The term "Manhattanhenge" was popularized by Neil deGrasse Tyson, an astrophysicist at the American Museum of Natural History and a native New Yorker. It is a reference to Stonehenge, a prehistoric monument located in Wiltshire, England, which was constructed so that the rising sun, seen from the center of the monument at the time of the summer solstice, aligns with the outer "Heel Stone".
In accordance with the Commissioners' Plan of 1811, the street grid for most of Manhattan is rotated 29° clockwise from true east-west. Thus, when the azimuth for sunset is 299° (i.e., 29° northward of due West), the sunset aligns with the streets on that grid. A more impressive visual spectacle, and the one commonly referred to as Manhattanhenge, occurs a couple of days after the first such date of the year, and a couple of days before the second date, when a pedestrian looking down the centerline of the street westward towards New Jersey can see the full solar disk slightly above the horizon and in between the profiles of the buildings. The date shifts are due to the sunset time being when the last of the sun just disappears below the horizon.
The precise dates of Manhattanhenge depend on the date of the summer solstice, which varies from year to year but remains close to June 21. In 2014, the "full sun" Manhattanhenge occurred on May 30 at 8:18 p.m., and on July 11 at 8:24 p.m. The event has attracted increasing attention in recent years.