the west, these maxims are only generally true. Because of axial tilt, the exact direction of the Sun’s daily arrival and departure changes as Earth tilts into and out of seasons. On the spring and autumn equinoxes, the Sun hits due east and west, but, on other days, the angles are off. In the Northern Hemisphere, the summer solstice marks the Sun’s northernmost point. Between the equinoxes and equally spread around the solstice, the Sun will rise and set in the same place. For example, the Sun will set in the same spot five days before the solstice and five days after the solstice. This phenomenon leads to Manhattanhenge happening twice between spring and summer. It happens generally around Memorial Day each year and then in early-to-mid July. If you’re reading this article in 2024 when it was published, the first instance of Manhattanhenge transpired on May 29; the second will happen on July 12.

You might wonder to yourself that there’s nothing inherently special about New York City’s grid, and you’d be correct. The Sun rises and sets all over the world, including many cities that feature street grids. We got Torontohenge and Baltimorehenge and Chicagohenge and Montrealhenge and Strasbourghenge and even MIThenge, when the Sun sets across the notorious “Infinite Corridor.” Of course, the grids in these cities are not all 29 degrees off, as in New York, so the dates vary for these other henges. In the British city of Milton Keynes, which was constructed in 1967, planners noticed their main street nearly aligned with the rising sun on the summer solstice, so they persuaded builders to shift everything a few degrees. Bam, Milton-Keyneshenge.

You might have also astutely noted that the situation of New York’s urban canyons would also align on certain dates with the rising sun. On equidistant sides of the winter solstice, Manhattanhenge occurs in reverse, as the rising sun comes directly between buildings from the Atlantic side of the city. These dates typically fall on December 5 and January 8.

Reverse Manhattanhenge, as the sun rises along West 32nd St - photo by Fred Hsu

Though the developers of New York intended to create a rectangular grid, they never meant for them to be giant clocks. The towers of America’s largest cities do not align, like Stonehenge, on the solstices. The city has become, however, a stopped clock that just happens, thanks to axial tilt, to be right not twice per year but four times!

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