Methodology
How we compute real eclipse visibility from your location
We combine topocentric astronomical computation, a 30 m digital elevation model and historical climatology to answer one simple question: will you actually see it from where you plan to be?
The problem
Official sources (NASA, IMCCE) publish the path of totality: the band within which the Sun is completely covered by the Moon. It is an exact astronomical calculation, but it ignores local terrain and atmosphere.
For the 12 August 2026 eclipse, the path crosses the north of the Iberian Peninsula and ends grazing Mallorca. On paper, 377 Spanish cities lie within it. In practice, in the vast majority the Sun will be less than 5 degrees above the horizon during totality — an “eclipsed sunset”. And a real topographic horizon (a mountain to the west, buildings, vegetation) can block the Sun before the event ends.
Our analysis identifies ten clean points where totality will actually be fully visible — concentrated in Galicia and Tenerife — versus the 377 that appear in a naive reading of the path map.
That difference, between “being inside the path” and “actually seeing it”, is what we compute at eclipses.app.
Our approach
1.Topocentric astronomical computation
We use `astronomy-engine`, an implementation of Jean Meeus’s algorithms (Astronomical Algorithms, 1998), with corrections for parallax, atmospheric refraction and proper motion. For each point on Earth we compute the exact position of the Sun and Moon in altitude and azimuth with sub-arcsecond angular precision.
From there we derive the eclipse contacts from your location: C1 (start of partial), C2 (start of totality if applicable), peak, C3 (end of totality) and C4 (end of partial), with their UTC and local timestamps.
2.30 m Digital Elevation Model
The key question is not “is the Sun geometrically above the horizon?” but “is it above the real topographic horizon?”. To answer it we need to know what surrounds the observer.
We use Copernicus DEM GLO-30 tiles, with 30-metre resolution (≈0.0003°) covering the entire Iberian Peninsula, the Balearic Islands and the Canary Islands. At each point we sample the horizon in 360° at several distances between 0.5 and 25 km, apply corrections for Earth’s curvature and atmospheric refraction, and obtain the maximum elevation angle the Sun would have to clear to be visible at each azimuth.
The difference (delta) between the Sun’s altitude and the topographic horizon’s altitude is what we call margin over the real horizon.
3.Historical cloud-cover climatology
Clouds are not computed, they are reproduced statistically. For each location and date we cross-reference:
Open-Meteo Forecast API when ≤ 16 days remain to the event (short-range numerical prediction).
ERA5 climatology (ECMWF reanalysis, 1991-2020 baseline) when more days remain, to answer “what is the historical probability of clear skies at that time of day and in that month”.
We sample in the direction of the Sun, not just at the zenith: at low solar altitude what matters are the clouds towards the west, not those overhead.
0-100 score
We combine the above factors into a single number between 0 and 100. A score > 80 means excellent conditions across all dimensions: totality, ample margin over the real horizon, significant duration and favourable weather. A score < 40 means it’s worth looking for another nearby spot: either because the Sun will be almost on the ground, because a mountain silhouette will hide the event from you, or because clouds historically cover the area in August at that hour.
The score combines four factors: eclipse type (total > annular > partial), margin over the real topographic horizon, duration of the phenomenon from your point, and climatology. The exact formula is not published to prevent SEO gaming, but the breakdown is always accessible via API with an authorised account (contact for academic access).
Honest limitations
What the score does not model:
- Specific cloud cover on the day of the event — climatology is statistical; the actual day may be an exception.
- Local air conditions — haze, suspended dust, humidity. They affect the appearance of the low Sun, not computed.
- Nearby obstacles — an adjacent building, a tree, a streetlight ruin the field of view and do not appear in the 30 m DEM.
- Unplanned events — wildfires, Saharan dust, exceptional contrails.
These limitations are not negligence, they are honesty about the model: the best way to reach the spot with confidence is to check the forecast 24 h beforehand and have a 60-90 minute drive alternative ready. For PRO users, the app includes a meteorological plan B: three alternative locations within a reasonable radius, with real-time updated forecast.
Open data
We publish:
- Press kit with the 2026 eclipse summary + JSON dataset by autonomous community.
- Public API to verify our calculations against external sources (no commercial use).
- Score map in raster format covering the entire eclipse corridor.
Validation
We are in contact with Spanish astronomical associations for cross-validation of the calculations at their observatories and recommended viewpoints. The up-to-date list of collaborators and endorsements is at . /colaboradores.
Are you a journalist or researcher?
If you need:
- Access to the full dataset (CSV/JSON without gating)
- Verification of a specific calculation before publishing
- Visual examples to illustrate an article
- Technical citations with attribution
Write to us at prensa@eclipses.app or check the press kit at /prensa. We answer within 48 hours.