How Sundrift works
A sunrise table answers one question: when does the sun cross the sea horizon. In a valley, behind a ridge, next to a mountain — that answer is wrong by hours, and in winter it can be wrong by months. Rjukan in Telemark loses direct sun for over a hundred days a year; the sunrise table never notices.
The horizon is the answer
When you tap a point, the Sundrift engine reads the terrain in every direction — 360 one-degree bins, out to 30 kilometres — and records the highest obstruction angle in each: the real horizon as seen from that exact spot, including Earth curvature and standard atmospheric refraction.
That profile is the whole trick. The sun's position for any instant of any date is pure astronomy, computed on your device. The sun shines on your point exactly when its elevation clears the measured horizon in its direction of travel. One profile answers today, tomorrow, and every other day of the year — which is why scrubbing through the day costs nothing and works offline.
What "free" includes
- Today's true sun window at any point you tap, with the horizon drawn.
- Scrubbing through the day — in sun, in shadow, sun below horizon.
- Split windows when a ridge interrupts the sun's path mid-day.
The paid report — the full year calendar, first and last sun dates, floor-by-floor comparison and a PDF — is in development, alongside the iPhone and iPad app.
What the answer does not include yet
The development preview models terrain everywhere and buildings in Norway (OSM footprints with heights where tagged). Vegetation is in progress, and until a factor is modelled at your point, the result says so rather than pretending. A terrace behind tall trees will read sunnier than it is until the vegetation tier lands — the label on every result is the contract.
Why the engine fails rather than guesses
If elevation data is missing for part of the horizon, Sundrift refuses to answer instead of silently assuming flat ground. A wrong "the sun arrives at 09:58" is worse than no answer — this rule is load-bearing in the engine and will not change.