The Hayden Planetarium provides some more background to Manhattenhenge:
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. On the equinoxes: the first day of spring and of autumn. Every other day, the Sun rises and sets elsewhere on the horizon. Had Manhattan’s grid been perfectly aligned with the geographic north-south line, then the days of Manhattanhenge would coincide with the equinoxes. But Manhattan’s street grid is rotated 30 degrees east from geographic north, shifting the days of alignment elsewhere into the calendar.
My 2010 and 2011 Manhattanhenge photos are below, and for 2012, see The Telegraph’s fun photoessay from July’s sunny spectacle.
Missed it? Next Manhattanhenge will be around 2013′s summer solstice (June 21, 2013).