Original – Lighthouse visibility defies Earth’s curvature logic.
The range at which various lighthouse beams can be seen at sea far exceeds what would be expected on a globe Earth with a 25,000-mile circumference. For example, the Dunkerque Lighthouse in southern France, standing at an elevation of 194 feet, is visible from a boat 10 feet above sea level and 28 miles away. According to spherical trigonometry, if the Earth were a sphere with a curvature of 8 inches per mile squared, this light should be obscured 190 feet below the horizon.
Versus – Refraction reveals objects hidden by Earth’s curvature.
The range at which various lighthouse beams can be seen at sea can indeed seem to exceed expectations on a globe Earth if atmospheric refraction is not considered. Refraction bends the light downward, extending visibility beyond the geometric horizon calculated using simple spherical trigonometry. For instance, the Dunkerque Lighthouse visibility can be explained through atmospheric refraction, which makes it possible to see objects below the geometric horizon. Mariners and navigators have long utilized this knowledge to account for observed discrepancies.