"Manhattanhenge (sometimes referred to as the Manhattan Solstice) is a biannual occurrence in which the setting sun aligns with the east-west streets of Manhattan's main street grid. The term is derived from Stonehenge, at which the sun aligns with the stones on the solstices. It was coined in 2002 by Neil deGrasse Tyson, an astrophysicist at the American Museum of Natural History. It applies to those streets that follow the Commissioners' Plan of 1811, which laid out a grid offset 28.9 degrees from true east-west.
At sunset, a traveler along one of the north-south avenues on the West Side looking east can observe the phenomenon indirectly, being struck by the reflected light of the many windows which are aligned with the grid. An observer on the East Side can look west and see the Sun shining down a canyon-like street.
The dates of Manhattanhenge are usually May 28 and July 12 or July 13 (spaced evenly around Summer Solstice). The two corresponding mornings of sunrise right on the center lines of the Manhattan grid are approximately December 5 and January 8 (spaced evenly around Winter Solstice). As with the solstices and equinoxes, the dates vary somewhat from year to year."