SunreachAbout the data
Every time on this site and in Sunreach comes from one engine: the sun's position from the NOAA/Meeus equations, and a skyline computed from terrain data around the spot. Answers state which terrain they used and how accurate they are.
How is "sun on the spot" defined?
The spot is in direct sun when any part of the sun's disc is above the skyline in the sun's direction, seen from 1.7 m above the ground. All definitions:
| Sun position | NOAA / Meeus low-precision equations, about 0.01° |
|---|---|
| Refraction | Saemundsson: apparent altitude = true altitude + R(true altitude) |
| Flat sunrise and sunset | True altitude of the sun's centre −0.833° |
| Sun on the spot | Apparent altitude + 0.2667° (the sun's radius) above the skyline angle at the sun's azimuth |
| Skyline | Marched from 1.7 m above the ground in 720 directions (0.5°); 20 m steps to 4 km, then 40 m, to 25 km |
| Skyline with lidar | Same march; within 2 km samples come from 2 m lidar (step max(2 m, 0.5 % of distance), at most 20 m); bare ground only, trees and buildings are not counted for now |
| Earth curvature | drop = d² / (2 R), R = 6,371 km / (1 − 0.13) for terrain refraction |
| Golden hour | True altitude −4° to +6° |
| Blue hour | True altitude −6° to −4° |
| Low warm light on the spot | Sun on the spot and apparent altitude below 6° |
| Times | Always in the spot's time zone; a day runs from local midnight to midnight (23 or 25 hours on daylight-saving days) |
How accurate are the times?
About ±1 minute where the skyline comes from 2 m lidar and ±5 minutes with 40 m global terrain, plus ±2–4 minutes from atmospheric refraction when the sun is very low.
- Angle error ≈ height error ÷ distance: 10 m at 2 km is 0.3°, about 2 minutes at Slovenian latitudes; at 20 km about 10 seconds.
- 30 m global elevation data reads summits up to about 100 m low, so nearby peaks matter most.
- Narrow gorges and overhangs can be below the resolution of the terrain model, and trees and buildings are not counted for now.
- At higher latitudes the sun moves more slowly near the horizon, so the same terrain error costs more minutes.
The engine is tested against reference values from an independent Python implementation. For Bovec (46.3378° N, 13.5521° E) on 18 October 2026 both give direct sun 08:34–17:40 and a flat-horizon day of 07:27–18:14.
Where does the terrain come from?
From open elevation data: 40 m global terrain everywhere, and lidar in supported areas of Slovenia, France, Austria, Switzerland, Italy, Spain, Germany and the USA (2 m; 5 m in Spain).
| Global terrain | AWS Terrain Tiles (Tilezen, terrarium tiles at zoom 12), resampled to 40 m over 50 × 50 km. Sources include EU-DEM (© Copernicus), SRTM and GMTED (USGS). |
|---|---|
| Slovenia | 2 m lidar: Slovenian Environment Agency (ARSO), Laser scanning of Slovenia 2011–2015, CC BY 4.0 (terms of use). Ground model (DMR1). The data is provided as is, without warranty. |
| France | IGN, LiDAR HD, Licence Ouverte 2.0. MNT ground model resampled to 2 m; source coverage varies. |
| Austria | © BEV – Bundesamt für Eich- und Vermessungswesen, CC BY 4.0. ALS ground model resampled from 1 m to 2 m. |
| Switzerland | © swisstopo (Federal Office of Topography), swissALTI3D and swissSURFACE3D. 2 m ground tiles read on demand. |
| Spain | © Instituto Geográfico Nacional (IGN), MDT05 from PNOA-LiDAR, CC BY 4.0 (scne.es). 5 m ground model interpolated to 2 m. |
| Germany | © Bayerische Vermessungsverwaltung, DGM1, CC BY 4.0 · © LGL, www.lgl-bw.de, DGM1, dl-de/by-2-0 · © Geobasis NRW, DGM1, dl-de/zero-2-0 · © GeoBasis-DE / LVermGeo ST, DGM1, dl-de/by-2-0. 1 m ground models averaged to 2 m. |
| Italy (Trentino) | © Provincia autonoma di Trento, LiDAR PAT 2014–2018, CC BY 4.0. 0.5 m ground model resampled to 2 m. |
| Netherlands | AHN4, © Rijkswaterstaat and Het Waterschapshuis, CC0. 0.5 m ground model cut to 2 m by PDOK’s coverage service. |
| Flanders | DHMV II DTM 1 m (2013–15). Bron: Digitaal Vlaanderen, Modellicentie Gratis Hergebruik. 1 m ground model resampled to 2 m. |
| Czechia | DMR 5G (2009–13), © ČÚZK, CC BY 4.0. 2 m ground model read from ČÚZK’s image service. |
