At Besselian Elements, our focus has always been the “science of the shadow”, providing the most rigorous data possible by accounting for the real lunar limb and Earth’s orography and expecially solar Radius. But data truly comes alive when it is placed in the hands of observers. This is why our collaboration with Christophe Durand and his remarkable project, Eclipsefan.org, has been so rewarding
The Story: From a Single Query to a Global Tool
Our journey began in early January 2026, when Christophe, an amateur astronomer and GIS expert, reached out to inquire about our shadow data for his own observation plans in Spain. What started as a technical exchange quickly blossomed into a deep integration of our core technologies into the Eclipsefan platform. Today, Eclipsefan serves as one of the most sophisticated “one-stop shops” for the 2026 eclipse, combining our high-precision geometry with advanced meteorological modeling
Eclipsefan.org: A Powerhouse for Observers
- Hyper-local Weather: Integration of high-resolution models like AROME and AROME-PI for short-range “nowcasting,” helping you decide where to drive in the final six hours before totality.
- Sun-Visibility Forecasts: An experimental feature that predicts whether the low-hanging Sun will actually be visible through the clouds at your specific location.
- 3D Horizon Profiles: A vital tool for this eclipse, given how low the Sun will be in the sky; it helps ensure a nearby hill or building won’t rob you of the show.
- Comprehensive Environmental Data: Layers for Saharan dust (calima) and wildfire smoke
- Eclipse Events: events in Spain and France
Besselian Elements "Under the Hood"
- Orographic Shadow Tiles: We provided high-resolution tiles that display exactly where mountain and terrain shadows will fall at the moment of maximum eclipse across Spain and France.
- Jagged Outlines: Unlike traditional maps that show smooth, idealized limits, the limits and centerlines on Eclipsefan (when using the BE engine) follow the actual topography of the Earth. This results in “jagged” lines that accurately reflect how the shadow “jumps” across ridges and valleys, especially as the Sun hangs low on the horizon
A New Standard for Precision: The IQP Engine
- The real lunar limb (using the latest LRO data).
- A revised solar radius of 959.95″.
- Detailed observer orography.
Peering Through the Clouds: Visibility Modeling
One of our most intense areas of shared research was the “Line of Sight” (LoS) sun visibility and expecially the cloud shadows. Because the 2026 eclipse occurs so low in the sky, standard “overhead” cloud forecasts are insufficient. We discussed about the approach and even asked Jay Anderson advice and ended up on a slight different method, both available on the site also featuring specific markers for cloud altitude—categorized as Low (1500m), Mid (4000m), and High (8000m). This allows observers to see not just if it will be cloudy, but whether the specific path between them and the low-hanging Sun will be clear
The Besselian Elements Volvelle
It might surprise some to learn that the original idea for the Eclipsefan website was rooted in a traditional astronomical instrument: the volvelle. Christophe envisioned a way to generate these rotatable paper calculators so observers could have a “no-battery companion” on the field. We customized our own volvelles generated on Eclipsefan with our branding and with our data. It’s a fun object, providing a tactile, reliable way to understand the eclipse’s geometry without needing a screen or a signal
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