# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Mathematical prediction of "Manhattanhedge" ??
From: Frank Reed
Date: 2022 Jun 29, 08:35 -0700

Joe Wong, you wrote:
"The general idea is to obtain the GHA and declination value by solving the astronomical triangle..."

While you can do it that way (and I don't see any problem with your analysis), the easy and standard way to do something like this is by direct simulation of the sky. If you want the Sun to line up with some altitude and azimuth, just run some trials until you get a hit. Attached below you'll see my results by simulation (using my USNO web app clone but many other tools are suited to the task) to get the time and date in May and then by symmetry the time and date in July when it repeats. I say "by symmetry" since I didn't have to spend any time hunting for that second date -- it should follow the solstice the same number of days as the date in May preceded the solstice.

When you analyze a scenario like this by simulation, you can easily assess "deltas" for various factors. For example, if you're a minute late, do you miss it? And if you try it a day earlier, will you see nothing or nearly the same thing? Your result yielded a declination that you recorded to the nearest second of arc. How much does one second of arc matter in the final result? And how sensitive is this to the chosen final altitude? This is actually the most important issue that we haven't addressed. There are hills across the Hudson River. Depending on the observer's location in Manhattan, we may want the Sun a little higher in the sky. So maybe the next day in May would be better, or the previous day in July. But don't mess with ritual! :) In the 25 years since Neil deGrasse Tyson started promoting this "Manhattanhedge" event, the requirement to view it on an exact date from certain ideal locations has become a sort of social ritual regardless of the actual viewing details.

I'm also including some images of me when I was a guest "expert in celestial navigation" on Tyson's StarTalk tv talk show from years ago. Just for entertainment!

Frank Reed

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