Much to our forecasting chagrin, clouds instead of sunshine have been the rule today. Luckily the clouds are breaking up a bit as tonight is one of Mother Nature's great astronomical spectacles. Yes, this evening is Williamsburghenge, the night when the sun's setting rays are parallel to the streets, well the numbered streets from North 3rd to North 15th, in Williamsburg.

Many readers have asked us how astrophysicist Neil De Grasse Tyson calculates the dates of Manhattanhenge. We don't know how he does it, and we made up the part about many readers asking us the question. We do know, however, how we figured out the date of Williamsburghenge and it's really quite easy.

Details after the jump...

To figure out a "-henge" date for your street you need to know two angles: the angle of the street and the angle of sunset. Both of these angles are known as the azimuth, and are expressed as degrees from true north. So east is 90 degrees, south is 180, west is 270 and from 14th Street northward in Manhattan the sunset azimuth is 290 degrees. That last bit of information makes finding your street's azimuth quite easy.

You can determine your street's azimuth with a good map, like a USGS topographic map, a straight edge, and a protractor but that reminds Gothamist too much of our high school geometry class (the one in which our teacher would have cute twin girls sit on his lap.)

But we digress. The way we found the angle was to bring a Google map centered on the East River into Photoshop. We rotated the map so the Manhattan streets were running directly east-west, then rotated the map a bit more so North 7th in Williamsburg was running east-west. Adding that angle, 11.5 degrees, to the Manhattan azimuth gives the Williamsburg azimuth of 301.5 degrees. Much of Bushwick between Cypress and Bushwick Aves. have the same azimuth so Bushwickhenge is tonight as well!

Once you have your street azimuth you need to find out what day sunset has the same angle. For that we used the U.S. Naval Observatory's sun and moon altitude/azimuth calculator. It's a bit of an iterative process to get the right day but look for when the sun's altitude is zero in the evening, as that's the moment of sunset, and read off the corresponding azimuth. The day when the sun azimuth equals your street azimuth is the henge day. Easy as pie!

bridge and sunset by plasmoNYC on Flickr.