About SunPathly

A set of tools for a question that comes up constantly and is almost never answered properly: where does the sun actually fall on this house, this room, this plot?

Why it exists

Sun position is one of the few things about the outdoors that can be known exactly. It follows from orbital mechanics, needs no measurements, no sensors and no licensed data, and is as valid for a date in 2043 as for this afternoon. Yet the question "will this room get evening sun?" is usually answered with a shrug, and the tools that can answer it properly are either built for developers or cost money.

So: the geometry, computed honestly, presented for people who are buying a house, planting a garden or wondering about a roof.

What it computes

• Sun position — altitude and azimuth for any location, date and time, using the NOAA formulation of Meeus's solar algorithms. Accurate to roughly one arc minute between 1800 and 2100.
• Rise, set and twilight — including the standard refraction allowance and the sun's upper limb, so times match published almanacs.
• Shadows — buildings and fences are projected from their top edges along the true sun vector onto flat ground; tree canopies are modelled as raised spheres.
• Room daylight — the window aperture is projected onto the floor plane and clipped to the room, giving the real sunlit patch rather than an illustration.
• Panel irradiance — plane-of-array totals from a clear-sky atmosphere model with an isotropic sky and ground reflection.

How accurate it is

Sunrise and sunset times computed here agree with published almanac figures to within about a minute across a wide range of latitudes:

What it does not do

Being clear about this is more useful than claiming more:

• Weather. There is no cloud, haze or rain in any of these models. Hours of direct sun are clear-sky maxima, which is an upper bound, not an average.
• Terrain. Ground is assumed flat. A garden that falls away from a house gets more sun than shown; one that rises gets less.
• Real horizons. The tools cannot see the hill behind your house. That is why several of them let you set a skyline obstruction angle by hand — it is the single largest correction most sites need.
• Partial transparency. Trees are treated as solid. Real canopies leak a great deal of light, and deciduous ones stop casting meaningful shade in winter.
• Surveys. Formal daylight, sunlight and right-to-light assessments use measured building heights and site levels. These tools use the numbers you type in, so the answers are exactly as good as your measurements.

Privacy

Every calculation runs in your browser. Your coordinates, room dimensions and garden layout never leave your device. The one exception is the location search box, which sends the text you type to a geocoding service — and you can avoid it entirely by entering coordinates directly. The privacy page has the detail.

Credits

• Solar position: NOAA Solar Calculator formulation, after Jean Meeus, Astronomical Algorithms.
• Air mass: Kasten & Young (1989). Clear-sky irradiance follows the Ineichen–Perez approach in simplified form.
• Geocoding: the Open-Meteo open geocoding API.
• No frameworks and no external fonts — the whole site is plain HTML, CSS and JavaScript. The only third-party script is Cloudflare Web Analytics, which is cookieless and counts page views without tracking you.

Contact

Corrections are welcome, particularly if you can point at a number that disagrees with a published source. Email hello@sunpathly.com.