How much sun do the mountains steal from you?

Băile Herculane, 1 January 2018, 11:24

A photograph tests the model

A terrain calculation can be put to the test: a photograph whose time and place were logged by the phone shows the sun on the ridge, and the calculation, made without the photograph, says when it should get there: the top edge of the sun clears the ridge at 11:21. The photograph was taken at 11:24. It is a single check, but one the model could have failed.

Where the mountains take the most

We ran the same calculation for the centre of each of Romania's 3,181 communes and towns, for every day of 2026. On the shortest day of the year, 6.7 million people, 35% of the population, live in communes and towns where, at the centre, the terrain takes at least half an hour of direct sun. In places with 2.7 million inhabitants, the loss at the centre is at least an hour. The calculation is not made at every house: everyone in a commune is counted at its central point.

The biggest losses are in narrow valleys: Băile Herculane (Caraș-Severin) loses 41% of the year's direct sun; Măguri-Răcătău (Cluj) loses 36% of the year's direct sun; Slănic-Moldova (Bacău) loses 30% of the year's direct sun. A single point can mislead: at Toplița (Hunedoara), top of the ranking below, on 21 December the sun clears the mountain at 12:37 at the village centre, but at 8:35 100 metres further north and at 14:04 100 metres further south. At Băile Herculane the result holds: at the centre and at the four points 100 metres away, the sun appears between 10:39 and 11:07. At the centre of Perișani (Vâlcea), at the foot of a hill to the south, the sun does not clear the hill at all on 25 days a year, from 9 December to 2 January. A hundred metres further south the run grows to 80 days; a hundred metres further north, 21 December has 4 hours 23 minutes of direct sun.

Over a whole year of clear skies, counting each person at the centre of their commune or town, a Romanian loses on average 157 hours of direct sun to the terrain. The big lowland cities lose around 1% or less: Bucharest under 0.1%, Timișoara 0.2%, Constanța 0.4%.

The mountains' shadow, minute by minute

The same calculation for the whole country at once: each place is lit according to how its slope faces the sun, and put in shadow if terrain rises between it and the sun. On the morning of 21 December the Carpathians' shadows stretch tens of kilometres across the basins; at midday in June they almost disappear.

Rankings

Where the sun appears latest on 21 December

The larger places: how much sun the terrain takes

The 200 most shaded places, over the whole year

How we calculated

Terrain

Terrain Tiles (Mapzen/Tilezen, on Amazon Web Services), a terrain model built from SRTM (NASA), EU-DEM and other sources. In mountain areas, within 8 km of you, the calculation uses cells of about 30 m; up to 60 km, cells of about 310 × 220 m; beyond that, up to 450 km within the grid (19° to 31° E, 41.5° to 49.3° N), cells of about 925 × 650 m. Outside areas with more than 250 m of relief within a few kilometres, the 30 m grid is not used, because there it would mostly see blocks of flats.

The horizon

From the chosen point, 1.7 m above the ground, the calculation follows 1,440 directions, one every quarter of a degree, and keeps the highest angle at which terrain appears. It subtracts the curvature of the Earth and allows for the bending of light in the air near the ground (coefficient k = 0.13).

The sun

The sun's position comes from Jean Meeus's formulas, as used by NOAA's solar calculator; compared with the JPL DE421 ephemeris at 1,561 random times and places, the sun's altitude differs by at most 0.012 degrees, a few seconds of clock time. The sun counts as "above the mountain" when its upper edge, with atmospheric refraction added, clears the ridge line, the same rule used for almanac sunrise.

Clock time

Romanian clock time: Eastern European Time, moving to summer time on the last Sunday of March and back on the last Sunday of October. The calculation is for 2026; from one year to the next the times shift by at most a minute, except on the days between the two years' clock-change dates.

Places

The centre of each of the 3,181 communes and towns is the OpenStreetMap point for its seat village or town, matched by the SIRUTA code of the administrative unit (3,022 points) or by name (65); for 94 we kept the coordinates from Wikidata and GeoNames. The population is from the 2021 census and is all counted at that point: a house on the sunny slope or at the edge of the village can have a different horizon.

Checks

The horizons were compared with those calculated by the European Commission's Joint Research Centre for PVGIS at 40 hill and mountain places picked at random: the typical difference is 1.3 degrees, and at 36 of them our horizon is lower. In 12 deep valleys, Băile Herculane among them, the typical difference is 2.2 degrees, up to 5.9, and our horizon is lower at 11. The sun losses on this page are, if anything, underestimated. The Băile Herculane photograph is a third check, independent of both.

What the calculation leaves out

Clouds, fog and winter temperature inversions, which in many basins hide the sun more than the mountains do. Buildings, trees and poles around you. The hours of "direct sun" are those possible under clear skies.

Further

Flat-horizon sunrise for any city in the world: Light at six. Which mountains you can see from home, with the same terrain model: Can you see the mountains from home?

Sources

1. Terrain: Terrain Tiles (Mapzen/Tilezen) on Amazon Web Services, from SRTM (NASA), EU-DEM and other sources; full source attribution.
2. Sun position: J. Meeus, Astronomical Algorithms, 2nd ed., Willmann-Bell, 1998; NOAA Global Monitoring Laboratory, Solar Calculation Details. Check: the JPL DE421 ephemeris, through the Skyfield library.
3. Control horizons: European Commission, Joint Research Centre, PVGIS 5.3, printhorizon service.
4. Terrestrial refraction: C. Hirt et al., "Monitoring of the refraction coefficient in the lower atmosphere using a controlled setup of simultaneous reciprocal vertical angle measurements", Journal of Geophysical Research 115, D21102, 2010.
5. Places: © OpenStreetMap contributors, ODbL, extracted through the Overpass API on 25 September 2026; Wikidata; GeoNames. Population: National Institute of Statistics, 2021 Population and Housing Census. Borders: Natural Earth.
6. Photograph: Leontin l, "Baile Herculane 2018 (7)", Wikimedia Commons, CC BY-SA 4.0; time and position from the file's EXIF data.