WORLDHENGE
This weekend is the September equinox. Time to take some henge photos, and for me to post this.
If you’re not familiar, henge photos are where the sun aligns with a street and a photographer captures the golden drama of our big lifegiving plasma ball near the horizon. It feels primal but concrete — a single image connecting our daily lives and the urban fabric with the celestial. They’re especially cool at the annual solar events of equinox — when the night and day are of equal lengths — and the solstice — when the day or night is at its longest point of the year.
Such photographs have been happening for a long time, and might just have remained idiosyncratic curiosities until astrophysicist Neil deGrasse Tyson coined the term “Manhattanhenge” for the times when the sun matches the tight east-west grid of Manhattan streets. The “henge” suffix referencing the Stonehenge’s megaliths, famous for aligning to solar events. Having a catchy name allowed the phenomenon to become more memetic and now lots of people make a point to participate. There are crowds and hashtags. Tourists flock. It’s a thing.
Chicago also has some solid grids to its downtown, so there are lots of Chicagohenge photos as well.
In San Francisco the most famous henge does not happen on a particular solar event. It happens down California street with a view of the Bay Bridge on September second and is appropriately called Californiahenge. Look at that. Gorgeous.
Note: Like a recipe blog, I’m about to detour into the story of how I made the henge photo opp maps and calendars. If you don’t want to follow me into these particular weeds, and you just want to see the map or subscribe to a particular city calendar, skip the next sections and jump down to “A photographer’s takeaway”. Everyone else: It’s gonna get a lil’ nerdy. Awww yeah.
Arbitrary henge calculations
My muse has ADHD, and last Friday she had me noodling on this phenomenon instead of, you know, any of the other projects I have going on. And it struck me that many of the long, straight roads in a city must have particular dates where they align with the sun. Surely, I thought, someone has captured more streets like this online. *Googles* OK. No one has. Can I?
Figuring this out manually for a particular street would take finding its azimuth. An azimuth is an astronomical term, which is an angle that is 0 at due North and proceeds clockwise around the horizon through 360 degrees. You could do this on a paper map by measuring how many degrees from North the street runs. (Curvy streets wouldn’t work.) Once you had the street’s azimuth, you could check against a solar calendar of the year for that location to see on what date, if any, the street azimuth matches the sun’s. It would take some number crunching, but it’s doable.
Sidebar: Why is it complicated?
If you’re wondering why this might take a solar calendar and why it would be particular to the location of the city, recall that the sun doesn’t follow the same course across the sky throughout the year. Earth rotates around its axis at a tilt of 23.5°. against the plane described by its orbit. That means from a point on the surface of the planet (bypassing the complication of the arctic circles), the sun’s daily path across the sky appears to rise and fall over the course of the year. This arc is lowest at the winter solstice and highest at the summer solstice. All the cities that share a latitude share the same solar calendar, just with different localized times for sunrise and sunset. But if you could teleport to a significantly different latitude you would see the azimuth of the sun against the horizon change accordingly. It would drop towards the south if you teleported north and vice-versa. And if you teleported your street with you, if it aligned just before it teleported, it wouldn’t align in the new latitude.
This problem fits computation
But doing this by hand for a whole city much less across the world would take way too much time and be way too error prone. But, I realized, if finding a street’s henge date is a number thing, it’s something that can be computized. I also happen to work in designing for AI. I have played with large language models like OpenAI’s chatGPT and Anthropic’s Claude for a few years now. Could I ask one of them to get me to the answer?
No, gen AI wouldn’t work…
I’m quite fond of Anthropic’s commitment to safe, responsible AI, so I hopped on to Claude.ai and asked it if it could do all these calculations. It cranked on the problem and made a list for me, but when I asked it to make links to Google Street view of the locations it had identified, they just didn’t look right. Large language models (LLMs) like this are known to confidently hallucinate. So for a brief moment I thought it was a dead end.
