Horizon lab

Line of sight d–

Along surface s–

With refraction–

Horizon dip–

Earth visible–

Everest visible to–

Live flight · Flightradar24

API key

Sandbox key Remember on this device

Advanced

Proxy URL (optional) Update every

Each update spends API credits. Updates use the cheaper "light" endpoint, and the view dead-reckons smoothly between them.

Cross-section

Distance vs height

Quick previews · altitude

Quick previews · location

Settings

Imagery View Time warp 3D terrain Horizon ring on globe HUD horizon lines Altitude hold Invert pitch

The math

Observer at height h above a sphere of radius R. The sight line to the horizon is tangent to the surface, so it meets the radius at a right angle.

d² + R² = (R + h)²
d = √(2Rh + h²) ≈ √(2Rh)

On Earth that is d ≈ 3.57 √h km with h in metres.

cos θ = R / (R + h)
s = R·θ dip = θ

θ is the angle at Earth's centre. It is also how far the horizon sits below eye level (dip). s is the distance you would travel over the ground to reach it.

R′ = R / (1 − k), k ≈ 0.13

Refraction bends light around the bulge a little. This is the same as a ~15% larger Earth: d ≈ 3.86 √h km. The 3D view is drawn without refraction, so the ring uses the geometric value.

fraction visible = h / (2(R + h))

Seeing a peak of height H: d_max = √(2Rh) + √(2RH).

Engine: CesiumJS (Apache-2.0). Imagery: Sentinel-2 cloudless by EOX (contains modified Copernicus Sentinel data) or © OpenStreetMap contributors. Terrain: AWS Terrain Tiles. Live flight data: Flightradar24 API (your own key).

W/S or ↑/↓ pitch · A/D or ←/→ roll · Q/E yaw · Shift/Ctrl throttle · 1–9 altitude presets · C view · H alt hold · T time warp · P pause · M panel

Horizon Flight Simulator

The higher you fly, the farther the horizon. Climb and watch the blue ring: it marks the exact edge of what you can see, worked out from d = √(2Rh + h²).

Got a Flightradar24 API key? Open the Horizon lab to ride along with a real flight and see its horizon live.

Keyboard: arrows or WASD, Shift/Ctrl throttle, number keys jump altitude. On touch screens, use the stick and throttle.