Manhattanhenge

So yesterday (May 30), and again on July 13 of this year (2015), the setting Sun will line up with Manhattan's roughly east-west street grid - an event dubbed Manhattanhenge (making the connection, of course, to Stonehenge).

Here's a picture I saw on my newsfeed this morning.

So why does this occur when it does?

As the year progresses, the rising and setting position of the Sun moves as well.  If you went out every morning at sunrise, and plotted the position of the Sun against the horizon each day, you would see the following.

Obviously, Manhattan's streets aren't aligned in a true east-west fashion or else Manhattanhenge would occur only on the equinoxes (March 20 and September 23, 2015) when the Sun is rising due east and setting due west.

The summer solstice this year is June 21.  The two Manhattanhenge dates (May 30 and July 13) occur 22 days before and 22 days after the solstice.  So what is the direction of the setting Sun on those dates?  These can be obtained from the U.S Naval Observatory Astronomical Data Services website (a great website for observational astronomy nerds like me, by the way).

For Manhattan, the azimuth (compass direction in 000°-360° where 000° is north, 090° is east, 180° is south, and 270° is west) direction of  sunset is 300° on both of these dates.

Let's take a typical cross-town street - the famous 42nd Street.  What is its western azimuth direction?  It's 300°, of course.

This works particularly well for Manhattan because of the clear view across the Hudson River toward New Jersey and the tall buildings framing the view.

It also brings up an interesting issue for archaeoastronomy - the study of astronomical alignments in archaeological structures (like Stonehenge). Manhattan's street grids were built according to the geographic alignment of the island and the Manhattanhenge effect is completely coincidental.  But what about ancient structures that appear to have alignments with the Sun, Moon, or other astronomical objects (prominent stars and constellations).

If you can find such alignments in ancient structures, were they intentional or coincidental?  That's not always an easy question to answer, I'll defer further discussion to the next post.