MANHATTANHENGE 2009
by Neil deGrasse Tyson
This weekend, and the second weekend in July, "Manhattanhenge" once again returns to New York City. Images, my full write-up, times, and other details are posted here:
http://www.haydenplanetarium.org/resources/starstruck/manhattanhenge
As you may know, Manhattanhenge takes place on two consecutive days, twice a year, when the setting Sun aligns precisely with the Manhattan street grid, creating radiant sunsets that burst across our brick and steel canyons, simultaneously illuminating both the north and south sides of every street. A rare and beautiful sight.
Unnoticed by many, the sunset point actually creeps day to day along the horizon: northward until the first day of summer, then returning southward until the first day of winter. In spite of what pop-culture tells you, the Sun rises due east and sets due west only twice per year - the first day of spring and of autumn. Every other day, the Sun rises and sets elsewhere on the horizon.
While we are on the subject, when viewed from all latitudes north of the Tropic of Cancer (23.5 degrees north latitude), the Sun always rises at an angle up and to the right, and sets at an angle down and to the right. That's how you can spot a faked sunrise in a movie: it moves up and to the left. Filmmakers are not typically awake in the morning hours to film an actual sunrise, so they film a sunset instead, and then time-reverse it, thinking nobody will notice.
The consecutive days for Manhattanhenge identify when half the Sun's disc and when the entire Sun's disc is setting on the grid. If the Sun were to set perpendicular to the horizon then these would be the same day. But since the Sun's path angles down and to the right, and shifts slightly from one day to the next, we get two days of Manhattanhenge, a half-sun sunset (my personal favorite for photographs) and full-sun sunset.
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