…not by itself anyway
But I happen to have been thinking a lot about agentic AI architectures recently. (And how they relate to agentive tech.) If you’re not familiar with agentic back ends, they are a way to use the natural language and planning capabilities of an LLM to make a plan to accomplish some task, and then it hands individual steps to agents that are specialized in doing a task well and accurately. There might be agents for web search, or for reasoning, or for mathematics, or visualization, etc, etc. I have a diagram hereabouts.
The LLM coordinates the output from the agents, synthesizes it together, and hands the answer back to the user. This architecture promises to get AI over the hallucinatory nature of LLMs. This is a new-ish architecture, and to my knowledge there aren’t any consumer LLMs that do it. Was it another dead end? No! I could act as the coordinator and even play the part of a couple of the “missing” agents. And once you make it a program, you don’t have to limit yourself to one street. You can have it look across an entire city for streets. And you can do cities across the world.
I am an organic agentic
I asked Claude to help me write some Python code that would find the answers. (Python is an open source programming language with a relatively easy-to-understand syntax and a huge number of libraries that could be used to do all this.) And it did! It wasn’t perfect, but it was close even after a first prompt. Then across 7 after-the-kids-went-to-bed evening sessions, I was able to iterate my way to some code, to which I could hand the name of the city, and after it ran, have a list of henge candidates for that city.
Crafting some usability details
I could have just published a table with dates, locations, and times for each city, but that would require photographers to remember that it was there, check it, and then grab the information as they needed it. I’m an interaction designer and I know it could be made better by using more modern platforms.
- The first thing to do was to put it into a calendar format. That way the information is available on the dates and times when it would be most useful — and the events could be made annually recurring, making for an evergreen resource. I thought about including alerts, but then decided it would be better to let individuals decide for which events they might want notification and when those should arrive. Google Maps also lets me publish individual calendars, so you only need to subscribe to the one that matters most to you.
- I also wanted to include a Google Maps Street View to help a photographer use their own working knowledge of the location to decide if they want to go there, to set their expectations of what area they’re headed toward, and to prime them with an image that will help them recognize when they’ve gotten to the right place.
- Also, while I could have found a henge opportunity for every single day of the year, that feels like too much; like the frequency of it would quickly overwhelm someone. Once a month seems like a good pace for local hobbyists. To keep the non-solar event months the same distance from the solar event months, they’re all on or around the 21st of the month.
- I live in the San Francisco Bay Area, where weather is always something of a crap shoot. So I included a link to the weather for the location that a photographer can check leading up to the event. Cloudy day? Stay home and maybe live prompt it?
- And lastly, despite its finding so very many examples, sometimes it couldn’t find a long, straight road at the right azimuth for a given date. In these cases I added a flag to the calendar event indicating that it’s not a particularly “hengy” option. Using that flag, photographers can look at the Street View and decide if it’s worth going and trying.
A photographer’s takeaway
So after all that, yes, I now have a collection of henge photo opps in map and calendar formats.
You can see the map view via this custom Google Map. In it, each place marker is titled with its matching date and has a link to the Street View so you can get a first-person visual.
Note that many places around the world do not have Google Map or Street View data, by which I ran the algorithm and verified and selected options, respectively. So, the collection has a huge availability constraint, and if you’re inferring something by these locations, you’ll encounter a similar availability bias.
Individual city calendars follow, grouped whimsically.
Home
Pop Music
I cannot wait to come back to…
Teams
And ciao, belli capelli!
Among the places I want to visit:
For friends there:
Want a city that’s not listed here? While I don’t have a ton of spare time to handle a flood of requests, if you’ve checked in advance that there is Google Maps data and Street Views for a given place, mention it in the comments and I’ll try to set aside some time to add it.
Please be careful
Oh and an important caveat. If you decide to try out some of these, keep in mind these henge locations were proposed by an algorithm and I selected from among them via Google Street views. I have little to no pragmatic knowledge of these places. Prioritize reason, your safety, the safety of others, and other people’s privacy when acting on them. If it seems dangerous to you, harmful or disrespectful to others, or questionable, don’t do it. Discretion will be the better part of valor, here.
