Solargraph Simulator

See the sun's path across the sky, from solstice to solstice, for 20 locations on Earth. ⬇ Explanation and details below ⬇

Instructions

Explore solargraphs in different locations at different times:

• Pick a station by clicking a point on the map. Hovering over a point will show you the name of the station and what data is available for it. Try the extremes, like Ny-Ålesund, Paramaribo, and the South Pole! The up/down arrow keys also switch between stations.
• Pick a season by clicking the bars in the information card on the right (or using the left/right arrow keys). Thicker bars represent seasons like summer/fall or winter/spring (solstice to solstice), thinner bars are entire years.
• Check out analemma mode by clicking a thin analemma/full year bar (or pressing a). Compare the analemmas of Seattle, WA, Lauder, NZ, and the South Pole.
• Switch to panorama mode by pressing the p key, then navigate the 360° view of the sky by dragging with your mouse (hold shift and drag up or down to zoom). Press f to go into fullscreen mode for a more immersive experience.
• Try out the various other modes by pressing the ? key on your keyboard to bring up the keyboard shortcuts box, and playing around with the options.

What are Solargraphs?

Solargraphs are long-exposure photographs that record the sun's trajectory, revealing patterns of sunlight and cloud cover throughout the year. Each line in a solargraph represents one day, as the sun moves across the sky. The sun's path differs from day to day due to the Earth's path around the sun and the tilt of its axis. This causes the fan-like vertical pattern.

The varying brightness in the image reflects the amount of sunlight measured at that point, or in other words: how sunny or cloudy it was. Consistently sunny locations, like Hanford, CA, have almost entirely contiguous streaks, while places like Seattle, WA or Tateno, Japan are much more mixed. Some places also show shadows from nearby mountains or structures, like Ny-Ålesund, Norway or the South Pole.

Traditional solargraphs are taken with a pinhole camera over several months. I wrote a whole blog post with more explanations and a few example images, if you want to learn more.

What is analemma mode?

When you click the thin bars in the info box, or press the a key, the display shows only one point for each hour. Instead of continuous streaks, you now see dots that connect to form a shape known as an analemma.

I talk more about analemmas in solargraphs in the blog post, but it's an interesting variation on the theme. The analemma is usually drawn as a diagram, but this way it can be shown directly. In a traiditional solargraph, it requires a mechanical shutter that has to keep working for an entire year. Here, it's a relatively straightforward filtering of the data.

Why the odd shape? The sun doesn't move like that!

The shape comes from flattening the dome of the sky into a single image, which is done with a cylindrical projection by default. Press j to switch to a stereographic projection instead, which can look a little closer to normal, depending on the latitude. Or press p for panorama mode, where you can look around the solargraph as it wraps across the sky.

Real solargraphs are often made with cameras built from tins or cans, whose curved paper bends the image into a cylinder. That captures a very wide horizontal field of view, but it's also what creates the characteristic distortion behind the familiar solargraph shape.

What does it mean to simulate solargraphs?

This app renders an image based on solar irradiance data measured by various stations across the world (see data sources below). The sun's position in the sky can be computed for any point in time and any location on the planet. For each data point, the app draws a dot (or technically, a Gaussian splat) in that computed location, with a brightness that reflects the measured irradiance value at that point in time. The dots get summed, and when there are enough of them, they form continuous streaks.

The rendering on this page is a simulation of a solargraph, but it's also its own thing. In addition to trying to be close to traditional solargraphs, it can do things that normally wouldn't be possible: animating the drawing, showing one point per hour for an analemma, changing the projection, wrapping it into a 360° space, and so on.

Data sources

The solargraph simulator uses solar irradiance data from three sources:

• University of Washington's Department of Atmospheric and Climate Science for Seattle, WA
• NOAA's SOLRAD network for other locations in the U.S.
• Baseline Surface Radiation Network (BSRN) for locations outside the U.S.

The line below the rendering shows which data source is being used for the current simulation and provides a link. In the case of BSRN, it links to the specific station data citation.

Data quality varies widely between stations and networks. The ones shown on the map above had enough contiguous data available for a few seasons (and also adjacent seasons for drawing analemmas). Some have data from the last few years, others are mostly older and more fragmented.

The data has been extracted from the various sources, filtered, and preprocessed to make for easier rendering. I have also imputed values where there were only small gaps (five minutes or shorter). The Seattle data in particular has frequent short drop-outs that would show up as noise otherwise. There are still larger gaps in a few data sets, but I decided to keep them so I can provide more global coverage.