Thread by @jfaherty

We have finally arrived at the first 2023 official #Manhattanhenge sunset! The weather looks excellent so get your cameras ready. Are you wondering what this phenomenon is? Why it happens? Where you should watch from? I've got a whole thread for you on it #NYC @AMNH

#Manhattanhenge is a name given to the days of the year when the sun sets perfectly aligned with the grid of Manhattan. So perfectly aligned with the cross streets where iconic buildings will beautifully frame the sun just as it touches the horizon. I also call it a "grid kiss"

Why does #Manhattanhenge happen? #NYC is a grid -- with 90 degree angles between the avenues and cross streets. At sunset you look west-ish across fairly wide cross streets and the Hudson river toward a relatively flat NJ skyline.

Important to note is that Manhattan is not a true North South facing island. It actually points 29.1 degrees to the NorthEast. That plays a critical role in deciding the dates of #Manhattanhenge. The actual grid orientation that I use for the date calculation is 299.1 degrees.

Another fun twist, there are two things you can thank for the shape of the grid of Manhattan. #1) Nature or specifically glaciers/erosion and #2)The grid plan of 1811 with the three responsible commissioners: Gouverneur Morris, Simeon De Witt, and John Rutherford. #Manhattanhenge

The story of the grid plan is fascinating. You can read more about it in The Greatest Grid: The Master Plan of Manhattan edited by Hilary Ballon and see the interactive maps on the Museum of the City of New York's website @MuseumofCityNY #Manhattanhenge thegreatestgrid.mcny.org/greatest-grid/

Fast forward to modern times, on April 1, 1997 cartoonist @realstanmack did a piece in the "Look" section of Natural History Magazine on the new @AMNH Hayden planetarium director @neiltyson. In cartoon form, this is the first ever publication of the dates of #Manhattanhenge

Then on January 1, 2002 Natural History Magazine ran a special issue called "City of Stars A New Yorkers Guide to the Cosmos". On page 73 was a picture taken by @neiltyson of 34th street. This is the first official #Manhattanhenge picture acknowledging the phenomenon.

It still took a while for people to catch on to the phenomenon. @neiltyson would promote to the @AMNH starstruck email list and a few astronomers like myself started to get really into it and also promoting it. #Manhattanhenge

In 2010 I took on the job of #Manhattanhenge date calculator and started doing public programs at @AMNH in the planetarium where I would describe the event and then take people onto the streets to watch it. You can see how #Manhattanhenge increased in popularity over time.

In September 2019, the word #Manhattanhenge was officially entered into the Oxford English Dictionary. @neiltyson had coined the word as an homage to Stonehenge which famously has a structure which marks the summer solstice sunrise. @OED

As for why it happens at all, it comes down to an Astronomy 101 lesson. The Earth's axis is tilted by 23.5 degrees relative to its orbit around the Sun. The impact of that is the seasons, and also a changing sunset position each day. #Manhattanhenge @CCNYPlanetarium

I refer to the May #Manhattanhenge as the "Welcome to the grid" sunset. Every day until the summer solstice -- June 21st-- the Sun will cross the grid a little higher (the highest being 2.9 degrees). I made a chart in 2019 you can follow: docs.google.com/spreadsheets/d…

After the June 21st solstice the sun crosses the grid a little lower until July 13th when we once again have official #Manhattanhenge sunset and that marks the end of the season. I refer to the days of May 30th - July 11th as #Manhattanhengeeffect days.

On #Manhattanhenge days, the Sun will hit the bullseye of the Manhattan grid no matter what cross street you are on. The Sun is 93 million miles away. The grid of Manhattan is <13 miles across. It's the same intersection on 145th street as it is on 14th street. @CCNYPlanetarium

So where to watch from? Any cross street where you can see to NJ is fair game. Excellent places are the wide cross streets: 14th, 23rd, 34th, 42nd, 72nd, 86th, 96th, 145th, etc. It's as much about the buildings that frame the sun so thats a good guide. #Manhattanhenge

You also have some good views in the outer Burroughs. I always recommend Gantry Plaza state park in Queens which can see straight across to NJ safely from a picnic spot. #Manhattanhenge

A bit of advice. You need to be in the middle of the street to watch it all happen so be safe, wait for the crosswalk and try to get the drivers to participate by pausing and looking. It's a chance to get strangers talking in #NYC. How often does that happen?#Manhattanhenge

Official dates and times for this year are May 29th 8:13 pm (Half Sun) May 30th 8:12pm (Full Sun) July 13th 8:21pm (Half Sun) July 12th 8:20pm (Full Sun). At @AMNH we will do a public program in July where there will be a viewing. Stay tuned for more information! #Manhattanhenge