If you’ve ever tried to figure out exactly where the sun is going to hit a building at 3 PM on a random Tuesday in November, you’ve probably fallen down a rabbit hole of clunky software. Most solar calculators feel like they were designed by people who hate architects. But then there’s the andrew marsh sun path tool. Honestly, it’s a bit of a legend in the building science world, even if it looks like it belongs in a 2014 time capsule.
Dr. Andrew Marsh is the guy behind it. He’s an architect and a coder who basically decided that if the industry wasn't going to make tools that actually worked for designers, he’d just build them himself. You might recognize his name from Ecotect—the software Autodesk eventually bought—but his web-based sun path apps are where the real "secret sauce" lives today.
These tools aren't just pretty pictures. They are mathematical beasts.
The Andrew Marsh Sun Path Tool: What Most People Get Wrong
People often think a sun path diagram is just a map of the sky. It’s not. It’s a projection of a 3D hemisphere onto a 2D surface, and how you do that projection matters a lot.
Most "sun seeker" apps on your phone use basic GPS and your camera's AR. They’re fine for seeing if your tomatoes will grow, but they aren't precise enough for professional overshadowing analysis. Marsh’s tool allows you to switch between stereographic, orthographic, and equidistant projections. If you’re an architect, those words mean the difference between a building that stays cool in July and one that turns into a literal oven.
The andrew marsh sun path app handles the "sub-solar point"—the exact spot on Earth where the sun is directly overhead—with scary accuracy. It’s not just drawing a yellow line. It’s calculating the orbital relationship between the Earth and the Sun, accounting for things like atmospheric refraction.
Yeah, the air actually bends light. Most apps ignore that. Andrew Marsh doesn't.
Why professionals still use a "free" web tool
You’d think with all the fancy BIM (Building Information Modeling) software out there, these web apps would be obsolete. Nope. Here is why they’re still the go-to:
- Speed. You don't have to wait for a 2GB Revit model to load just to check a shadow.
- The Interactive 3D Sun-Path. You can drag the location marker anywhere on a Google Map and watch the sky dome update in real-time. It’s incredibly responsive because it uses WebGL and SVG.
- Educational Nuance. It shows the analemma—that weird figure-eight shape the sun makes over a year.
It’s about "performance-driven design." If you can see the sun path while you’re still sketching the floor plan, you make better decisions. You place windows where they actually belong, not just where they look symmetrical.
How to actually use the 2D and 3D sun path diagrams
Basically, you’ve got two main versions of the andrew marsh sun path tools.
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The 2D version is a classic. It looks like a polar chart. You’ve got your compass directions around the edge and altitude circles going toward the center. It’s great for creating those "shadow masks" you see in professional site reports. You can literally overlay it on a site plan and know exactly when the neighbor's house is going to block your light.
Then there’s the 3D version. This is the one that really shines on a tablet. It gives you a "god-view" of the Earth. You can toggle on the Arctic Circle, the Tropics, and even the twilight bands. It’s sort of a reality check for your brain. It helps you visualize why the sun stays so low in the sky in London versus why it’s almost vertical in Singapore.
A quick tip on "Solar Noon"
One thing that trips everyone up is the difference between "Clock Noon" and "Solar Noon." Your watch says 12:00, but the sun says something else. Marsh’s tool calculates the solar noon for your specific longitude. It’s a small detail, but if you’re trying to design a passive solar house, being off by 20 minutes can mess up your entire shading strategy.
What really happened with these tools?
There was a bit of a scare a few years back. The original tools were built on Java applets. When browsers stopped supporting Java, the internet basically broke for architectural students.
Marsh didn't just let them die. He rebuilt them using HTML5 and Javascript. He’s been hosting them on his own site and through platforms like Bitbucket. Honestly, he does this mostly as a service to the community. He’s mentioned in his blog that he uses these tools to test his own solar calculation code.
He’s a bit of a perfectionist. That’s good for us.
Actionable insights for your next project
If you're using the andrew marsh sun path tools for a real design, don't just look at the summer solstice. Everyone looks at June 21st (or December 21st in the south). That’s the easy part.
The real pro move is checking the equinoxes.
March and September are when the sun is moving the fastest in the sky. If your shading works on the equinox, it’ll likely work for most of the year.
Next Steps:
- Go to the 2D Sun-Path Map. Drag the marker to your specific lot.
- Export the JSON. You can actually save your settings and load them back in later. This is a lifesaver if you're working on multiple sites.
- Check the "Overshadowing" tool. If you have a 3D model of your site, Marsh has a separate "Site Designer" tool that uses the same sun-path engine to calculate actual solar radiation on surfaces.
Don't just trust the default settings in your CAD software. They're often "good enough," but "good enough" is why we have so many glass buildings that need massive air conditioning units. Use the tools that the experts use.