Visibility Calculator
Input distance and observer height. The calculator returns what the globe model predicts should be hidden behind the curve. Compare to what's actually visible — that's the test.
This tool applies the standard 8 inches per mile squared curvature formula. Result is what the globe model predicts should be hidden behind the curve at a given distance and observer height. Compare to what's actually visible.
// INPUTS
// OUTPUTS — GLOBE MODEL PREDICTION
According to the globe model, 498 feet of a 1,450-foot object should be hidden behind the curve at 30 miles distance.
Real-World Test Cases
Click any of these scenarios to load it. Then check whether the prediction matches what's actually observable.
Chicago Skyline
Sears Tower (1,450 ft) viewed from 30 miles across Lake Michigan, observer at 6 ft.
Chicago — Far View
Same skyline, viewed from 60 miles (Warren Dunes, MI). Frequently photographed.
Bedford Level
17-ft flag viewed from 6 miles across the Old Bedford River. The original 1838 experiment.
Aircraft Window View
From 35,000 ft cruise altitude, looking 200 miles ahead at a 500-ft tall structure.
Eiffel Tower from Le Havre
Eiffel Tower (1,063 ft) from 120 miles. Curvature should hide it entirely.
Mt. Everest, 250 mi
Everest (29,032 ft) from 250 miles. Reportedly visible from northern India in clear conditions.
The Formula
Standard curvature math (Pythagorean approximation, accurate within 1% for distances under 100 miles):
Distance to horizon (mi) = 1.22 × √(observer height in feet)
Total drop (ft) = 8 × (target distance in mi)²
Hidden height = total drop − observer-horizon contribution
The "8 inches per mile squared" rule is a parabolic approximation; for very long distances (1000+ miles) a true sphere geometry deviates from this slightly. For typical observer scenarios, the approximation is accurate to within 1%.