EarthCurve: Horizon Calculator

EarthCurve turns observer height, target height, and distance into hidden height, horizon range, and a cross-section you can read at a glance.

What it helps with
- Hidden height of a ship, island, tower, or shoreline beyond the horizon.
- Horizon distance from standing, boat-deck, cliff, and tower eye heights.
- Standard refraction (7/6 Earth radius) versus a pure geometric model.
- A distance ladder so you can compare nearby ranges at the same eye height.
- Saved sightings for repeats you check from the same spot.