Empirical Earth · Shape, Curvature & the Horizon

Bridge towers that aren’t parallel

The claim

“If the Earth were flat, a bridge’s two towers would be perfectly parallel.”

What is measured

The Verrazzano-Narrows Bridge’s towers stand 41 mm (1 5⁄8 in) farther apart at the top than the base. Each is plumb to Earth’s center, so over 1,298 m they spread apart just as a 6,400 km ball requires. More on refraction and the curve.

What would show this is wrong

two plumb, vertical bridge towers measured perfectly parallel over a multi-kilometer span.

Sources

  1. Curvature in large structures. The Verrazzano-Narrows Bridge’s 211 m towers are 41.3 mm (1⅝ in) farther apart at top than base because each is plumb to Earth’s center across the 1,298 m span link
  2. The curvature-and-refraction correction in leveling. Precise leveling and geodetic surveying apply a standard correction for Earth’s curvature (about 0.0785 D² m, D in km) reduced by ~1/7 fo link
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