Twice a year, the sun setting over the Hudson River aligns perfectly with New York City's streets, creating rare and striking views of the orb of light passing through the shadowy canyon formed by the city's skyscrapers.

Called 'Manhattanhenge' by the American Museum of Natural History's Neil deGrasse Tyson, the phenomenon occurs not on the summer solstice itself but rather just before and just after, because Manhattan's grid doesn't perfectly match up with compass directions: What we call "uptown" isn't due north.

Here NYU Tandon Professor of Computer Science and Engineering Cláudio T. Silva, who previously mapped the shadows of every building on every block of the city, explains why Manhattanhenge is so dramatic and shares his top spots for viewing it.