Last week saw the second and last occurrence of Manhattanhenge this year. If you've been blissfully slumbering under a rock, or for some reason just not using Manhattan as the measure of the world, Manhattanhenge is when the setting sun lines up with the (approximately) East-West grid of Manhattan streets.
They say it's best to observe the 'henge from as far East as you can get on a wide street that has a clear shot through to the Hudson river. I don't really relish standing in the middle of such a street to get a shot, so I chose 72nd street at Central Park West, where I could stand just inside the park and get a clear shot down 72nd at least to Riverside Park. There were dozens of others with just the same idea, but we were all foiled by summer haze.
I mentioned the phenomenon to a friend who asked, "is it only in Manhattan?" I suppose you can observe something like this anyplace with a regular enough street grid, or even on any single straight street that at some point lines up with the setting sun, but of course (most of) Manhattan has a famously regular street grid, and famously deep canyons of tall buildings.
I also wonder why it's only at the sunset and not the sunrise - that would give you two more occasions a year for dramatic photos. I guess New York is the city that never sleeps, but not one that gets up very early.
There are probably more elegant ways to figure these things out, but I rather like the view you can get from Sundroid, an app that, among other cool functions, shows the angle at which the sun rises and sets for a given place on a given day. You can find your own personal 'henge this way.