| Poland | NMT 1 m, © GUGiK, free for any use. 1 m ground model resampled to 2 m. |
| Norway | NHM DTM 1 m, © Kartverket, CC BY 4.0. 1 m ground model resampled to 2 m. |
| Finland | Contains data from the National Land Survey of Finland, elevation model 2 m, 2018, CC BY 4.0, via CSC Paituli. 2 m ground model. |
| Estonia | Kõrgusandmed (elevation data), Maa- ja Ruumiamet, open data licence. 1 m ground model resampled to 2 m. |
| USA | Bare ground from the U.S. Geological Survey 3D Elevation Program (public domain; the survey is named in the app). |
| Place names (app) | © OpenStreetMap contributors |
| Weather (app) | Apple Weather, with the Apple Weather mark and legal link shown next to forecasts in the app. |
| Aurora (app) | NOAA SWPC: OVATION nowcast, planetary Kp and its 3-day forecast, real-time solar wind, WSA-ENLIL and the 27-day outlook (public domain). GFZ Potsdam: Hp30 (CC BY 4.0). What a spot needs comes from a magnetic dipole (IGRF 2025 pole), NOAA’s Kp table and the spot’s skyline: a guide, not a promise. |
Which places have high-resolution coverage?
Slovenia: 2 m ground model within 2 km in the bundled Velika korita Soče and Ihan areas; wider coverage requires additional hosted sheets.
France: 2 m ground model from IGN LiDAR HD where processed sheets are available in metropolitan France and Corsica; coverage depends on published source data and available sheets.
Austria: 2 m ground model from BEV airborne laser scanning where processed sheets are available; coverage depends on available sheets.
Switzerland: 2 m ground model (swissALTI3D) fetched from swisstopo for spots in Switzerland and Liechtenstein.
Italy: 2 m ground model from the Province of Trento’s lidar in the bundled Pale di San Martino area (Baita Segantini); the rest of Trentino, Valle d’Aosta and Friuli Venezia Giulia require additional processed sheets.
Spain: 5 m ground model (IGN MDT05, from PNOA lidar) fetched for spots in mainland Spain and the Balearic and Canary Islands.
Germany: 2 m ground models from the 1 m state terrain models (DGM1) of Bavaria, Baden-Württemberg, North Rhine-Westphalia and Saxony-Anhalt; other states use global terrain for now.
USA: 2 m ground models within 2 km where USGS 3DEP has suitable high-resolution coverage.
Netherlands: 2 m ground model from AHN4 (0.5 m lidar), fetched for spots anywhere in the Netherlands.
Belgium (Flanders): 2 m ground model from the Digital Elevation Model of Flanders II (1 m lidar), fetched for spots in Flanders; Brussels and Wallonia use global terrain for now.
Czechia: 2 m ground model (ČÚZK DMR 5G, from lidar), fetched for spots anywhere in Czechia.
Poland: 2 m ground model from GUGiK’s 1 m lidar terrain model (NMT), fetched for spots anywhere in Poland.
Norway: 2 m ground model from Kartverket’s 1 m national lidar terrain model, fetched for spots anywhere in mainland Norway.
Finland: 2 m ground model from the National Land Survey of Finland’s 2 m elevation model, fetched for spots anywhere in Finland.
Estonia: 2 m ground model from Maa- ja Ruumiamet’s 1 m lidar terrain model, fetched for spots anywhere in Estonia.
Ground models describe bare terrain. Trees and buildings are not counted for now: every skyline is bare ground, so a tree, roof or wall close to the spot can still block the sun.
Does the homepage show measured sunlight?
No. It illustrates a computed day at Tunnel View in Yosemite on 21 June 2027. Times are calculated for the marked viewpoint under clear-sky conditions, not for the distant cliffs.
The animation draws simplified 80 m terrain from a 40 m sample grid. Its clock uses the separate skyline calculation with 2 m USGS ground data nearby. Atmospheric conditions, trees and buildings can change what you see. Read the example and compare seasons.
Who makes this data possible?
Explore the national mapping agencies, terrain providers, map contributors and photographers behind Sunreach. Our attributions page records the source, licence, processing changes and current coverage for each layer.
Can I quote these numbers?
Yes. Please link the page you quote, and keep the terrain source and accuracy with the time.