Eclipse Horizon Checker

What is this?

On 12 August 2026, a total solar eclipse will cross northern Spain. Because the eclipse occurs near sunset, the sun will be very low — between 2° and 13° above the horizon depending on your location. Even a small hill to the west-northwest could block your view.

This tool uses satellite elevation data (SRTM, ~30 m resolution) to compute the terrain horizon from any candidate observation point, and tells you whether the eclipsed sun will be visible or blocked.

New in v0.2

• Totality duration — computed for each pin using NASA/Espenak path data (centerline coordinates, path width, and the standard duration formula).
• Tree canopy analysis — detects nearby trees that might block the low sun, using 1 m resolution height data from Meta/WRI downsampled to 30 m tiles. Loaded on demand — only the tiles around your pin are fetched.
• Solar disk visibility — reports whether the full corona is clear, or whether the lower limb is clipped by terrain. The sun's disk is ~0.53° across, so a site with +0.2° clearance will show a truncated view. Upper and lower limb lines are shown on the chart.

How to use it

• Click the map to drop a pin at a candidate viewing location
• Or paste coordinates from Google Maps into the fields above
• Set the observer height — your eye height above ground. Use 2 m for flat ground, 5–10 m if standing on a castle wall or rooftop.
• Optionally disable 🌲 Tree canopy analysis if not needed
• Click "Analyze all pins" to run the horizon computation
• Results show the clearance margin, solar disk visibility, totality duration, and tree warnings

Eclipse geometry

The sun's position at the moment of totality depends on where you are within the path:

• North coast (A Coruña – Bilbao): sun at ~10–13° altitude, ~278° azimuth
• Meseta (Palencia, Valladolid): ~8–10° altitude, ~281° azimuth
• South edge (near Madrid): ~6–8° altitude, ~283° azimuth
• Mediterranean / Balearics: ~2–5° altitude, ~286° azimuth

This tool automatically computes the correct sun position for each pin based on NASA eclipse path data.

Accuracy notes

• Elevation data: AWS Terrain Tiles (~30 m resolution, derived from SRTM). Adequate for landscape-scale terrain; individual buildings are not resolved.
• Earth curvature: Fully corrected, with standard atmospheric refraction (k=0.13).
• Tree canopy: Meta/WRI Global Canopy Height Map, tiled at 30 m resolution. Scans within 500 m of the observer. At 9° sun altitude, a 25 m tree blocks only within ~160 m. The Python script uses the full 1 m source data.
• Duration: Interpolated from NASA/Espenak path table (120-second intervals). Accuracy ±2 seconds — use Xavier Jubier's maps for precise timing.
• Solar disk: The sun's angular radius (~0.267°) is accounted for. "Full corona clear" means the entire disk clears the horizon; "Lower limb clipped" means the centre is visible but the bottom edge grazes terrain.
• Ray tracing: Horizon computed at 0.5° azimuth intervals across ±20° around the sun, sampling every ~40 m out to 40 km.

Python version

For a full 360° horizon analysis with higher-resolution SRTM1 data, download the Python script from the GitHub repository. It produces detailed plots and supports custom analysis.