Also let me know how it goes. I’m happy to correct or replace any of these. And I’m eager to see your henge photos. Comment here or ping me on mastodon. Tag #worldhenge if you’re so inclined for others to make them easier to find.
Keep in mind the sun’s path is almost always angled
When I went to do my first test in the San Francisco Bay Area on September solstice 2024, the algorithm pointed me to a location in relatively nearby Walnut Creek. When we first got there, I was worried that the math was off, because the sun looked nowhere near the street. But I had to remind myself that because of our latitude, the sun would angle down. Sure enough it slid diagonally to a crook in the treeline at the end of the road. Had those trees not been there, it would in fact have been a perfect henge. Good going, Claude.
Some life takeaways
I have long been a student of randomness, especially as a creative tool. I have a very very long running project about it I‘ve spoken about on stage. In the realm of generative randomness, there is a school of thought known as Latourex, or Experimental Travel, which uses “games” or formulas to guide them to experiences in a place beyond the ordinary tourist spots. (I’ve designed for it before.) If you were looking for a way to get yourself out of the house, out of a routine, and to “touch grass” you can do a lot worse than LaTourEx. I humbly submit these calendars as an additional tool in that particular backpack.
Some AI design takeaways
At first I had the algorithm just run. It randomly picked one of the six top matches it found. But when I looked at the output, some of it was weird, or wouldn’t even load. So one of my iterations instantiated a human-in-the-loop interaction where the algorithm would show me six options for each date and let me pick the one I wanted. I’m a bit embarrassed I had to be reminded of it, but until we get to general AI, human-in-the-loop should be the default.
I’m convinced by how fast and successful this was that the agentic model is the model of the future. I mean, I suspected it from a priori reasoning, but this was a nice confirmation. If you’re in the design space, keep an eye out for this. There are some deep design implications to it and I predict it’s going to boost the next evolution of the technology.
Other things that might be made in this vein
- For a very large city like Tokyo or Mexico City, it might be possible to get a full year of henge locations into a calendar, but that would take reviewing over 2000 street views in the process. So, maybe someday?
- I thought it might be cool to do a landmark-centric map of streets that point to particular landmarks, like, say the Golden Gate Bridge, but it turns out that — especially in a hilly area like the San Francisco Bay Area — calculating sight lines is much much less computizable than azimuth and sunset times.
- I do have code that looks for a particular henge opportunity centered on a particular address within a given radius, but that’s so idiosyncratic it doesn’t feel worth sharing.
- I also have code that can look at an address and find the date on which a specific road henges, if any. If there is some spectacular road you’d like me to run, comment.
- I could make an algorithm that finds the henge of any two arbitrary geolocations but I haven’t found a use case for that yet.
- I also thought it might be fun to create a globe trotter itinerary whereby one could set aside a year and travel across the globe to capture the best or most perfect henges. But that would be only of use for the idle rich, who could hire anyone to figure that out for them. So I haven’t bothered with that.
- The lunar calendar also might provide some opportunities for henge pics (especially for people like me who sunburn within minutes), but moonrise and moonset happen at times around the clock, so would need an additional filter for times when photographers are likely to be active. Maybe there are more insomniac photographers than I realize, and if so, let me know and I can make a few of those city moonhenge calendars as well.
- In order to find henge candidates, there is a margin of wiggle room built into the code of 2°. It might be fun to find absolute perfect henges from across the world for every day of the year, but searching through that much data would take a very long time. If there’s interest I may set it up to run at night while I sleep. Poor little laptop. It’s trembling in fear at the thought.
Anyway thanks for reading this far. I hope you found it enlightening and inspirational and that I can see some of your cool #worldhenge photos in the future.
Happy solstice. ☀️
