The noun Manhattanhenge designates a phenomenon in which the sun rises or sets in alignment with the streets that run east to west on the street grid of Manhattan, a borough of New York City, New York, USA.

The alignments occur at sunset on certain dates before and after the summer solstice (late May and mid July) and at sunrise on certain dates before and after the winter solstice (early December and early January).

The noun Manhattanhenge is composed of Manhattan and -henge, in Stonehenge, the name of a megalithic monument on Salisbury Plain in Wiltshire, England, because the effect denoted by Manhattanhenge is thought to resemble that of the sun rising in alignment with the central upright stones of Stonehenge at the summer solstice.

Manhattanhenge—photograph published in Newsday (New York City, New York) of Tuesday 29^th May 2018:

The noun Manhattanhenge was coined by the U.S. astrophysicist Neil deGrasse Tyson (born 1958). This is what he wrote in City of Stars: A New Yorker’s Guide to the Cosmos, published in October 2002 in a special issue of the U.S. magazine Natural History:
—Source: American Museum of Natural History:

What will future civilizations think of Manhattan Island when they dig it up and find a carefully laid out network of streets and avenues? Surely the grid would be presumed to have astronomical significance, just as we have found for the pre-historic circle of large vertical rocks known as Stonehenge, in the Salisbury Plain of England. For Stonehenge, the special day is the summer solstice, when the Sun rises in perfect alignment with several of the stones, signaling the change of season.
For Manhattan, a place where evening matters more than morning, that special day comes twice a year, when the setting Sun aligns precisely with the Manhattan street grid, creating a radiant glow of light across Manhattan’s brick and steel canyons, simultaneously illuminating both the north and south sides of every cross street of the borough’s grid. A rare and beautiful sight. These two days happen to correspond with Memorial Day and Baseball’s All Star break. Future anthropologists might conclude that, via the Sun, the people who called themselves Americans worshiped War and Baseball.
For these two days, as the Sun sets on the grid, half the disk sits above and half below the horizon. My personal preference for photographs. But the day after also offers Manhattanhenge moments, but at sunset, you instead will find the entire ball of the Sun on the horizon.

Neil deGrasse Tyson wrote the following in an email sent on Friday 11^th July 2003—as quoted by the Oxford English Dictionary (3^rd edition, 2019):

Sunset on Manhattanhenge begins at 8:25 PM, at a cross-street near you.

In 2006, for example, the phenomenon known as Manhattanhenge occurred on Sunday 28^th May. This was announced by Justin Rocket Silverman, AM New York staff writer, in an article published in several U.S. newspapers on Saturday 27^th May 2006—for example in Newsday (New York City, New York):

“Manhattanhenge” occurs Sunday, a day when a happy coincidence of urban planning and astrophysics results in the setting sun lining up exactly with every east-west street in the borough north of 14^th Street.
Similar to Stonehenge, which is directly aligned with the summer-solstice sun, “Manhattanhenge” catches the sun descending in perfect alignment between buildings. The local phenomenon occurs twice a year, on May 28 and July 12.
“People never stop to think that this will happen, but it’s happening this Sunday, so take the opportunity to appreciate it,” said Neil deGrasse Tyson, an astrophysicist at the American Museum of Natural History. “New Yorkers need to take advantage of as many opportunities to look at the sky as they can get.”
If the streets of Manhattan were lined up in true east-west lines as Stonehenge is, the sun would set directly along them on June 21. But what New Yorkers call “east” and “west” are actually about 30 degrees off.
“It was a disturbing day when I learned everything we call ‘north’ in New York is actually northeast,” Tyson said. “But what we are left with is an inadvertent homage to the sky.”
The astrophysicist also pointed out some people wishing to appreciate Manhattanhenge mistakenly stand on 12^th Avenue and end up watching the sun set over New Jersey.
The real spectacle, of course, is reserved for those who stand on First Avenue and gaze west as the sun sets neatly between the buildings.

The day after Manhattanhenge, i.e., Monday 29^th May 2006, the Daily News (New York City, New York) published The sun stars in big show, by Derek Rose and Jane H. Furse, Daily News writers:

Manhattan Memorial Day weekend revelers enjoyed a special finale to the glorious day yesterday as the sun washed crosstown streets with golden light before sinking below the horizon.
The cosmic effect occurs when the sun aligns precisely with the east-west grid of the streets, in what’s known as the rare “Manhattan-henge” phenomenon.
“We’re so bogged down in crass reality, we want to transcend this mundane existence,” mused Joseph Drexel, 54, a Chelsea artist, as he took in the view. “We have to love Mother Nature.”
“It was like a huge rubber ball making its way across the horizon of the street and then sinking immediately,” said Mark Harris, 55, who joined about 100 others at the Tudor City overpass at 42^nd St. and First Ave.
The sun was the star all day as New Yorkers enjoyed the weather and honored the nation’s war dead.
“These soldiers will always be in our hearts. Freedom has a price, and this is it,” said veteran Henry Bohman, 58, at the Memorial Day Parade in College Point, Queens.